Linkage and Crossing-over.pptx

LINKAGE AND CROSSING-OVER SMG
A brief description of Linkage - Bateson and Punnett's Experiment on Sweet pea, Lathyrus odoratus, Coupling and Repulsion Theory, Complete and Incomplete Linkage, Significance of Linkage, Crossing-over: Cytological basis, Types, Factors influencing the frequency , Significance, Mitotic crossing-over

1. LINKAGE AND CROSSING-OVER Dr Saji Mariam George Associate Professor (Retired) Assumption College Autonomous Changanacherry

2. LINKAGE AND CROSSING-OVER Genes (segment of genetic material – DNA in most organisms and RNA in some viruses) or hereditary units that control a particular character are located in chromosomes. In other words, chromosomes are the physical bearers of hereditary units called genes(Chromosome theory of heredity- proposed independently by Walter Sutton - based on studies in Grasshoppers and Theodor Boveri - based on studies in Sea urchins, 1902 - 1903). Walter Sutton Theodor Boveri https://biology-igcse.weebly.com

3. British geneticists, Bateson, Saunders and Punnett (1906) had carried out hybridization experiments in two varieties of Sweet pea ( Lathyrus odoratus ). In this experiment, they had focused the inheritance of two pairs of contrasting characters such as flower colour (purple vs. red) and shape of pollen grains (long vs. round), [a dihybrid cross]. One Sweet pea plant was with purple flowers and long pollen grains and the other with red flowers and round pollen grains . Sweet pea ( Lathyrus odoratus ) Edith Rebecca Saunders , William Bateson & Reginald Crundall Punnett   Creative Commons Attribution 4.0 International , https://www.researchgate.net , © 2014 Cornell University, https://pollen.tstebler.ch Pollen grains of Sweet pea

4. Let ‘ P ’ stands for the allele for purple flower colour and ‘ L ’ , for the allele for long pollen grains . So, the genotype (genetic constitution) of the pure breeding homozygous Sweet pea parent plant with purple flowers and long pollen grains can be represented as PPLL . From previous hybridization experiments in Sweet pea, it had been found that, purple flower colour is dominant over red and long pollen grain is dominant over round . Hence the genotype of the other Sweet pea plant with red flowers and round pollen grains (both recessive traits) can be represented as ppll . All the F1 (First filial generation) plants had purple flowers and long pollen grains (hybrids, Genotype in heterozygous condition, PpLl ) as expected. On further crossing ( selfing , F1 x F1 – PpLl x PpLl ), the F1 plants produced the F2 generation (Second filial generation ). [Alleles : Two or more alternative forms of the same gene having different phenotypic effects ]

5. Bateson, Saunders and Punnett had found that the F2 phenotypic ratio deviated significantly from the expected typical dihybrid F2 phenotypic ratio , 9:3:3:1 ( That is, 9 purple long : 3 purple round : 3 red long : 1 red round - This ratio is obtained when there is independent assortment of two genes that control the two traits). They had found that two parental classes (purple flowers with long pollen grains and red flowers with round pollen grains) were formed in large numbers ( over represented ) and the non- parentals (i.e., recombinants - purple flowers with round pollen grains and red flowers with long pollen grains) were formed only in lesser numbers ( under represented ) than expected.

6. Image :https://www.chegg.com F2 : Purple long (parental type, formed in more numbers ) , purple round (non-parental type or recombinants , formed in less numbers ) , red long (non-parental type or recombinants, formed in less numbers ) , red round (parental type, formed in more numbers than expected ). F2 phenotypic ratio deviated significantly from the expected 9:3:3:1 – Indicates lack of independent assortment between the two genes that control the two traits , flower colour and shape of pollen grains.

7. A test cross between F1 dihybrid and the homozygous double recessive parent ( PpLl x ppll ) also showed deviation from the expected dihybrid test cross ratio 1:1:1:1 (i.e., 1 purple long : 1 purple round : 1 red long : 1 red round . That is, both parental types of progeny and the non-parental types or recombinants are formed in equal proportions). Instead, the test cross progeny included more number of parental types ( purple long and red round ) and less number of non-parental types or recombinants ( purple round and red long ). Bateson, Saunders and Punnett had analysed their observations and proposed the ‘ Coupling and Repulsion Theory ’ to explain the lack of independent assortment . [ Test cross: A cross between F1 hybrid and the homozygous recessive parent )

8. Coupling and Repulsion Theory Coupling Phase According to Bateson, Saunders and Punnett , in Sweet pea, there might be a connection between the parental alleles for flower color and shape of pollen grains which resulted in lack of independent assortment and deviation from the expected dihybrid F2 phenotypic ratio, 9:3:3:1. That means, the genes for flower colour and pollen shape tend to remain together and do not assort independently as per Mendel’s law of independent assortment.

9. Coupling phase can be defined as the condition in which the two dominant alleles tend to enter the gametes together in greater than random proportion. The Sweet pea double heterozygote ( dihybrid ) PpLl (Purple long) in coupling phase has two dominant alleles , P and L from one parent (Genotype PPLL – Sweet pea with purple flowers and long pollen grains) and two recessive alleles, p and l from the other parent (Genotype ppll - Sweet pea with red flowers and round pollen grains) and they tend to enter the same gamete and to be transmitted together and hence there is no independent assortment .

10. The double heterozygote , PpLl in Coupling Phase ( PPLL x ppll → PpLl – Coupling double heterozygote) Here, the dominant alleles , P and L are on one chromosome and their recessive alleles p and l are on the other chromosome of a homologous pair – i.e., in Coupling phase . [ Homologous pair : The maternal and paternal chromosomes ]

11. The genotype of the F1 double heterozygote, PpLl can be written as PL/pl to denote the coupling phase, where the slash (/) separates alleles inherited from different parents. This means that the alleles on the left and right of the slash are on different homologous chromosomes, one from each parent . The coupling might have prevented their independent assortment in the F 1 . Hence the parental combinations of alleles P (dominant allele for purple flowers) and L ( dominant allele for long pollen grains) and p (recessive allele for red flowers) and l (recessive allele for round pollen grains) were more prevalent than the non-parental combinations P and l and p and L in the gametes of the F1 plants.

12. Self fertilization of F1 therefore, produced more number of plants with parental types (purple flowers with long pollen grains and red flowers with round pollen grains) and less number of non-parental types or recombinants (purple flowers with round pollen grains and red flowers with long pollen grains ) in the F2 generation than expected. The non-parental types of progeny or recombinants have one character from one parent and another character from the other parent. That is, in purple round types, the trait purple flower colour from one parent and the round pollen grain shape from the other parent. Similarly, in red long types, the trait red flower colour from one parent and the long pollen grain shape from the other parent.

13. Repulsion Phase Repulsion denotes the phase where  the double heterozygote , PpLl has two dominant alleles P and L and recessive alleles p and l from two different parents . That is, from a cross between a Sweet pea plant with purple flowers and round pollen grains (Genotype PPll ) and another one with red flowers and long pollen grains (Genotype ppLL ) and they tend to enter different gametes and to remain apart . This is called  gametic repulsion . In this case, the dominant allele P and the recessive allele l are found on the same chromosome and the dominant allele L and the recessive allele p are found on the other chromosome of a homologous pair. In other words, the non-allelic dominant alleles “repelled” each other – This is called repulsion . That means, each homologous chromosome has one dominant allele and one recessive allele of the two genes. That is, there is a tendency for one dominant allele of one trait and one recessive allele of another trait to enter the gametes in greater proportion .

14. The double heterozygote , PpLl in Repulsion Phase ( PPll x ppLL → PpLl – Repulsion double heterozygote) Here, dominant allele of one gene and the recessive allele of the other gene are found on each homologous chromosome . i.e., in Repulsion phase .

15. The genotype of the double heterozygote, PpLl in Repulsion phase can be denoted as Pl/ pL which means that two dominant alleles P and L or recessive alleles p and l are introduced into the double heterozygote by two different parents and hence are located in two different chromosomes of a homologous pair and are inherited separately. Thus, the double heterozygote PpLl may be either in coupling phase ( PL/pl ) or in repulsion phase ( Pl/ pL ). Haldane (British scientist, 1942)used the term Cis for coupling phase and Trans for repulsion phase. J.B.S Haldane (John Burdon Sanderson Haldane)

16. Even though Bateson, Saunders and Punnett had tried to explain their results in Sweet pea breeding experiments involving two pairs of contrasting characters (Flower colour : Purple vs. Red ; Shape of Pollen grains : Long vs. Round) by proposing the Coupling and Repulsion Theory , they had failed to give a precise explanation for the deviation from the typical Mendelian ratios (i.e., dihybrid F2 phenotypic ratio, 9:3:3:1 and dihybrid test cross ratio, 1:1:1:1 )and lack of independent assortment. Later, the Coupling and Repulsion Theory was replaced by the Theory of Linkage and Crossing-over put forward by Morgan et.al., (1911).

17. Morgan’s Theory of Linkage and Crossing-over Thomas Hunt Morgan (1910) and his associates (William Ernest Castle, Alfred Henry Sturtevant, Hermann Joseph Muller and Calvin Blackman Bridges, Columbia University, New York) had carried out breeding experiments in fruit fly (vinegar fly), Drosophila melanogaster . Eventhough , it was Sutton and Boveri (1902-1903) who proposed the chromosome theory of heredity, the first experimental evidence for it was given by Morgan et. al ., – the studies on inheritance of white-eyed trait in Drosophila . T. H. Morgan W. E. Castle A. H. Sturtevant H. J. Muller C. B. Bridges

18. Morgan et. al ., had found that Coupling and Repulsion are two aspects of the same phenomenon called Linkage . They had proposed that two genes are found in coupling phase because they are present on same chromosome and are said to be   linked . Similarly , two genes are in repulsion phase because they are present on two different homologous chromosomes.

19. Chromosome Theory of Linkage Morgan and Castle (1911) had proposed the Chromosome Theory of Linkage based on studies in Drosophila . The important postulates of the theory are The genes are linearly arranged in a chromosome . Each gene is located at a specific point called locus. (Plural: Loci). Genes located in the same chromosome tend to remain together and are inherited as a single unit. This is known as linkage . In other words, linkage can be defined as the tendency of the genes present in the same chromosome to be inherited together as a single unit during meiosis. All the genes located in a single chromosome constitute a linkage group . The total number of linkage groups in an organism corresponds to the haploid chromosome number (or equal to the number of homologous chromosome pairs ). Linked genes tend to stay in parental combinations.

20. The strength of the linkage depends upon the distance between linked genes in a chromosome. Genes that are located very close to each other show stronger linkage than the genes that are far apart from each other. That is, the strength of linkage is inversely proportional to the distance between linked genes .

21. Linkage Groups – Number of linkage groups is same as that of the haploid chromosome number of a species : Examples Organism Haploid Chromosome Number Linkage Groups Sweet Pea ( Lathyrus odoratus ) 7 7 Maize ( Zea mays ) 10 10 Fruit fly or Vinegar fly ( Drosophila melanogaster ) 4 4 Onion ( Allium cepa ) 8 8

22. Types of Linkage There are two types of linkage viz. complete and incomplete . Complete Linkage   The genes that are very closely located in a chromosome show complete linkage and are inherited together as a single unit . Hence only parental types are produced in succeeding generations. There is no independent assortment and crossing over . Hence no recombinants are produced . However, complete linkage is very rare - reported only in male Drosophila melanogaster and female silk moth, Bombyx mori . Drosophila melanogaster male fly https://www.semanticscholar.org Female silk moth Saved from  aureus-butterflies.de Saved by  Carmen Gómez https://www.pinterest.com Female silk moth, Bombyx mori

23. Example : Complete Linkage in male Drosophila melanogaster Morgan and Lynch (1911) had found that in Drosophila, the genes for body colour (gray vs. black) and wing size (long vs. vestigial ) were linked to each other. It is also to be noted that the allele for gray body colour is dominant over black and the allele for long wings is dominant over vestigial wings. Gray body, long wings Black body , vestigial wings https://sciencemusicvideos.com [Long wings : Normal functional wings Vestigial wings : Shriveled or crumpled, non-functional wings ]

24. Let ‘ G ’ represent the allele for gray body colour and ‘ g ’ for black body colour . Similarly, let ‘ L ’ represent the allele for long wings and ‘ l ’ for vestigial wings . Let us consider a cross between a male Drosophila fly with gray body and long wings (Genotype GGLL ) and a female fly with black body and vestigial wings ( Genotype ggll ). Since the gene for gray body colour is dominant over black and the gene for long wings is dominant over vestigial wings, all the F1 flies have gray body and long wings (Genotype GgLl ).

25. A test cross between a F1 male fly with gray body and long wings (Genotype GgLl , can be represented as GL / gl in Coupling or Cis linkage phase) and the double recessive female parent fly with black body and vestigial wings (Genotype ggll ) , produced only parental types of progeny (i.e., with gray body, long wings and black body , vestigial wings ) in a 1 : 1 ratio (That means, 50% of the progeny with gray body, long wings and 50 % with black body , vestigial wings) with no recombinants instead of the normal dihybrid test cross ratio 1: 1: 1: 1 . (That is, 1 gray body, long wings : 1 gray body, vestigial wings : 1 black body, long wings : 1 black body, vestigial wings, where both parental types and non-parental types or recombinants are produced in equal proportions).

26. In test cross, non- parentals or recombinants (flies with gray body , vestigial wings and black body , long wings) were not produced because of complete linkage between the genes for gray body colour and long wings and between black body colour and vestigial wings in the F1 double heterozygote male fly so that they remained together during inheritance (GL/ gl ). Hence, there is lack of independent assortment and crossing-over. [Test cross to detect linkage: Test cross (Hybrid x homozygous recessive parent) can be used to detect linkage. A deviation from the typical test cross ratio (where both parental phenotypes and recombinant phenotypes are formed in equal proportions) is an indication of linkage ].

27. Complete Linkage in male Drosophila melanogaster In this figure, the chromosomes are drawn as rod shaped .The results of the test cross between F1 male Drosophila fly ( GgLl – can be represented as GL/ gl in coupling or cis linkage phase, Gray long) with double recessive female parent fly ( ggll , Black vestigial) deviated from the expected dihybrid test cross ratio, 1:1:1:1(1 gray long [ parentals ] : 1 gray vestigial [recombinants] : 1 black long [recombinants] : 1 black vestigial [ parentals ] ). Instead , the progeny include only parentals , Gray long and Black vestigial , produced in equal proportions. That is, 50 % Gray long : 50% Black vestigial i.e. in a ratio, 1:1 and no recombinants are produced . This is due to complete linkage and hence absence of crossing-over in F1 male Drosophila fly.

28. 2. Incomplete Linkage The genes that are distantly located in a chromosome are said to be incompletely linked . This phenomenon is called incomplete linkage ( Partial linkage ) . Incompletely linked genes in a chromosome have   chances of separation by a process called crossing-over . Most hybridizations involving incompletely linked genes produce both parental types of progeny (in large numbers) and non-parental types or recombinants (in small numbers - due to certain degree of crossing-over between the linked genes) .

29. Example: Incomplete Linkage in female Drosophila melanogaster Calvin Bridges crossed a wild type female Drosophila fly with gray body and long wings (Genotype GGLL ) with a male having black body and vestigial wings (Genotype ggll ) . The F1 flies were wild type with gray body and long wings (Genotype GgLl , can be represented as GL/ gl in coupling or cis linkage phase ). A test cross between F1 female fly and the double recessive male parent fly produced progeny with more parental types ( gray long and black vestigial , 83 % ) and a few non-parental types or recombinants ( gray vestigial and black long , 17 % ) and thus deviated from the typical dihybrid test cross ratio,1: 1: 1: 1, where both parental types and recombinants are produced in equal proportions(i.e., 1 gray long : 1 gray vestigial : 1 black long : 1 black vestigial). [ Wild type : The most common phenotype in a natural population ].

30. The deviation from the typical dihybrid test cross ratio, 1 : 1 : 1 : 1 and the occurrence of more parental types in the test cross progeny shows that the genes are linked . However, occurrence of a few non-parental types or recombinants in the progeny shows that these genes are incompletely or partially linked so that certain degree of crossing-over occurred during meiosis which resulted in the production of recombinant progeny .

31. Incomplete Linkage in female Drosophila melanogaster The test cross between F1 female Drosophila fly ( GgLl , in coupling or cis phase, GL/ gl , Gray long) with double recessive male fly ( ggll , Black vestigial) is shown. The presence of a few recombinants (Gray vestigial and Black long ) in the progeny indicates that , these genes are not so tightly linked (Incompletely or partally linked ) and have undergone certain degree of crossing-over during Pachytene stage of prophase of first meiotic division at the time of gametogenesis in F1 female fly. ( During Pachytene , each chromosome has 2 chromatids called sister chromatids . Crossing over occurs between non-sister chromatids ).

32. Thus, we have found that in Drosophila melanogaster , the linkage between the factors or alleles for body colour (gray vs. black) and wing size (long vs. vestigial ) were linked to each other and the linkage is complete in the male fly and incomplete in the female fly .

33. Significance of Linkage Linkage is the phenomenon where the genes located very closely in a chromosome have a tendency to be inherited together as a single unit during meiosis. This enables the preservation of parental gene combinations which in turn help to maintain the constancy of a species from generation to generation.   Linkage play an important role in the preservation of beneficial combinations of alleles as the tightly linked genes rarely undergo crossing-over. Linkage   between two or more genes in a chromosome that control different desirable characters is beneficial for simultaneous genetic improvement of two or more characters . On the other hand, in some chromosomes, a gene that control a desirable character may be closely linked with another gene that is responsible for an undesirable one. In such cases, along with the gene for the desirable trait, the other gene for the undesirable trait may also be inherited together, which may reduce the fitness of the progeny ( i.e., Linkage drag ). Such a condition will be a hindrance for a breeder to genetically improve those traits.

34. Linkage analysis is an effective method to locate a gene of interest in a chromosome . Linkage analysis has helped to find out the location of several genes on human chromosomes and to study the genetic basis of many human diseases. Linkage restrict genetic variability in a population . [ Genetic variability is essential for hereditary improvement of traits ].

35. Crossing-over Though all the genes in a chromosome are said to be linked, there is a possibility for those genes which are distantly located in a chromosome to get separated by crossing-over. Crossing-over occurs in all sexually reproducing organisms during the first meiotic division.

36. Crossing-over involves the reciprocal exchange of equivalent portions between non-sister chromatids of a pair of homologous chromosomes. Morgan and Cattell (1912) had called this process of interchanging, "crossing-over". The crossing-over break the linkage between genes that leads to shuffling and genetic recombination. Therefore, crossing-over results in new combination of alleles in the gametes. That means, crossing-over alters the pattern of genes in the chromosomes and thus creates genetic variability among population. [Population : A group of organisms of one species that interbreed and live in the same place at the same time]. Image :https://www.toppr.com

37. Cytological basis of Crossing-over (Physical mechanism of Crossing-over ) Studies in Maize (Corn, Zea mays ) by Harriet Baldwin   Creighton and Barbara Mc Clintock (1931) and in Drosophila by Curt Stern (1931) had proved that crossing-over involves physical exchange ( physical swapping ) of segments between non-sister chromatids of paired homologous chromosomes . Harriet Baldwin Creighton Barbara Mc Clintock & Curt Jacob Stern 

38. © 2014  Nature Education  Adapted from Pierce, Benjamin.  Genetics: A Conceptual Approach , 2nd ed. https://www.nature.com

39. The major cytological events that lead to the occurrence of crossing- over are the following. Pairing of homologous chromosomes known as synapsis occurs during Zygotene ( Zygonema ) stage of Prophase of the first meiotic division. Paired homologous chromosomes form a bivalent . The two homologous chromosomes do not fuse during pairing but remain separated by a space which is filled with a proteinaceous substance called synaptonemal complex. © 2014  Nature Education  Adapted from Pierce, Benjamin.  Genetics: A Conceptual Approach , 2nd ed. https://www.nature.com https://biology4isc.weebly.com Zygotene

40. During the next stage Pachytene ( Pachynema ), the chromosomes become shorter and thicker. Each chromosome split length wise (divide) to form two chromatids (sister chromatids ), held together by the undivided centromere . Thus , the bivalent consists of four chromatids (four strand stage or tetrad). Crossing-over occurs at this stage. Crossing-over takes place by symmetrical breakage, reciprocal exchange and reattachment of the pieces between any two non-sister chromatids of the tetrad. [Double-strand breaks: Programmed double-strand breaks by an endonuclease , Spo11( DNA topoisomerase VI subunit A) at specific chromosomal positions into segments – Formation of double-strand breaks does not occur uniformly across the genome. There are certain regions which are favorable for double-strand break formation known as double-strand break hotspots such as intergenic and/or promoter regions where nucleosomes are more widely dispersed . The union of segments of chromosomes is catalysed by an enzyme , DNA ligase . Hotspots : Location of some intense activity ] © 2014  Nature Education  Adapted from Pierce, Benjamin.  Genetics: A Conceptual Approach , 2nd ed. https://www.nature.com https://biology4isc.weebly.com Pachytene

41. Breakage and union between any two of non-sister chromatids result in two sets of chromatids , one set with unaltered parental gene combinations called the non-cross over chromatids or non-recombinant chromatids or non-crossovers and the other two with altered gene combination called crossover chromatids or non-parental or recombinant chromatids or crossovers where shuffling of genes occurred. As a result of crossing-over, non-parental combinations of genes appear in the gametes. Such gametes are called crossover gametes or recombinant gametes. © 2014  Nature Education  Adapted from Pierce, Benjamin.  Genetics: A Conceptual Approach , 2nd ed. https://www.nature.com

42. Joan E. Bailey-Wilson Ph.D. National Human Genome Research Institute https://www.genome.gov/genetics-glossary/Crossing-Over

43. Crossing-over results in the production of recombinant chromosomes - Leads to gene shuffling or genetic recombination Image credit : Snustard & Simmons , 2012

44. Sites of crossing-over (the point of interchange of chromosomal segments between non-sister chromatids ) can be cytologically recognized by a cross-shaped configuration called chiasma (Plural: Chiasmata - a term coined by Frans Alfons Janssens,1909 - a Belgian cytologist. ( Janssens ’ chiasmatype hypothesis). In other words, chiasmata are the cytological manifestation of crossing-over. Frans Alfons Janssens

45. Chiasma may be either terminal (located at the end) or interstitial (located in the middle part). Chiasmata can be seen during the Diplotene ( Diplonema ) stage of Prophase of the first meiotic division. During Diplotene stage, chromosomes repel each other slightly, maintaining close contact only at the centromere and at each chiasma . This partial separation makes it possible to count the chiasmata accurately. During Diplotene , the interstitial chiasma begin to be displaced along the length of the chromosomes by a process called terminalization .  uploaded by  Liangran Zhang https://biology4isc.weebly.com Diplotene

46. Types of Crossing-over i ) Single crossing-over In this type, a single chiasma is formed at one point between non-sister chromatids of a pair of homologous chromosomes. That means, only one crossing-over occurs. This results in the formation of single crossover gametes. Purves et al.,  Life: The Science of Biology , 4th Edition, by Sinauer Associates ( www.sinauer.com ) and WH Freeman ( www.whfreeman.com ) 

47. ii) Double crossing-over In this case, two chiasmata are formed simultaneously between any two points in the same pair of homologous chromosomes. That means, two crossing-over occur simultaneously in a pair of homologous chromosomes. This process had been named by Sturtevant as ‘double crossing-over’. Double crossing-over occurs at a frequency which is equal to (or less than) the product of two single crossing-over frequencies. Both the chiasmata may be formed between the same chromatids or between different chromatids . Thus two (two-strand double crossing-over), three (three-strand double crossing-over) or four (four- strand double crossing-over) chromatids of the tetrad may be involved in the process of double crossing-over. Two-strand double crossing-over (Here, only two strands of the tetrad are involved in the formation of a double crossing-over.) http://bio3400.nicerweb.net

48. Types of Double crossing-over

49. iii) Multiple Crossing-over Crossing-over occurs at three ( triple crossing- over ), four ( Quadruple crossing-over ) or more points in the tetrad. Frequency of multiple crossing-over is extremely low. Snustard & Simmons , 2012

50. Factors affecting the frequency of crossing-over Frequency of crossing-over (i.e., the number of crossing-over at meiosis I ) and recombination varies in chromosomes and also among species. In most of the species, the number of crossing- overs is much lower than the number of DNA double-strand breaks. This is because, all DNA double-strand breaks may not be repaired as crossovers. Some DNA double-strand breaks are repaired as non-crossovers. For example, in the plant Arabidopsis thaliana , out of 150 to 300 DNA double-strand breaks, only around 10 are repaired as crossovers. Similarly in Maize, out of 500 DNA double-strand breaks, only about 20 are repaired as crossovers. In humans, approximately 10% of DNA double-strand breaks are repaired as crossovers. The decision, which DNA double-strand breaks will be repaired as crossovers and which ones will be repaired as non-crossovers is called crossover-designation.

51. The number of crossing-over is strictly regulated to form an optimum number of physical links (i.e., chiasmata ) between homologous chromosomes to ensure their accurate segregation during anaphase of meiosis I. Even if there is fluctuation in DNA double-strand break formation, the frequency of crossing-over is maintained more or less constant (Crossover homeostasis). The formation of crossing-over is regulated in such a way that their number is kept at a very low level. This may help to maintain the DNA  sequences in cells with very little change from generation to generation. In most of the species, the average number of crossing-over per chromosome rarely exceeds three per bivalent. However, several genetic , epigenetic (= relating to or arising from non-genetic influences on gene expression e.g. DNA methylation ) and environmental factors can influence the frequency of crossing-over. [DNA methylation : Addition of methyl (CH 3 ) group from  S - adenyl methionine to the fifth carbon (C5 position) of a cytosine to form 5-methylcytosine in DNA. Methylation can change the activity of a segment of DNA without changing the sequence. DNA methylation play a role in epigenetic gene regulation. ]

52. Let us consider some factors that influence the frequency of crossing-over during meiosis. Distance between linked genes Since genes are arranged linearly on chromosomes, the strength of linkage depends upon the distance between the genes involved. That means, genes that are located very close to each other in a chromosome are strongly linked. That is, there is high degree of linkage. Therefore, in closely spaced gene loci, the probability of crossing-over will be very less and they are less likely to recombine .  So the frequency of recombination might be very little.

53. Copyright © 1997. Phillip McClean If the distance between two linked genes is more, the strength of linkage decreases and there is greater chance of occurrence of crossing-over. In other words, the probability of crossing-over between two genes is directly proportional to the distance between them . i.e., greater the distance between the two genes, the greater will be the probability of formation of crossing-over. If the genes are located very far away, there is the possibility of double cross-over, triple cross-over etc. and the frequency of recombination may also increase. Random assortment of genes will occur if the two genes are far apart on the same chromosome(or on separate chromosomes) and may produce 50% recombination (i.e., The progeny will contain both parental types and recombinants in equal proportions. That means in a 1:1 ratio ). Thus, the strength of linkage is inversely proportional to the distance between two genes .

54. The percentage of crossing-over (recombination) or Cross over value can be calculated as Where, total number of progeny = Total No. of parental types + Total No. of recombinants. (As already stated, when there is random assortment (i.e. independent assortment) of genes, the parental types and the recombinants will be formed in equal proportions. That means in a 1:1 ratio ). On substitution, we get 1/(1+ 1) x 100 = 50% . [ i.e. 1 (Total No. of Recombinants) / 1 (Total No. of parental types) + 1 (Total No. of recombinants ) x 100 ] . Thus, the  recombination frequency between two independently assorting genes cannot exceed  50%. In other words, the maximum recombination frequency between two independently assorting genes is 50%.

55. Sturtevant(1914) , had suggested that frequency of recombination could be used to estimate the relative distance between two genes. 1% Recombination frequency = 1 map unit = 1 cM. In other words, 1  cM indicates a 1% recombination potential. That means, in 100 meioses, there will be one recombination event . Thus, two genes with a low recombination frequency are likely to be closer together while those with a high recombination frequency are likely to be farther apart on a chromosome. Based on this, Sturtevant created the first genetic map ( Linkage map) of genes on the  X chromosome of Drosophila melanogaster . [ cM – centiMorgan ,(in honor of Thomas Hunt Morgan), the unit of map distance. Map distance : According to Sturtevant(1914), percentage of crossing-over is used as an index of the distance between any two pairs of genes. That is, the unit of distance is taken as a portion of the chromosome of such length that, on an average, crossing-over occurs in it in 1% of the germ cells(reproductive cells). i.e., one map unit is equal to 1% recombinant phenotypes. In other words, a spacing of 1 cM indicates a 1 % chance that two genes will be separated by crossing-over ] .

56. Crossover Assurance, Interference and Coincidence Crossover Assurance Irrespective of size, at least one crossing-over ( obligate crossing-over ) occurs per bivalent, generating the ‘obligate chiasma ’ that ensures proper disjunction of homologous chromosomes during anaphase I of Meiosis I. This phenomenon is called ‘ Crossover Assurance ’ . Failure to maintain at least one crossing-over per homologous pair of chromosomes increases the probability of non-disjunction which in turn may result in the formation of aneuploid gametes. [Disjunction : Normal separation of chromosomes toward opposite poles during anaphase. Non-disjunction : The failure of one or more pairs of homologous chromosomes or sister chromatids to separate normally during nuclear division, usually resulting in an abnormal distribution of chromosomes in the daughter nuclei. Aneuploidy : The loss or gain of one or a few chromosomes ] .

57. Interference (Crossover Interference or Genetic Interference or Chiasma Interference) The occurrence of one crossover in a given chromosome pair tends to prevent the occurrence of another one in that pair. This phenomenon is called interference . In other words, crossing-over in one region interferes with the crossing-over in the nearby regions. That is, crossing-over at one point in a chromosome reduces the probability of another crossing-over at a nearby position in the same chromosome. This is because, the chiasma formed during one crossing-over affects the ability to form another chiasma in the nearby region. Thus crossover interference is a measure of the independence of crossing- overs from each other and it is a patterning phenomenon that ensures that crossing- overs are widely spaced along a chromosome as well as a more or less even distribution of chiasmata along chromosomes. That is,   crossover interference is non-random placement of crossovers along chromosomes. The net result of crossover interference is fewer double-crossovers than would be expected according to map distances.

58. One way in which genome-wide crossover reduction could be achieved without forming univalents is by an increase in the strength of crossover interference. Increasing interference could reduce crossover numbers without affecting the formation of at least the single ‘obligate’ crossover (Jones and Franklin, 2006). The strength of interference varies in different regions of the chromosome.   Interference decreases with distances up to 46 map units ( centiMorgans ) from the initial crossing-over . Crossover interference has been observed in most organisms.   The level of crossover interference is influenced by sex, age, length of chromosomes, etc.   In mice and cattle, interference is stronger in females than in males ( Szatkiewicz et al ., 2013; Wang et.al., 2016) where as in humans, interference is stronger in males than in females (Campbell et al ., 2015). The exact mechanism by which interference is exerted is not fully understood. It may be due to multiple levels of recombination regulation.

59. Coincidence The simultaneous occurrence of two single crossing-over events in the same pair of homologous chromosomes to form a double crossing-over is called coincidence . Coefficient of coincidence ( CoC ) = Observed frequency of double crossovers / Expected frequency of double crossovers in target intervals . [ In other words, Number of double recombinants in the progeny / Number of double recombinants expected in the progeny ; Expected double crossovers can be calculated as Expected double crossovers = Recombination frequency in region 1 X   Recombination frequency in region 2. i.e., the probability of a double crossover is the product of their separate probabilities ] . Coefficient of Coincidence ( CoC ) + Interference (I) = 1 Therefore, interference , I can be calculated as I = 1 – CoC .

60. When interference is complete, i.e., I = 1, CoC = 0. i.e., there is no coincidence. That means, there will be no double crossing-over. That is , a crossing-over in one region will interfere the occurrence of another crossing-over in a nearby region. Sturtevant had found that interference decreases further away from the locus of the first crossing-over. Thus, double crossing-over is much less likely to occur in short distances than in longer ones . Therefore, strength of interference is a function of map distance. Interference is stronger over short distances (i.e., stronger over map distances less than 20cM) and decreases if the gene loci are at a greater distance. Thus, the intensity of interference is inversely proportional to the distance between gene loci.

61. At a certain map distance, interference disappears (i.e., interference, I = 0. That means, there is no interference. So, the coincidence, CoC = 1. That is, coincidence is complete. Hence double crossing-over will occur at the expected frequency. That means, a crossing-over at one region occurs independently of a crossing-over at a nearby region. Thus, coincidence is an inverse measure of interference. If the value of coefficient of coincidence is between 0 and 1, there is partial interference and there will be a few crossing- overs .

62. Position of gene The correlation between distance between genes and the probability of crossing-over may not be applicable to all genes in a chromosome. This is because, formation of chiasma does not occur at random throughout the length of a chromosome. Crossing-over occur in preferential regions of the genome. That means, certain regions of some chromosomes have significantly higher rates of crossing-over or recombination called crossing-over hotspots or recombination hotspots , which serve as localized stimulators of general recombination and other regions which have unusually low recombination called coldspots , which depress the level of recombination. Recombination hotspots are interspersed with recombination coldspots . [Crossing-over hotspots: DNA fragments of a few kilobases in length with a higher rate of recombination than the surrounding DNA - Hotspots occur in very small regions, approximately 25% of the genome, where 80% of the crossing-over occur. For instance, in many primates, mice etc., 80 % of recombination occur in 10-20 % of the genome. In humans, recombination hotspots are approximately 10% of the genome where 40-60 % recombination occur. In many plant species also, recombination events are not evenly distributed along the length of the chromosomes. For instance, in wheat ( Triticum aestivum  L.) chromosome 3B, it has been estimated that 90% of the crossing-over occur in distal sub- telomeric regions. In Arabidopsis , crossing-over hotspots are closely associated with DNA double-strand break hotspots ( Choi et. al., 2018) ] .

63. Crossing-over is generally suppressed near the centromere ( Centromere effect - first described in  Drosophila   ) and telomeres (Telomere effect) of a chromosome ( Coldspots ). The mechanism of suppression of crossing-over at centromere is not fully understood. Presence of heterochromatin as well as DNA methylation at centromere may result in the suppression of crossing-over. If crossing-over occurs at centromere region, it may cause aneuploidy during meiosis in females. In regions adjacent to centromeres , double-strand breaks may occur but they do not resolve into crossing- overs . The presence of large blocks of heterochromatin at telomere may affect the formation of chiasma and thus suppress crossing-over.

64. Mostly crossing-over occur at chromosomal arm regions. In most plant species, crossing-over mainly occur in distal euchromatic regions than in central regions. Linkage analyses in Maize ( Zea mays , Gore et. al ., 2009) and Wheat ( Triticum aestivum ,   Saintenac et. al ., 2009) revealed that the frequency of crossing-over  is higher in sub- telomeric regions and lower in interstitial regions.  Size of chromosomes The length of a bivalent influences its ability to form chiasmata . So, the frequency of crossing-over is higher in a long bivalent than in a short one. Short bivalents exhibit strong interference.

65. Chromatin Structure Frequency of crossing-over can be influenced by chromatin structure. In Maize, frequency of crossing-over is more in the distal gene-rich euchromatic regions. Crossing-over can be suppressed by methylation of DNA and histones ; the amount and distribution of highly repetitive heterochromatin etc. The occurrence of crossing-over is rare or absent in heterochromatic regions. Genomic regions with repetitive sequences have low frequency of crossing-over. Recombination frequency is positively correlated with G-C content in certain organisms like Drosophila melanogaster ,   Saccharomyces cerevisiae  (  brewer's yeast or baker's yeast), honey bee etc. There are   also reports that regions with low G-C content have high recombination (Lynn et.al ., 2004). But, in   Arabidopsis thaliana, there is no correlation between G-C content and the frequency of crossing-over. [ G -for Guanine and C -for Cytosine, the nitrogen bases in DNA ]

66. Recombination frequency is also influenced by transposable elements. Generally, crossing-over is suppressed in highly repetitive genomic regions that are made up of transposable elements (Underwood and Choi , 2019). Crossing-over hotspots identified from Maize have low DNA methylation and transposons . High transposable element densities have been reported in low recombination regions of Drosophila species. On the other hand, Theobroma cacao  and rice populations show largely divergent hotspot locations influenced by retrotransposon abundance and genetic divergence ( Marand et. al. , 2019). [ Transposable elements ( Transposons or Jumping genes) : They can move from one location on the genome to another - first discovered by Barbara Mc Clintock in Maize . Retrotransposon : They transpose through RNA intermediates. First, the transposable DNA is copied into RNA and then into DNA by reverse transcription by an enzyme reverse transcriptase and inserted into the target site ]

67. Generally, there is a positive correlation between gene density and frequency of crossing-over. Formation of crossing-over is affected by sequence homology. For instance, in the case of inversion heterozygotes and translocation heterozygotes , where one chromosome  has the normal sequence and the other chromosome with altered sequence, the pairing of homologous chromosomes will be disrupted in the regions of chromosomes where such changes have occurred , which will in turn reduce the probability of crossing-over. That is, reduction in sequence homology between chromosomes will lessen the probability of crossing-over and thereby recombination. [Gene density : The number of genes per million base pairs, called a megabase , Mb ; For instance, the gene density of the human genome is roughly 12–15 genes/Mb. ] Inversion : A type of structural change in a chromosome in which two breaks occur and the broken segment is reinserted after rotating 180° so that the gene order is reversed. Translocation: A type of chromosomal aberration which involves the transfer of a segment of a chromosome to a different part of the same chromosome or to a different chromosome, which change the arrangement of the genes ].

68. Autopolyploidy Autopolyploids generally exhibit reduced crossing-over rates when compared with their diploids (Shaver, 1962; Watanabe, 1983; Gillies et al ., 1987; Yant et al ., 2013). Sex of the individual In most of the organisms, crossing-over occur in both males and females. But, there are exceptions. Crossing-over is absent in male Drosophila melanogaster (Morgan, 1914 ; but Drosophila ananassae males undergo a few crossing-over )and female silk moth Bombyx mori (Tanaka, 1914) ( Achiasmy – Complete suppression of recombination in one sex). Sex-specific differences in the frequency of crossing-over ( Heterochiasmy ) and its distribution along chromosomes have been reported in many species. [ Autopolyploidy : The phenomenon where all the genomes in a polyploid organism are identical Polyploid : An organism with more than two sets of chromosomes ].

69. Frequency of crossing-over may be a) Higher in female meiosis – E.g. Eutherian mammals - For instance, crossing-over frequency in oocytes of human females is generally higher than in spermatocytes of human males, which   correlates with differences in synaptonemal complex length - The synaptonemal complex is considerably longer in oocytes in comparison to spermatocytes . (Tease and  Hultén , 2004 ; Petkov et.al ., 2007). b) Higher in male meiosis - In some metatherian mammals, Sheep, Arabidopsis thaliana etc. In the plant Arabidopsis thaliana, crossing-over rates in distal regions of chromosome 4 are very high in male meiosis but very low in female meiosis ( Drouaud et.al ., 2007). c) Almost same - There is no significant sex-specific differences in the rate of crossing-over . e.g. Tomato, Barley , Rape seed or Canola etc. [ Eutherian mammals : Placental mammals that give birth to well-developed young ones. Metatherian mammals : They give birth to partially developed young ones e.g. Marsupiales ] .

70. Maternal Age The frequency of crossing-over and recombination is found to vary among different age groups. The frequency of crossing-over may either increase or decrease as the maternal age advances. A reduction in frequency of crossing-over with regard to advancing maternal age was reported in oocytes of mice, Drosophila etc. A decrease in the frequency of chiasmata , a change in their location on the chromosome and an increase in the frequency of univalents have been found in mice oocytes as the age advanced. (Henderson and Edwards, 1968).  In female Drosophila flies also, the frequency of crossing-over decreased as the age advanced (Bridges,1927; Whittinghill and Hinton, 1950) . They had reported that the highest rate of crossing-over takes place in the eggs laid during the first 4 to 5 days and gradually declines till the 12th-16th day. Bridges found a 20% decline in recombination rate as the age advanced   in female Drosophila flies. Such changes in a chromosome may be due to local rearrangements. However, an increase in crossing-over in older age (> 16 days) has also been reported.

71. In humans, both increase and decrease in recombination rates with advancing maternal age have been reported. In older women, reduction in crossing-over and genetic recombination may cause non-disjunction which in turn may result in aneuploidy and may cause many problems like infertility, miscarriages, birth defects(e.g . Down syndrome) etc.   Down syndrome : Trisomy 21 – The individual has three copies of the chromosome 21 instead of the normal two copies. https://www.savedownsyndrome.com/blog/theneweducationalgraphics-csyye

72. Temperature The frequency of crossing-over may decrease or increase with regard to variations in temperature. The effects of temperature on the frequency of crossing-over and recombination are species-specific. In Rice plants, crossing-over and recombination increases with a rise in temperature. In Wheat, high temperature within the fertility threshold (i.e., between 10°C and 26°C ) has a positive impact on the frequency of crossing-over and thereby meiotic recombination ( Coulton et al ., 2020).

73. In Arabidopsis thaliana , both low (8°C) and high (28°C) temperatures (Cold and heat stresses), increase the frequency of crossing-over. An increase in temperature within the fertility-tolerable range (28°C) promotes crossing-over. For instance, frequency of crossing-over increased when Arabidopsis plant grown at a temperature of 20°C was shifted to 28°C. The frequency of crossing-over was approximately  10% higher at the extreme of the temperature range, 8–28 °C. An increase in chromosome axis length may account for the rise in frequency of crossing-over at low temperature in Arabidopsis thaliana (Lloyd et. al ., 2018). A higher temperature (32°C - 38°C) disrupts central element of synaptonemal complex and causes asynapsis and thus affects bivalent and chiasma formation which in turn result in the suppression of crossing-over (De Storme and Geelen , 2020). [ Asynapsis : Failure of pairing of homologous chromosomes ]

74. In Barley ( Hordeum vulgare ), changes in temperature from 22°C to 30°C caused reduction in frequency of crossing-over. The distribution of crossing-over was also altered, and there were significantly more crossing-over in the interstitial regions at higher temperature (Higgins et . al. , 2012). However, Phillips et. al ., (2015) have reported that in Barley, heat causes an increase in the rate of crossing-over and with a redistribution of crossovers from distal toward more proximally located chromosome regions.  

75. In female Drosophila flies , the frequency of crossing-over is less at temperatures between 22°C and 25°C. But, more crossing-over is observed if the temperature is either lowered or raised (Plough, 1917; Stern, 1926; Graubard , 1934 and Smith, 1936). Crossing-over was induced in male Drosophila flies at a temperature of 35°C, applied during the larval period Whittinghill (1937) . In Drosophila, heat shock results in crossovers on chromosome 4. Position of chiasma ( e.g . centromere proximal versus distal) can also change with variations in temperature (Abel, 1964; McNelly-Ingle et al ., 2009).

76. Starvation or Nutritional deficiency Nutritional stress like starvation or nutritional deficiency increases recombination in  Drosophila melanogaster . Extreme changes in larval nutrition affected the recombination frequency in the third chromosome of Drosophila melanogaster (Neel, 1941). The ionic status of the cells of an organism also influences the frequency of recombination. For instance, the presence of metallic ions such as Calcium and Magnesium ions in the food reduced the frequency of crossing-over and recombination in Drosophila . But, the removal of such chemicals from the diet increased the rate of crossing-over. High level of Sodium ions also caused reduction in crossing-over ( Griffing and Langridge 1963). Phosphate treatment increased chiasma number in both diploids and tetraploids of the grass Festuca pratensis ( Deniz and Tufan , 1998). High levels of phosphate increased chiasma frequency in two strains of diploid rye ( Secale cereale , Bennett and Rees ,1970).

77. Chemicals Treatment with mutagenic chemicals like alkylating agents was found to increase the frequency of crossing-over in female Drosophila fly. Ethyl methane sulphonate is known to induce somatic crossing-over. Exposure to Ethylene diamine tetraacetic acid (EDTA) increase the recombination rate in female Drosophila (Levine, 1955). Colchicine prevents crossing-over by preventing synapsis (pairing of homologous chromosomes). High dose of Selenium reduces the frequency of crossing-over. Antibiotics such as Mitomycin -C and Actinomycin -D increase the frequency of crossing-over . The anticancer drug (the chemotherapeutic agent) Cisplatin ( cis -platinum(II) diamine dichloride) is highly recombinogenic in assays with some model organisms such as Candida albicans , Saccharomyces cerevisiae and somatic cells of  Drosophila melanogaster .   Cisplatin and UV exposure cause different forms of DNA damage that can be processed by the homologous recombination pathway and form  crossing-over during meiosis.

78. Plasma genes ( Cytoplasmic Genes) In some species, plasma genes may cause reduction in crossing-over. For example, Tifton male sterile cytoplasm (Tift 23 A1 cytoplasm) in Pearl millet ( Bajra - Pennisetum glaucum ). [Tift 23 A : Cytoplasmic-genic male sterile (CMS) line of Pearl millet with short stature, profuse tillering , uniform flowering and good combining ability, evolved at Tifton, Georgia. In Pearl millet , the first reported CMS system, A1 was based on the Tift 23A1 cytoplasm (Burton, 1965, Burton and Athwal 1967) ; used in commercial hybrid seed production in Pearl millet. ]

79. Radiations Investigations on effect of radiation on frequency of crossing-over gave conflicting results. The different responses of organisms to radiation may be due to various factors such as biological ( i.e., species-specific differences and variation in the developmental stage during which irradiation was done) and physical ( i.e., nature of the radiation employed and the temperature when irradiation was done). Generally, there was a decrease in the frequency of crossing-over after irradiation. Plants irradiated just before the start of meiosis had much lower chiasma frequency . Generally, crossing-over is absent in male Drosophila flies. But, extremely low frequency of crossing-over had been reported in male Drosophila flies (Muller, 1916; Bridges and Morgan, 1919 ; Sturtevant, 1929 ; Patterson and Suche , 1933). It has been found that crossing-over can be induced in male Drosophila flies by X-ray irradiation of immature germ cells. Rifenburgh (1935) had reported that irradiation of young larvae by ultra-violet radiation induced crossing-over between the black and vestigial loci in Drosophila male fly . [Irradiation : Exposure of a biological material to any one of the radiations ]

80. X-ray and Gamma ray irradiation can increase the frequency of crossing-over in female Drosophila fly. Jain and Basak (1965) had reported that radiation treatments induced cryptic structural changes in some of the chromosomes of Delphinium which restricted pairing which in turn reduced chiasma frequency.

81. Significance of Crossing-over Creation of Genetic variability Crossing-over and genetic recombination in the first meiotic division during gametogenesis is an essential feature of sexual reproduction that increase genetic variability among the progeny, which is essential for effective selection (both natural and artificial). Genetic variability is a prerequisite for the evolutionary process. However, the low number of crossing- overs often limits the genetic variation that can be utilized in breeding programs(Plant Breeding and Animal husbandry).   [Plant Breeding : Applied branch of Botany that deals with the genetic improvement of crops for the service of man. Animal husbandry : The science of breeding and caring of domesticated animals ] .

82. Increase /decrease of fitness and adaptability of the progeny Crossing-over and recombination may bring together the beneficial or desirable alleles from both the parents which equip the progeny with either more vigour , reproductive potential, survival ability or adaptability. Such beneficial combinations of alleles may spread through a population in several generations and eventually, may become the distinctive features of the species. On the other hand, crossing-over and recombination may break down the association between certain beneficial alleles, which may reduce the vigour or fitness and adaptability of the progeny. In some other cases, crossing-over and recombination may break the association between one beneficial and another deleterious alleles allowing selection to take advantage of the beneficial one.   In crossing-over poor regions, desirable combinations of alleles are preserved but it will be very difficult to get rid of the undesirable ones.

83. 3. Regular or irregular segregation of paired homologous chromosomes(Bivalents) at Anaphase I during Meiosis I Crossing-over is required for the normal segregation of pairs of homologous chromosomes during anaphase I of meiosis I. Failure to maintain at least one crossing-over per homologous pair of chromosomes(i.e., obligate crossing-over) increases the probability of non-disjunction. This is because, the chiasma formed during crossing-over provide the physical attachment between the paired homologous chromosomes which is necessary for the proper alignment of the bivalents on the metaphase I plate and their normal segregation at anaphase I. Absence of chiasmata or inappropriately located chiasmata may result in non-disjunction which in turn may cause aneuploid variations in chromosome number and consequently form aneuploid gametes.

84. Preparation of Linkage maps (Chromosome maps, Genetic maps or Crossover maps) Under standard environmental conditions, the recombination frequency of a pair of linked genes is constant and characteristic for that pair of genes. So, the frequency of crossing-over or recombination or crossover value between gene pairs on the same chromosomes can give an estimate of the relative distance between them. As we have already found, 1% Recombination frequency = 1 centiMorgan ( cM ) = 1 map unit. Thus, the frequency of crossing-over or recombination between different genes on a chromosome can be find out and can be used to estimate their relative distances and order. Based on this data, we can construct a linkage map. It is a linear representation of the gene order and relative distance on a chromosome. That is, chromosome maps are diagrammatic representation of chromosomes in the form of a straight line showing genes as points separated by distance proportional to the crossover value. In chromosome maps, one member of a homologous pair of chromosome is represented as a straight line proportional to its length with the position of genes marked on them.

85. Example : Bridges and Olbrycht’s map of seven X-linked genes in Drosophila . Map distances in centimorgans ( cM ) Snustard & Simmons , 2012

86. Mitotic Crossing-over Crossing-over that occurs in somatic cells (i.e., body cells) during mitosis (i.e., somatic crossing-over). It is a rare event that occurs with a frequency of 10 -4 to 10 -5 per cell division ( Gunther , 1984). Mitotic crossing-over was first reported by Curt Stern (1936) on X chromosome of a female Drosophila melanogaster fly with the heterozygous genotype + sn /y+ (Repulsion or Trans linkage phase) for body colour (wild type, gray ‘ + ’ vs. yellow, ‘ y ’) and bristles ( normal ‘ + ’ vs. singed ‘ sn ’ – gnarled or short twisted bristles). That is, one  X chromosome carry the recessive allele ‘y’ for yellow body and the other X chromosome carry the recessive allele ‘ sn ’ for singed bristles. [ Female Drosophila fly has two X chromosomes.   i.e., XX ].

87. Stern had observed that most of the female Drosophila flies were gray bodied and with normal bristles, as gray body colour is dominant over yellow and normal bristles is dominant over singed. However, Stern had also observed that some female flies had single yellow spots, singed spots and twin yellow-singed spots ( i.e.,twin sectors, yellow adjacent to singed) on gray body. This is because of mitotic crossing-over and segregation that occured in a cell with heterozygous genotype, + sn /y+ , which results in cells homozygous for y and for sn . Thus mitotic recombination resulted in the expression of recessive genes in small areas in wild type gray bodied female fly. [Twin spots : Paired alterations visible in adjacent areas – mosaic patches or spots. Organisms that are composed of cells of more than one genotype are referred to as genetic mosaics] .

88. Gray-bodied Drosophila Yellow- bodied Mutation - Singed bristle , sn ( short twisted bristles ) Image credit: https://www.carolina.com Botaurus . https://researchguides.library.vanderbilt.edu https://www.biologie.uni-halle.de http://rose-annemeissner.blogspot.com Normal bristle

89. Jones(1937) had reported paired alterations visible in adjacent areas(twin spots) and unpaired spots in the aleurone layer of the endosperm of maize. According to Jones(1937), the twin spots in the triploid aleurone layer of the maize endosperm is the result of a shift of known color and texture genes C, C', Pr, P, Wx and Su due to somatic crossing-over. Later, mitotic crossing-over had been reported in a wide variety of organisms such as Aspergillus nidulans , Saccharomyces cerevisiae , Nicotiana tabacum , Antirrhinum majus , humans etc.

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