Chromosomal aberrations, utilization of aneuploids, chimeras and role of allopolyploid in evolution

Chromosomal aberrations, utilization of aneuploids, chimeras and role of allopolyploid in evolution

1. WELCOME

2. ASSIGNMENT PRESENTATION (UNIT- 3) Submitted By:- Vaghela Gauravrajsinh K. Patel Chinmaykumar G. Patel Chiragkumar R. Makwana Hitendrasinh B. Submitted To:- Dr. N.B.Patel sir, Asso. Professor, Dept. of GPB, C.P.College of Agriculture, SDAU, SKNagar.

3. WHAT IS CHROMOSOME? Chromosomes are the rod-shaped , filamentous bodies present in the nucleus, which become visible during cell division. They are the carriers of the gene or unit of heredity . Chromosomes were first described by Strausberger ( 1875). The term “Chromosome”, given by Waldeyer (1888). Chromosome ( Chroma means colour and Soma means body ).

4. WHAT IS CHROMOSOMAL ABERRATIONS ? The somatic (2n) and gametic (n) chromosomes numbers of a species ordinarily remain constant due to extremely precise mitotic and meiotic cell divisions . But occasionally, spontaneous variation in chromosome number or structure do arise in nature, these are called chromosomal aberrations . Chromosomal aberrations are of two types: Structural chromosomal aberrations Numerical chromosomal aberrations

5. STRUCTURAL CHROMOSOMAL ABERRATIONS Structural chromosomal aberrations alter the chromosome structure, i.e., the number, the sequence or the kind of genes present in chromosome ( s ). There are four common types of structural aberrations :- Deletion or Deficiency Duplication or Repeat Inversion Translocation

6. Deletion is Intra-chromosomal types aberration. Duplication is Intra-chromosomal types aberration . Inversion is Intra-chromosomal types aberration . Translocation is Inter-chromosomal types aberration.

7. ORIGIN OF STRUCTURAL ABERRATIONS All chromosome aberrations are produced following chromosome breakage. Chromosome breakage occurs spontaneously in a low frequency (~ 1% of cells in almost all tissue studied). The frequency of spontaneous chromosome breakage is modified by several factors, viz ., age, oxygen availability , temperature and metabolic stage of cell. When a break occurs in a chromosome, the two broken ends thus produced often join with each other producing the same original chromosome; this is known as restitution.

8. DELETION OR DEFICIENCY Loss of a chromosome segment is known as deletion or deficiency . The term deficiency was coined by Bridges in 1917. It involves the one break . Term deletion was coined by Painter and Muller in 1929. It involves the two break. Deletion was the first structural aberration detected by Bridges in 1917 from his genetic study on the X chromosome of Drosophila . Types of Deletion or Deficiency :- Terminal deletion:- L oss of either terminal segment of a chromosome. Interstitial deletion: - Loss of intercalary segment of a chromosome .

9. DELETION

10. TERMINAL DELETION A chromosome has two ends or terminals. Loss of either terminal segment of a chromosome is known as terminal deletion . The deletion occurs in only in one chromosome of homozygous pair, it is known as terminal heterozygous deletion. T he deletion occur in both the chromosome of a pair , it is termed as homozygous deletion.

11. INTERSTITIAL DELETION Some times there is loss of a segment of chromosome from the intermediate portion or between telomere and centromere . Loss of intercalary portion of chromosome is known as interstitial deletion.

12. ORIGIN OF DELETION Deficiency originates spontaneously or it may be induces artificially like physical mutagen, chemical mutagen. Terminal deficiency required a single break, Intercalary deficiency required two break.

13. EFFECT OF DELETION It could affect eye sight, smell, skin tone or loss of weight. Small deletions are less likely to be fatal; large deletions are usually fatal - there are always variations based on which genes are lost. Some medium-sized deletions lead to recognizable human disorders, e.g. Williams syndrome Deletions are responsible for an array of genetic disorders, including some cases of male infertility.  Deletion of part of the short arm of chromosome 5 results in Cri du chat syndrome.

14. “Cri-du-chat” (cry-of-cat ):- A specific deletion of a small portion of chromosome 5; these children have severe mental retardation, a small head with unusual facial features, and a cry that sounds like a distressed cat . “Philadelphia 22”:- Deletion of the chromosome 22. P rader-Willi syndrome:- Deletion on long arm of chromosome 15 . Wolf- Hirschhorn syndrome:- Caused by partial deletion of the short arm of chromosome 4 .

15. USES OF DEFICIENCY Deficiency may be used for the study of chromosome pairing and its behavior during cells division. They may be used for locating a gene on a particular chromosomes. Deficiency can be used to resolve special problems, such as, the relationship between chiasma and crossing over.

16. DUPLICATION Duplication refers to the occurrence of a segment twice in same chromosome. It result in addition of one or more genes in to chromosome. Duplication is also known as repeat and is first reported in drosophila by Bridge in 1919. Played important role in evolution of gene families . According to Ohno(1970) duplication are the source of all the new genes, and thus the basis of organic evolution.

17. DUPLICATION

18. TYPES OF DUPLICATION Interchromosomal duplication:- T he duplicated segment of a chromosome is present in another chromosome of the genome. Intrachromosomal duplication:- The duplicated segment remains in the same chromosome . It may be present at different location . It has a two types:- Direct tandem Reverse tandem

19. DIRECT TANDEM DUPLICATION In this case sequence of gene in the duplicated segment is similar to the sequence of genes in the original segment of a chromosome. Here the sequence of gene in the duplicated segment is reverse to the sequence of genes in the original segment of a chromosome. REVERSE TANDEM DUPLICATION

20. DISPLACED When the duplication is found away from the original segment but on the same arm of the chromosome, it is known as displaced duplication.

21. REVERSE DISPLACED Such duplication is also away from the original segment but on the other arm of a chromosome. These two types are known as non-adjacent duplication, because they are away from the segment which shows duplication .

22. ORIGIN OF DUPLICATION Origin of duplication involves chromosome breakage and reunion of chromosome segment with its homologous chromosome . As a result , one of the two homologous involved in the production of a duplication and up with a deficiency, while the other has a duplication for the concerned segment .

23. EFFECT OF DUPLICATION Duplication may produce specific effects when the phenotype is affected due to a change in the position of a gene, it is called position effect. Duplication may lead to a more intense effect of a duplicated gene. Crossing over is suppressed in the duplicate region due to lack of corresponding duplicated segment in the normal chromosome . The gene number is increased in the chromosome having duplication . Presence of duplication leads to reduction in pollen fertility in plant species.

24. USES OF DUPLICATION Duplication can be used to study the chromosome behavior during meiosis, such as, chromosome pairing, crossing over and their consequences. Duplication offers number of possibilities in plant breeding. They can be used to increase the dosage of certain desirable genes for increasing disease or pest resistance, enzymatic activity or other characteristics. Duplication has an advantage over polyploidy because the genetic disbalance due to the duplication of chromosomal segments is lesser as compared to polyploidy where the whole genome is duplicated. In case where genes for resistance to diseases or pests are linked to some undesirable genes, or the genes for resistance to various races are allelic, a combination of resistance to different races can be obtained through duplication.

25. USES OF DUPLICATION Duplication can be used to study the chromosome behavior during meiosis, such as, chromosome pairing, crossing over and their consequences. Duplication offers number of possibilities in plant breeding. They can be used to increase the dosage of certain desirable genes for increasing disease or pest resistance, enzymatic activity or other characteristics. Duplication has an advantage over polyploidy because the genetic disbalance due to the duplication of chromosomal segments is lesser as compared to polyploidy where the whole genome is duplicated. In case where genes for resistance to diseases or pests are linked to some undesirable genes, or the genes for resistance to various races are allelic, a combination of resistance to different races can be obtained through duplication.

26. INVERSION When a segment of chromosome is oriented in the reverse direction, such segment said to be inverted and the phenomenon is termed as inversion. The existence of inversion was first detected by Sturtevant and Plunkett in 1926 . Inversion occur when part of chromosomes become detached, turn through 180 ͦ and are reinserted in such way that are in reversed order. When they rejoin, the wrong ends may become connected. The part on one side of the loop connect with broken end different from the one with which it was formerly connected. This leaves the other two broken end to become attached.

27. INVERSION

28. TYPES OF INVERSION Single Inversion Pericentric inversion Paracentric inversion Complex Inversion Independent inversion Direct tandom inversion Reverse tandom inversion Included inversion Overlapping inversion

29. SINGLE INVERSION In this case, only one segment of the chromosome is inverted. There are two types of single inversion . It has a two types:- Pericentric inversion Paracentric inversion

30. PARACENTRIC INVERSION The inversion in which centromere is not involved is called Paracentric inversion. In this type of inversion both breaks occur in one arm of the chromosome. When only the chromosome of a homologous pair has inversion it is called inversion heterozygote. When both the members of homologous pair have similar type of inversion, it is called inversion homozygote.

31. PERICENTRIC INVERSION When centromere is involved in the inversion, it is known as Pericentric inversion. When a break occurs in each of the two arms of a chromosome, the centromere is included in the detached segment resulting in a Pericentric inversion.

32. Para centric Pericentric

33. COMPLEX INVERSION Occurrence of more than one inversion in a chromosomes is called complex inversion. Based on the mutual relationship of a inverted regions. It has a five types :- Independent inversion Direct tandom inversion Reverse tandom inversion Included inversion Overlapping inversion

34. TRANSLOCATION One way or reciprocal transfer of segments between non homologous chromosome is known as translocation . Integration of a chromosome segment into a non homologous chromosome is known as translocation. Three types: Simple translocation Shift Reciprocal translocation

35. SIMPLE TRANSLOCATION In this case, terminal segment of a chromosome is integrated at one end of a non-homologous chromosome. Simple translocation are rather rare. Such translocation only a simple break occur in one chromosome.

36. SHIFTS TRANSLOCATION I n shift, transfer of an intercalary segment from one chromosome to the intercalary position in a non-homologous chromosome . Thus two breaks occur in a loser chromosome and one break in the gainer chromosome for transfer and integration of such segments . Such translocation are known in the population of Drosophila, Neurospora etc .

37. RECIPROCAL TRANSLOCATION Mutual exchange of segments between non-homologous chromosomes. In such translocations, one break occur in each chromosome before exchange of segments. Such translocation are very common and have great evolutionary significance Reciprocal translocation are of two type, viz. homozygotes and heterozygotes. Translocation of this type is most common. e.g. Oenothera , Tradescantia etc.

38. Classification : on the basis of number of breaks involved Simple translocation: One break Reciprocal translocation or Interchange : Two break Shift type translocation or Transposition: Three break Complex translocation :More than three breaks

39. EFFECT OF TRANSLOCATION In general, no phenotypic effects of translocations are visible, but in case there is damage to the DNA during translocation, recessive mutation may arise . Translocation may also act as recessive lethal. Position effect may be produced by certain translocation.

40. USES OF TRANSLOCATION Study of chromosome behavior during meiosis. Assignment of centromere position. Interchange as genetic markers. Determination of unknown locus of a gene and chromosome mapping. Association of linkage groups or genes to specific chromosomes. Testing of the independent of linkage groups. Association of different linkage groups.

41. Determination of the initiation of chromosome pairing. Production of duplications Evolution of allopolyploids. Gamete selection. Enlarging the genome. Evolution of karyotype. Interchange as a source of trisomic. Interchange may be used for pest control. USES OF TRANSLOCATION

42. NUMERICAL CHROMOSOMAL ABERRATIONS Numerical aberrations are those that cause a change in the number of chromosomes and this chromosomes produced two types of cells or individuals. Those whose somatic genome (chromosome complement) is the exact multiple of the basic number characteristic of the species (EUPLOID ). Those in which the somatic number is an irregular multiplication of the basic number. Occasionally there could be reduction in basic chromosomes number also (ANEUPLOID).

43. NUMERICAL CHROMOSOMAL ABERRATIONS Euploidy Aneuploidy Haploidy (n) Diploidy (2n ) Polyploidy Hyperploidy Hypoploidy (3n,4n,5n) Trisomy Monosomy Tetrasomy Nullisomy

44. EUPLOIDY The change in chromosome number which involves entire set of chromosomes. Euploids have one or more complete genomes, which may be identical with or distinct from each other . The somatic chromosome number of a euploid individual is exact multiple of basic chromosome number of that species . Euploidy includes monoploids, diploids and polyploids.

45. MONOPLOIDS Monoploids contain a single chromosome set and are characteristically sterile . In other words monoploid have the basic chromosome number (x) of a species . Monoploid (x) differ from haploids (n) which carry half or gametic chromosome number . In a true diploid species, both monoploid and haploid chromosome number are same (i. e. x=n ).

46. HAPLOID Haploid is a general term used to designate the individuals or tissues with a gametic chromosome number. Single set of chromosome(denotes by “n”) Differences between monoploids and haploids : Monoploids Haploids 1) Represent gametic chromosome number of a diploid species Represent gametic chromosome number of any species 2) Monoploids are always haploids Haploids cannot always be monoploids 3) Contain single set of genome May contain one or more copies of genome.

47. ORIGIN OF HAPLOID Parthenogenesis and Apogamy. Somatic reduction and chromosome elimination. Anther, Pollen and Ovule culture. DOUBLE HAPLOID:- Double haploid plants are developed from haploids by doubling the chromosome number of haploid plants by colchicine treatment.

48. CHARACTERISTIC FEATURES OF HAPLOID PLANTS Haploids are smaller, less vigorous than their diploid phenotypes . Haploids are sterile, as the chromosomes have no regular pairing partner homologous chromosomes during meiosis and they are found as univalent at metaphase I of meiosis. The meiotic products are deficient in one or more chromosomes. Haploids can be produced through anther culture, parthenocarpy, delayed pollination etc.

49. DIPLOID Diploidy is characterized by presence of two genomes in each somatic cell of the diploid organism. Most animals and plants are diploids. The Diploidy is related with fertility, balanced growth, vigour, adaptability and survival of diploid organisms.

50. POLYPLOIDY The organisms with more than two genomes are called polyploids. Among plants, polyploidy occurs in multiple series of 3, 4, 5, 6, 7, 8 etc. of the basic chromosome number and thus resulting in triploids, tetraploid, pentaploid, hexaploid, heptaploid, octaploid etc ., respectively. Generally ploidy levels higher than tetraploid are not commonly encountered in the natural population. However there are some exceptions. E.g. hexaploid (6x) wheat, octaploids (8x) straw berries, many commercial fruits and ornamental plants, liver cells of man etc.

51. KINDS OF POLYPLOIDS Polyploids are distinguished on the basis of source of chromosomes into three main kinds. Auto polyploids Allopolyploids Segmental allopolyploids

52. TYPES OF POLYPLOIDY Autopolyploid:- Multiple copies of identical chromosome sets; usually develop normally; cells are proportionately larger than diploid Alloploidy :- multiple copies of non-identical (homologous) chromosome sets; includes genomes of two different species ; usually display “hybrid” characteristics Segmental allopolyploids: genome present in an individual are partially homologous

53. AUTOPOLYPLOIDS In a plant, when same set of chromosomes of a genome are increased in number, autopolyploid are obtained. The prefix “auto” indicates that the ploidy involves homologous chromosome sets . Genetical and morphological characters expressed by autopolyploid depend on the genetic constitution of parent plant. In many species, autopolyploids show an increase invigour and size this phenomenon is known as gigantism. However, autopolyploid occur rarely in natural populations.

54. MORPHOLOGICAL FEATURES OF AUTOPOLYPLOIDS Large cell size than diploid (larger stomata but freq. lower than normal ). Larger pollen grains. Slower growth and late flowering. Larger leaves, flower and fruits but generally less in number. Reduced fertility due to meiotic irregularities. Increase vigour and vegetative growth but yield is lower. Lower dry matter.

55. AUTOTRIPLOIDS Autotriploid have three complete sets of genomes of the same species in somatic cell . Generally, in nature they originate by the fusion of a haploid gamete with a diploid gamete . Triploids are generally highly sterile due to defective gamete formation. But triploid of some species are highly fertile e.g. spinach The triploid plants do not produce true seeds . Triploids are useful only in those plant species which propagate asexually like banana, sugarcane, apple, sugar beet.

56. AUTOTETRAPLOIDS: In autotetraploid, four copies of the genome of same species (AAAA or BBBB) are present . They may arise spontaneously or can be induced artificially by doubling the chromosomes of a diploid pieces with colchicine treatment. Autotetraploid are usually larger and more vigorous than the diploid species . E.g. Rye, alfalfa, grasses, groundnut, potato, coffee.

57. ALLOPOLYPLOIDS A polyploids containing genetically different chromosome sets from two or more species is known as allopolyploid . Natural allopolyploids most likely originate through chromosome doubling of F1 hybrid produced by chance through natural hybridization between two distinct species of the same genus or from different genera . Amphidiploid: It is an allopolyploid (allotetraploid) which arises by combining genomes of two different species.

58. Natural allopolyploids :- Inter-specific crossing followed by chromosome doubling in nature have resulted in origin of some natural allopolyploid crops like cotton, tobacco, mustard , wheat etc. Artificial allopolyploids :- Artificial allopolyploids have been synthesized in some crops either to study the origin of naturally available allopolyploids or to explore the possibilities of creating new species. Some examples of artificial allopolyploids are Triticale, Raphanobrassica.

59. ANEUPLOIDY This condition can be expressed either as an addition of one or more entire chromosome or as a loss of such chromosomes to a genomic number . Aneuploidy can be due to Loss of chromosomes in mitotic or meiotic cells due to laggards (lagging chromosomes), which are characterized by retarded movement during anaphase . Irregularities of chromosome distribution during meiosis of polyploids with uneven number of basic genomes like triploids and pentaploid. The occurrence of multipolar mitosis resulting in irregular chromosome distribution during anaphase.

60. CLASSIFICATION OF ANEUPLOIDY SR NO TYPE FORMULA SOMATIC CHRO. COMPLEMENT (A,B,C represent nonhomologous chromosomes) Normal disomic 2n AA BB CC Aneuploid A. Hypoploid 1. Monosomic 2n-1 AA BB C_ 2. Double monosomic 2n-1-1 AA B_ C_ 3. Nullisomic 2n-2 AA BB _ _ 4. Double nullisomic 2n-2-2 AA _ _ _ _

61. CLASSIFICATION OF ANEUPLOIDY SR NO TYPE FORMULA SOMATIC CHRO. COMPLEMENT (A,B,C represent nonhomologous chromosomes) Normal disomic 2n AA BB CC Aneuploid B. Hyperploid 1. Trisomic 2n+1 AA BB CCC 2. Double trisomic 2n+1+1 AA BBB CCC 3. Tetrasomic 2n+2 AA BB CCCC 4. Pentasomic 2n+3 AA BB CCCCC 5. Hexasomic 2n+4 AA BB CCCCCC

62. TYPES OF ANEUPLOIDY Monosomy Nullisomy Trisomy Tetrasomy

63. MONOSOMY The diploid organism which lacks one chromosome of a single homologous pair is called Monosomics. Genomic formula 2n-1 . The Monosomics are usually weaker than normal diploids . The number of possible Monosomics in an organism will be equal to the haploid chromosome number. Double Monosomics (2n-1-1) or triple Monosomics (2n-1-1-1) could also be produced in polyploids . In double Monosomics the missing chromosomes are non-homologous.

64. NULLISOMY Diploid organisms which have lost a pair of homologous chromosomes are called Nullisomics. Genomic formula 2n-2 . Nullisomics are not usually found in natural populations, but have to be obtained by intercrossing or selfing of Monosomics (2n-1 ). Nullisomics series are not of great agronomic importance, but used for genetic studies. They exhibit reduced vigour, fertility and survival . Double Nullisomy (2n-2-2) involves loss of two pairs of homologous chromosomes.

65. TRISOMY Trisomics are those organisms which have one extra chromosome (2n+1). E xtra chromosome may belong to any one of the different chromosome pairs, the number of possible Trisomic in an organism will be equal to the haploid chromosome number . An individual having two extra chromosomes each belonging to a different chromosome pair is called double trisomic (2n + 1 + 1 ). Depending on the nature of extra chromosome, simple Trisomics are of three types.

66. Primary Trisomics:- The additional chromosome is normal one in primary Trisomics. Secondary Trisomics:- Trisomics having isochromosome as additional chromosome. Tertiary Trisomics:- When additional chromosome in a trisomic is translocate one, it is known as tertiary trisomic.

67. EFFECT OF TRISOMY Down syndrome (trisomy 21): The result of an extra copy of chromosome 21. Down syndrome affects 1:700 children Characteristic:- facial features, short stature about 120 cm; heart defects. Susceptibility to respiratory disease, shorter lifespan. Prone to developing early Alzheimer's and leukemia Often sexually underdeveloped and sterile, mental retardation.

68. TETRASOMY Tetrasomics have a particular chromosome represented four times. Tetrasomics are those organisms which have two extra chromosome ( 2n+2). Therefore the general chromosome formula for Tetrasomics is 2n+2. Trisomics and Tetrasomics are together known as hyperploid or polysomic, which refers to addition of one or two chromosomes to a single or two different homologous pairs.

69. NUMERICAL CHANGE IN CHROMOSOME AND THEIR SYMBOLS SR NO. Term Type of change Symbol Heteroploid A change from diploid A. Euploid Number of genomes or copies of a genome is more or less than two a) Monoploid One copy of a single genome x b) Haploid Gametic chromosome complement n c) Diploid Two copies of genome 2x d) Polyploidy More than two copies of one genome or two copies each of two or more genomes

70. 1. Autoployploid Genomes are identical with each other i. Autotriploid Three copies of one genome 3x ii. Autotetraploid Four copies of one genome 4x iii. Autopentaploid Five copies of one genome 5x Iv. Autohexaploid Six copies of one genome 6x v. Autoheptaploid Seven copies of one gnome 7x vi. Autooctaploid Eight copies of one genome 8x

71. 2. Allopolyploid Two or more distinct genomes I Allotetraploid Two copies each of two distinct genomes (2x1 + 2x2) ii Allohexaploid Two copies each of three distinct genomes (2x1 + 2x2 + 2x3) iii Allooctaploid Two copies each of four distinct genomes (2x1 + 2x2 + 2x3 + 2x4) B. Aneuploid One or few chromosomes extra or missing from 2n 2n + few a) Monosomic One chromosome missing 2n-1 b) Double monosomic One chromosome from each of two different chromosome pairs missing 2n-1-1

72. c) Nullisomic One chromosome pair missing 2n-2 d) Trisomic One chromosome extra 2n+1 e) Double trisomic One chromosome from each of two different chromosome pairs extra 2n+1+1 f) Tetrasomic One chromosome pair extra 2n+2

73. UTILIZATION OF ANEUPLOIDS IN GENE LOCATION

74. USE OF ANEUPLOID FOR LOCATING GENES Aneuploidy Loss or gain of one or few chromosomes as compared to normal somatic chromosome complement. Generally as a view of locating the genes we use nullisomic, monosomic or Trisomics as per convenience.

75. TRISOMIC ANALYSIS Use of Trisomics for locating the genes and preparing chromosome mapping called trisomic analysis. BRIDGES (1921) was first to use Trisomics to locate the gene ey (eyeless) has its locus in the fourth chromosome of Drosophila melanogaster. Similar identifications followed shortly in Datura: the gene “ White ” was identified with the trisomic type “ Poinsettia ” ( BLAKESLEE and FARNHAM 1923 ).

76. ANALYSIS Assigning linkage groups to specific chromosome using primary trisomic Mapping of genes on specific chromosome arms using secondary, tertiary and telocentric Trisomics, Location of genes on specific chromosome arm Location of centromere and orientation of linkage group Conversion of recombination values into map distances

77. ASSIGNING LINKAGE GROUPS TO CHROMOSOMES Complete set of trisomic Strain carrying gene on particular chromosome Trisomic Disomic Selfed or testcross Discard Ratio analysed

78. Genotype of trisomic or triploid Chromosomal Segregation Max. eq. segregation 50% 25% 50% 25% 50% Aaa 17:1 11:1 14:1 41:4 67:5 Aaa 2:1 11:7 29:16 3:2 23:13 Theoretical trisomic F2 ratios obtained in duplex and simplex genotypes due to three. Different modes of segregation. Segregation types and transmission of X+1 gametes Normal F2 ratio 3:1

79. Theoretical trisomic test ratios obtained in duplex (AAa) and simplex (Aaa) genotype due to three different modes of segregation. Test cross Chromosomal Segregation Max. eq. segregation 50% 25% 50% 25% 50% 25% AAa X aa 5:1 3:1 4:1 11:4 19:5 35:13 aa X AAa 2:1 2:1 2:1 2:1 2:1 2:1 Aaa X aa 1:1 5:7 7:8 2:3 11:13 19:29 aa X Aaa 1:2 1:2 1:2 1:2 1:2 1:2 Normal test ratio 1:1 Segregation types and transmission of X+1 gametes

80. LOCATING GENE ON SPECIFIC CHROMOSOME ARM Secondary , tertiary or telocentric Strain carrying gene on particular chromosome Trisomic Disomic Discard Selfed or testcross Ratio analysed If dominant allele located on extra chromosome arm then in all:0 ratio observed… if not normal disomic ratio observed

81. EXPECTED TRISOMIC RATIO FOR DUPLEX

82. MONOSOMIC & NULLISOMIC ANALYSIS Monosomic analysis Used for locating genes in Polyploid species and in maize (can tolerate monosomic Monosomic analysis For monogenic trait For digenic trait Using intervarietal Ch. substitution Locating on ch . arms Analysis of F1 For dominant gene Analysis of F2 For recessive genes Analysis of F3 For hemizygous ineffective genes

83. LOCATING DOMINANT GENE BY ABSENCE OF EXPRESSION IN NULLISOMICS When dominant gene located on chromosome. Nullisomic for that chromosome will not show that character. E.g., genes for red seed colour-on chromosome 3D. Gene for awn suppression- on chromosome 4B and 6B both for wheat.

84. LOCATING DOMINANT GENE BY ANALYSIS OF F1 Cross whole set of monosomic using as female containing dominant marker with normal male containing recessive for that trait. Locating recessive gene by analysis in F2 Cross between whole set of monosomic containing recessive trait with normal male carrying homozygous dominant for that gene. In F1 all plants show dominant phenotype for both critical and non critical lines.

85. LOCATING HEMIZYGOUS INEFFECTIVE GENE THROUGH ANALYSIS OF F3 In some cases effect of aa differs from abut AA and Aa show dominance. aa shows recessive character a- is known as hemizygous ineffective. Ex , spheroccocum character in wheat. It shows same ratios in F2 for critical and non-critical lines. But in F3 study the ratio of progeny of plants which showed dominance in F2.

86. FOR DIGENIC TRAIT When character controlled by two genes, it may have various types of interactions. Critical lines have AAB- , A-BB, aab- , a-bb. Non-critical lines have AaBb, AABB, aabb.

87. LOCATING GENES USING INTERVARIETAL CHROMOSOMAL SUBSTITUTIONS Use of whole chromosome substitutions Monosomic series crossed with donor variety. F1monosomics selected cytologically. Selfed to get disomic for the univalent chromosome of donor. Then disomics are back-crossed to recipient to recover recipient. If this substitution leads to major morphological changes then we can conclude that gene located on that chromosome. They use intervarietal chromosome substitutions for locating characters like awning, earliness, lodging, plant height, protein content, 1000 kernel weight They used recipient variety: Chinese spring Donor varieties: Thatcher, Hope and Timstein

88. LOCATING GENES ON CHROMOSOME ARMS Locating gene on one of two arms of chromosomes by using telocentric chromosomes in form of Monotelosomics - 20 II + 1 t I Monoisosomics - 20 II + 1 i I Ditelosomics- 20 II + 1 t II Monotelodisomics- 20 II + 1 hetero II • Example : In Chinese spring wheat monotelosomics for chromosome arm 6BS are awned (Chinese spring is awnless), so it concluded that awn inhibitor gene B2 is present on 6BL (Sears, 1962) Use of chromosome banding techniques

89. SOMATIC SEGREGATION AND CHIMERAS SOMATIC SEGREGATION: When an organism produces several types of offspring the nor mal moment of segregation is at the reduction division , in which case segregation usually obeys Mendel's laws. But other types of segregation may occur. Much less is known about them than about the normal type, but a distinction may be made between three types of event, mutation, break-up of a chimaera, and plastid inheritance. Somatic segregation is more important in plants than in animals, because in the latter the germ cells are usually early differentiated from the soma, and whereas in a plant with white and green branches the gametes will of ten be different on the two branches, an insect whose right and left wings differ will commonly produce only one type of gamete.

90. CHIMERAS Definition: “A plant composed of tissues of two or more genotypes or ideotypes; as a consequences of mutation, fusion of different zygotes or grafting.”

91. TYPES OF CHIMERAS a)According to their structure : I) Sectorial Chimera : It is a plant in which a part of one cell layer has a different genotypes . crimson sectorial-chimera

92. b) Periclinal Chimera: It is a plant in which whole of one cell layer has different genotypes. Sandwich Periclinal chimera Pale leaf edge Periclinal chimera

93. ORIGIN There are often authentic accounts of the origin of chimeras: by spontaneous or induced mutations. by the sorting-out from variegated seedlings after plastid mutation. by grafting, and by the layering of mixed populations of cells within callus tissue cultures. by somatic hybridization through protoplast fusion.

94. ENDOMITOSIS Definition: “It is the phenomenon of replication of chromosomes without cell or nuclear division leading to polyploidy”

95. SOME IMPORTANT FEATURES The process can be induced in isolated tissues by treatment with colchicine, which prevents spindle formation so the centromeres of the daughter chromosomes are unable to move apart into separate nuclei. It may occur as an error in part of a plant, producing, for example, a tetraploid branch on a diploid plant. It occurs as a normal feature in some tissues of higher plants, e.g. the phloem cells of some leguminous plants are Polyploid. This type of polyploidy, where some of the cells of a plant have more than the normal complement of chromosomes for the species, is known as endopolyploidy. If endomitosis occurs in cells in the germ line or during the second division of meiosis then unreduced gametes may result.

96. Definition:- “ It is the phenomenon which results in the reduction of somatic chromosome complements and involves the segregation of whole genome.” Somatic Reduction

97. IMPORTANT FEATURES Its mechanism may involves the abnormalities of spindle such as multipolar spindle formations . It was first described in insects and later it was found to occur spontaneously in plants and plant tissues . It can be induced by certain chemicals in different plants, e.g., by chloramphenicol in barley root tip and by paraflouorphenylalanine in grape seedling . The most completely analysed case of somatic reduction in plants is that recorded recently by Brown in cotton.

98. EVOLUTIONARY SIGNIFICANCE OF CHROMOSOMAL ABERRATIONS Polyploidy played a significant role in the origin and evolution of many plant species. The primary phylogenetic effect of polyploidy is to stabilize selected hybrid genotypes. It also provides a mechanism by which daughter and parental populations become immediately isolated from each other. Polyploidy also buffers genotypes against the shock of absorbing foreign genomes, making hybridization possible between species that are otherwise genetically isolated from each other. Polyploidy, frequently termed ‘‘whole genome duplication’’, is a major force in the evolution of many eukaryotes.

99. ALLOPOLYPLOIDY Origin: Interspecific hybridization followed by chromosome doubling Meiotic irregularities – unreduced gamets Effect of Allopolyploid on plant: Allopolyploids are highly vigorous than diploids with some exceptional. Apomictic Adaptability differs from parents Ex: Raphanobrassica and triticale

100. Fig. 16-7 ALLOPOLYPLOIDS ARISE FROM INTERSPECIES HYBRIDIZATION + GENOME DUPLICATION

101. GENETICS OF ALLOPOLYPLOIDS: Allopolyploid speciation often results in more individual genetic variation than in a diploid species. Increased heterozygosity, the generation of novel heteromeric enzymes, and the formation of new gene combinations . Alloploids arise from the combination and subsequent doubling of different genomes, a cytological event is called Alloploidy. The genomes that are combined differ in degrees of homology, some being close enough to pair with each other, whereas others are too divergent to pair . Some of the chromosome of one genome may share a function in common with some chromosomes in a different genome. Wheat - pairing between homologous chromosome is prevented by Ph locus Allopolyploids shows meiotic behaviour as that of diploids – formation of bivalents & produce disomic ratios

102. ROLE OF ALLOPOLYPLOID IN EVOLUTION Wheat The bread wheat ( Triticum aestivum ) is an allopolyploid. It is believed that A genome of wheat has come from Triticum monococcum (2n=14), D genome from Triticum tauschi (2n=14) and B genome from unknown source probably from an extinct species (2n=14). Thus hexaploid wheat has two copies of the genomes from three species. First allotetetraploid Triticum turgidum developed from a cross between Triticum monococcum and unknown species of B genome. Then cross between T. turgidum and T. tauschi resulted in in the development of hexaploid wheat T. aestivum .

103. BRASSICA Three basic species:- 1 ) Brassica nigra (BB, n=8) 2 ) B. oleracea (CC, n=9) and 3 ) B. campestris (AA, n=10). The cross between Brassica nigra and B. oleracea gave rise to B. carinata . Cross between B. campestris and B. oleracea led to the development of B. napus , and cross between B. campestris and B. nigra resulted in the development of B. juncea . All the resulting species are amphidiploids.

104. “TRIANGLE OF U”

105. APPLICATIONS OF ALLOPOLYPLOIDY IN CROP IMPROVEMENT: Bridging cross: Nicotiana digluta Creation of new crop species Interspecific Gene Transfer Tracing the origin of crop species

106. LIMITATIONS OF ALLOPLOIDY It is not possible to predict the effect of alloploids. There may have defects in the newly synthesized alloploids. There are only a small proportion of alloploids are promising. Chances of developing new species are very low.

107. BALANCED LETHAL AND CHROMOSOME COMPLEXES Balanced Lethal Systems Lethal genes Some genes in an organism are known to cause the death of the organism. These genes can be either dominant or recessive, but the organism must be homozygous for them . These genes are essential, and are therefore, important for survival. Hence, alleles of a lethal gene show a deviation from the normal Mendelian inheritance. For instance, the Mendelian inheritance shows a phenotypic ratio of 3:1, whereas in the case of expression of lethal alleles there is a deviation from this ratio.

108. Lethal alleles are produced when mutation in a usual allele distorts the function of an essential gene. This in turn results in a phenotype, which when expressed, is fatal to the organism carrying them. An example is the gene expressing the coat color in mouse. A French geneticist found that yellow mouse was dominant over the brown mouse, and the color was determined by a single gene “C”. He also found that yellow mouse was never homozygous for the “C” gene. Lethal alleles are either dominant or recessive. When the allele is fully dominant it can cause the death of the organism in both heterozygous and homozygous conditions . But when the allele is recessive in nature the organism dies only when the lethal allele is in the homozygous condition .

109. TYPES OF LETHAL ALLELES Recessive lethals, D ominant lethals, C onditional lethals, B alanced lethals, and G ametic lethals.

110. BALANCED LETHALS Two different lethal systems can operate in an organism, and balance each other's effect. Both genes are tightly linked in the repulsion phase of linkage where a dominant allele of one gene is linked to a recessive allele of another gene. In such cases, one parent contributes the dominant allele of gene one and recessive allele of gene two, while the other parent contributes the recessive allele of gene one and dominant allele of gene two. This arrangement of lethal alleles maintains a heterozygous combination, and ensures that homozygotes for lethal chromosomes do not occur.

111. The SMC (Structure Maintenance of Chromosome) complexes are the key components of higher-order chromatin fibers and play important roles in genome stability. Three SMC complexes are present in most eukaryotic cells: cohesin (SMC1/3), condensin (SMC2/4) and SMC5/6 complex. Cohesin can make internal loops or embrace two sister chromatids (feature essential for proper chromosome segregation); condensin interconnects loops to condense chromatin during mitosis. The SMC5/6 complex is involved in the homologous recombination-based DNA repair, in replication fork stability and processing, and in cohesin regulation.

112. Transcription factors control cell-specific gene expression programs by binding regulatory elements and recruiting cofactors and the transcription apparatus to the initiation sites of active genes. One of these cofactors is cohesin, a structural maintenance of chromosomes (SMC) complex that is necessary for proper gene expression . We report that a second SMC complex, condensin II, is also present at transcriptional regulatory elements of active genes during interphase and is necessary for normal gene activity. Both cohesin and condensin II are associated with genes in euchromatin and not heterochromatin. The two SMC complexes and the SMC loading factor NIPBL are particularly enriched at super-enhancers, and the genes associated with these regulatory elements are especially sensitive to reduced levels of these complexes. Thus, in addition to their well-established functions in chromosome maintenance during mitosis, both cohesin and condensin II make important contributions to the functions of the key transcriptional regulatory elements during interphase.