Ppt on the Linkage and crossing over.PPTX

Linkage and crossing over

1. LINKAGE Researchers found that g enes inherit in groups rather than individually. The tendency of two or more genes to remain together in the same chromosome during inheritance is referred to as linkage. or L inkage is the tendency of genes to be inherited in groups. The strength of linkage is determined by distance between two genes- The greater the distance, lower will be the linkage strength and vice-versa

2. Un-linked VS Linked

3. Features of Linkage of Genes: 1. Linkage involves two or more genes which are located in the same chromosome in a linear fashion. 2. Linkage may involve either dominant genes or recessive genes or some dominant and some recessive genes. 3. Linkage usually involves those genes which are located closely. 4. Presence of linkage leads to higher frequency of parental types than recombinants in a test cross progeny. 5. Linkage may involve either two or more desirable traits or all undesirable traits or some desirable and some undesirable traits.

4. 6. Linkage is observed for both oligogenic traits as well as polygenic traits. 7. Besides pleiotropy, linkage is an important cause of genetic correlation between various plant characters. 8. The strength of linkage depends on the distance between the linked genes. Lesser the distance higher the strength and vice versa. 9. If crossing over does not occur, all the genes located in one chromosome are expected to be inherited together. Thus the maximum number of linkage groups in an organism is equal to its haploid chromosome number. 10. Linkage can be broken by repeated inter-mating of randomly selected plants in segregating populations for several generations.

5. TYPES OF LINKAGE nn

6. Based on Chromosome Involved: ( i ) Autosomal Linkage: It refers to linkage of such genes which are located in other than sex chromosomes (autosomes). (ii) X-Chromosomal Linkage: It refers to the linkage of genes which are located in sex chromosomes.

7. Linkage Groups: Linkage group refers to a group of genes which are present in one chromosome OR all genes which are located in one chromosome constitute one linkage group. The maximum number of linkage groups is equal to the haploid chromosome number of an organism. I n case of species having dissimilar sex chromosomes, linkage groups are one more than the haploid chromosome number, e.g - T here are ten linkage groups in corn, 7 in garden pea, 7 in barley, 4 in Drosophila melanogaster and 23 in man.

8. Detection of Linkage of Genes: Test cross is the most common method of detecting the linkage. F 1  heterozygous at two loci (say AaBb ) is crossed to a double recessive parent ( aabb ) and the phenotypic ratio of test cross progeny is examined. If the phenotypic ratio of test crosses progeny shows 1:1:1:1 ratio of parental and recombinant genotypes, it indicates absence of linkage. If the frequency of parental types and recombinant types deviate significantly from the normal test cross ratio of 1 : 1 : 1 : 1, it reveals presence of linkage between two genes under study.

9. 7. Linkage and Pleiotropy: A close association between two or more characters may result either due to linkage or pleiotropy or both. Pleiotropy refers to the control of two or more characters by a single gene. Inter-mating in segregating populations may break a tight linkage, If a crossover product is not found in-spite of repeated inter-mating, there seems to be the case of pleiotropy rather than linkage.

10. CROSSING OVER Crossing over refers to the interchange of parts between non-sister chromatids of homologous chromosomes during meiotic division. OR Crossing over is the swapping of genetic material between non-sister chromatids, involving breakage and reunion at precise point. The term crossing over was first used by Morgan and Cattell in 1912.

11. Feature of Crossing Over: 1. Crossing over takes place during meiotic prophase, i.e., during pachytene. Each pair of chromosome has four chromatids at that time. 2. Crossing over occurs between non-sister chromatids. Thus one chromatid from each of the two homologous chromosomes is involved in crossing over. 3 . Each crossing over involves only two of the four chromatids of two homologous chromosomes.

12. 5. D ouble or multiple crossing over is very rare,may involve all four, three or two of the four chromatids 6 . Crossing over leads to re-combinations or new combinations between linked genes. 7. Crossing over generally yields two recombinant types or crossover types and two parental types or non-crossover types. 8 . The value of crossover or recombinants may vary from 0-50%.

13. C hiasma and Crossing Over: The point of exchange of segments between non-sister chromatids of homologous chromosomes during meiotic prophase is called chiasma (pleural chiasmata). Chiasma was first discovered by Janssens in 1909. Depending on the position, chiasma is of two types, viz., terminal and interstitial. The number of chiasma per bivalent may vary from one to more than one depending upon the length of chromatids. When two chiasmata are formed, they may involve two, three or all the four chromatids.

14. Molecular Mechanism of Crossing Over: There are two important theories: 1. Copy choice theory and 2. Breakage and reunion theory Copy Choice Theory: This theory was proposed by Belling. This theory states that the entire recombinant section or part arises from the newly synthesized section. The non-sister chromatids when come in close contact they copy some section of each other resulting in recombination.

15. Thus, recombinant chromosome or chromatids have some alleles of one chromatids and some of other. According to this theory, physical exchange of preformed chromatids does not take place. The information may be copied by one strand or both the strands.

16. Breakage and Reunion Theory: This theory states that crossing over takes place due to breakage and reunion of non-sister chromatids. The two segments of parental chromosomes which are present in recombinants arise from physical breaks in the parental chromosomes with subsequent exchange of broken segments The breakage results due to mechanical strains that result from the separation of paired homologous chromosomes and chromatids in each chromosome during pachytene stage. The broken ends of non-sister chromatids unite to produce chiasmata resulting in crossing over.

17. CROSSING OVER IN MAIZE In the linkage experiment in maize, it is seen that the genes for seed colour C and full seed S remain associated in the parental combination in about 96 per cent but break apart in about 4 per cent. This recombination of linked genes to interchange parts between homologous chromosomes is termed as crossing over. Crossing over takes place in the segment of the chromosome between the loci of the genes C and S in some cells but not in others, so that about 96 per cent of the gametes contain the parental gene combination and 4 per cent contain recombination’s.

18. Mechanism of Crossing Over: During the zygotene stage of the first prophase of meiosis, the homologous maternal and paternal chromosomes start pairing and lie closely side by side. This phenomenon is called synapsis. This pairing of homologous chromosomes is brought about by the mutual attraction between the allelic genes. The paired chromosomes are known as bivalent.

19. Synapsis and chiasma formation is facilitated by a highly organized structure of filaments called synaptonemal complex Synapsis is followed by the duplication of chromosomes which change the bivalent nature of chromosome pair into tetravalent During this each of the homologous chromosomes in a bivalent split longitudinally into two sister chromatids attached to the undivided centromere Thus, four chromatids are formed which remain side by side as two pairs

20. Later, in pachytene stage crossing over takes place during which the non-sister chromatids of homologous pair twist over each other, the point of contact of cross over chromatids being called as chiasma In crossing over two or three chromatids are involved and accordingly two or more chiasmata are formed. At each chiasma the chromatid breaks and the broken segment rejoin a new chromatid, Thus exchange of parts of chromatids brings about alteration of original sequence of genes in the chromosome.

21. After crossing over is completed, the non-sister chromatids repel each other due to lack of attraction between them. The repulsion or separation of chromatids starts from the centromere towards the end just like a zipper and this separation process is named as terminalization. The process of terminalization continues through diplotene, diakinesis and ends in metaphase I.

22. Types of Crossing Over: Depending upon the number of chiasmata involved, crossing over may be of three types, viz., single, double and multiple: i . Single Crossing Over: It refers to formation of a single chiasma between non-sister chromatids of homologous chromosomes. Such cross over involves only two chromatids out of four. ii. Double Crossing Over: It refers to formation of two chiasmata between non-sister chromatids of homologous chromosomes. Double crossovers may involve either two strands or three or all the four strands.

23. iii. Multiple Crossing Over: Presence of more than two crossovers between non-sister chromatids of homologous chromosomes is referred to as multiple crossing over. Frequency of such type of crossing over is extremely low. 6. Factors Affecting Crossing Over: The frequency of crossing over is influenced by several factors: i . Distance: The distance between genes affects the frequency of crossing over. Greater the distance between genes higher is the chance of crossing over and vice versa.

24. ii. Age: Generally crossing over decreases with advancement in the age in the female Drosophila. iii. Temperature: The rate of crossing over in Drosophila increases above and below the temperature of 22°C. iv. Sex: The rate of crossing over also differs according to sex. There is lack of crossing over in Drosophila male and female silk moth. v. Nutrition: Presence of ions like calcium and magnesium in the food caused reduction in recombination in Drosophila. However, removal of such chemicals from the diet increased the rate of crossing over.

25. vi. Chemicals: Treatment with mutagenic chemicals like alkylating agents was found to increase the frequency of crossing over in Drosophila female vii. Irradiation: Irradiation with X-rays and gamma rays was found to enhance the frequency of crossing over in Drosophila females. ix. Centromere Effect: Generally genes that are located adjacent to the centromere show reduced frequency of crossing over.