Theories of Linkage:

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                                                                 Theories of Linkage:

VIEWS OF CLASSICAL GENETICISTS ON LINKAGE:

Mendel could not notice the phenomenon of linkage because fortunately the seven pairs of factors or alleles studied by him in pea were located on seven different pairs of chromosomes. It was noticed and discovered by some other post-Mendelian geneticists who during their genetic investigations came across to linked genes. The evolution of linkage concept took place by the views of following classical geneticists:

1. SUTTON'S VIEWS ON LINKAGE:

Sutton (1903) was the first classical geneticist who made certain predictions about the linkage merely by performing certain cytological investigations. He suggested that each chromosome must bear more than a single gene and that the genes represented by one chromosome must be inherited together. However, he could not prove his predictions by genetic experiments.

2. COUPLING AND REPULSION HYPOTHESIS OF BATESON AND PUNNETT :

Bateson and Punnett (1905-1908) formulated the 'hypothesis of coupling and repulsion' to explain the unexpected F, results of a dihybrid cross between a homozygous sweet pea (Lathyrus odoratus) having a dominant alleles for blue or purple flowers (BB) and long pollen grains (LL) with another homozygous double recessive plant (bbll) with red flowers and round pollen grains. When they test crossed a heterozygous blue or purple long (BbLl) plant with recessive parent (bbll), besides getting the 1:1:1:1 test cross ratio, they received phenotypic ratio of 7:1:1:7 as has been illustrated here:

 The 7:1:1:7 test cross ratio clearly indicated that there was a tendency in the dominant alleles (BL) to pass together to the same gamete. Similar was the case with recessive alleles (bl). This tendency of dominant or recessive alleles to inherit together was explained as "gametic coupling" by Bateson and Punnett, Further, when they crossed blue or purple round (BBll) with red long (bbLL), the F, hybrids were found to be heterozygous blue or purple long (BbLl). The F1 hybrid when test crossed with recessive (bbll) parent, the test cross ratio was blue or purple long: 7 red long: 7 blue or purple round: 1 red round, as has been illustrated in following figure:

 Test cross progeny: 1/16 Blue or purple long (BbLl): 7/16 Blue or purple round (Bbll): 7/16 Red long (bb Ll): 1/16 Red round (bbll) or 1:7:7: 1.

Hence, the two dominant pairs of alleles repelled each other. The tendency of both dominant or both recessive alleles to repel each other, so that the gametes of genotypes of Bl and bL are formed more frequently, was termed repulsion. Bateson and Punnett could not explain the exact reasons of coupling and repulsion, and it was T.H. Morgan who while per forming experiments with Drosophila, in 1910, found that coupling or repulsion was not complete. He further suggested that the two genes are found in coupling phase or in repulsion phase, because they are present on the same chromosome (coupling) or on two different homologous chromosomes (repulsion). Such genes are then called linked genes and the phenomenon of inheritance of linked genes is called linkage by Morgan.


3. MORGAN'S VIEWS ON LINKAGE:

Morgan stated that the pairs of genes of homozygous parent tend to enter in same gametes and to remain together, whereas same genes from heterozygous parents tend to enter in different gametes and remain apart from each other. He further stated that the tendency of linked genes remaining together in original com bination is due to their location in same chromosome. The degree or strength of linkage depends upon the distance between the linked genes in the chromosome. Morgan's concept about the linkage developed the theory of linear arrangement of genes in the chromosomes which helped the cytogeneticists in the construction of genetic or linkage maps of chromosomes.

4 . Differential Multiplication Theory:

Bateson (1930) assumed that linkage is due to the differential multiplication of cells containing different combinations of genes. After the segregation of genes during gamete formation, the set of gametes possessing parental combinations multiplies rapidly than the set having non-parental combination. This theory was disproved because after segregation of genes, there is only one division.

5. Chromosome Theory of Linkage:

Morgan and Castle proposed the chromosomal theory of link. age. The main features of the chromosomal theory of linkage are the following:

1. The genes are arranged in a linear fashion on the chromosomes.

2. The genes on a chromosome are linked

3. Genes showing linkage are located on the same chromosome

4. Linked genes will remain together during inheritance.

5. The distance between the genes will determine the strength of linkage. The closely located genes show strong linkage. Distantly located genes show weak linkage.

 

 

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