The genes located on the same chromosome are called linked genes. The phenomenon of staying together of all the genes of a chromosome is called linkage. Every organism possesses numerous characters. These characters are controlled by thousands of genes. But they have limited number of chromosomes. Therefore. each chromosome carries many genes on it. So the gene present on the same chromosome is linked to each other. Gene linkage is a physical relationship between genes. Linked genes cannot assort independently during meiosis. So they do not obey Menders law of independent assortment. Gene linkage also minimizes the chances of genetic recombination and variations among offspring.
Work of William Bateson and R.0 Punnet
William Bateson and R.0 Punnet were study flower colour and pollen shape in sweat pea in 1906. Purple (PP) flower colour is dominant over red (pp) flower colour. Similarly. long pollen grain (LL) is dominant over round pollen grain (11). They cross pure line of purple long (PPLL) with pure line of red, round (pp11). They obtained purple long in F1 They then intercrossed the F1. But they obtained strange results in F. They obtained following phenotypes:
- Purple. long
- Purple. round
- Red. long
- Red. round
These results do not give 9:3:3:1 ratio. Thus they do not follow
Menders law of independent inheritance. The progeny of F2 has large number of parental types (Purple. long and red. round) than recombinant type (Purple. round and red, long). Bateson and Punnett suggested that the appearance of large number of parental types in F, showed that some physical connection is present between these characters. They believed that some attractive forces are present between parental alleles. So they tend to remain together. They named this process as coupling. But they believed some negative forces are produced during recombination. These are called repulsion.
Work of T.H Morgan
Morgan performed experiments on Drosophila. He studied the affect of linkage on the inheritance of two di Ifere.,t characters. He followed two characters, body colour and wing size ,h7 drosophila:
(a) Wild type: Wild-type flies have gray bodies and normal ‘sings. (I)) Mutants: Mutant phenotypes for these characters are black body
and vestigial wings. The vestigial wings are much smaller than
The alleles for these traits arc represented by the follow ing symbols: = gray, b = black; vg = normal wings. vg vestigial wings. They are not sex linked. Their loci are on autosomes.
Morgan crossed female di-hybrids (h h vg’ vg) with males. These males %%ere mutant with black bodies and vestigial wings (b b vg N g). It is a Mendelian testcross. According to Menders la); of independent assortment. Morgan’s Drosophila testcross should produce four phenotypic classes of offspring. These classes should – be equal in number:
I. I gray-normal (wings)
2. I black-vestigial
- 1 gray-vestigial
- 1 black-normal.
OBSERVED 965 944 206 185
The actual results were verb different. There were disproportionate numbers of wild-type (gray-normal) and double mutant (black-vestigial) flies among the offspring. These two phenotypes corresponded to the phenotypes Of the two parents. Morgan concluded that body color and wing shape are inherited together in a specific combination. The genes for these two characters are located on the same chromosome. The other two phenotypes (gray-vestigial and black-normal) had much less number. These phenotypes were present among the offspring of Morgan’s cross. These new combinations of the two characters were formed by crossing over.
Linkage in corn
The phenomenon of linkage is more obvious in corn. Hutchion performed experiments on corn. lie crosses coloured, normal (full endosperm) seed (CCSS) with colourless and shrunken seeds (ccss). lie obtained coloured normal in F1. Then he test crossed this F1 with recessive (ccss). He obtained four classes of progeny. But these classes were not present in I: I:1:1 as expected by Mendel. It has more parental types (CCSS and ccss) than recombinant type (CCss or ccSS). It shows that both genes are linked.
A chromosome carries its linked genes en bloc in the form of a linkage group. The numbers of linkage groups are equal to the number of homologous pairs of chromosomes. Man has 23 linkage groups. Genes for colour blindness, haemophilia, gout etc form one linkage group on human X — chromosome. Similarly, gene for sickle cell anaemia, leukemia and albinism make another linkage group on human chromosome II. Number of linked groups in Drosophila are 4 and pea has 7.