Question

In Morgan's experiments on linkage, the parental types red eyed, normal wings and white eyed, miniature wings which were used for test-cross. The percentage of red eyed, normal wings and white eyed, miniature winged recombinants in F${}_2$ generation is

• A. 1.3
• B. 25
• C. 62.8
• D. 37.2
• E. 98.7

Answer 1

The correct option is D 37.2

In Morgan's experiments on linkage, the percentage of white eyed, miniature winged recombinants in F2 generation is 37.2.

Linkage is the phenomenon of certain genes staying together during inheritance through generations without any change or separation due to their being present on the same chro­mosome. Linkage was first suggested by Sutton and Boveri (1902-1903) when they pro­pounded the famous “chromosomal theory of inheritance.”

Bateson and Punnet (1906) while working on Sweet Pea found that the factors for certain characters do not show independent assortment. They then argued that since Sweet Pea has only a few pairs of homologous chromosomes, a chromosome must possess several factors which should be transmitted together.

It was Morgan (1910) who clearly proved and defined linkage on the basis of his breeding experiments in fruitfully Drosophila melanogaster. In 1911, Morgan and Castle pro­posed chromosome theory of linkage. It states that

(i) Linked genes occur in the same chromosome.

(ii) They lie in a linear sequence in the chromosome.

(iii) There is a tendency to maintain the parental combination of genes except for occa­sional crossovers.

(iv) Strength of the linkage between two genes is inversely proportional to the distance between the two, i.e., two linked genes show higher frequency of crossing over if the distance between them is higher and lower frequency if the distance is small.

Linked genes are those genes which occur on the same chromosome while unlinked genes are the ones found on different chromosomes. Linked and unlinked genes can be easily known from breeding experiments. Unlinked genes show independent assortment, a di-hybrid ratio of 9: 3: 3: 1 and the di-hybrid or double test cross ratio of 1: 1: 1: 1 with two parental and two recombinant types.

The linked genes do not show independent assortment but remain together and are inherited en block producing only parental type of progeny. They give a di-hybrid ratio of 3: 1 and a test cross ratio of 1: 1.

Incomplete Linkage:

Genes present in the same chromosome have a tendency to separate due to crossing over and hence produce recombinant progeny besides the parental type. The number of recombinant individuals is usually less than the number expected in independent assortment. In independent assortment all the four types (two parental types and two recombinant types) are each 25%. In case of linkage, each of the two parental types is more than 25% while each of the recombinant types is less than 25%.

Example:

Morgan and his students have found that linked genes show varied recom­binations, some being more tightly linked than others, (i) In Drosophila, crossing of yellow bodied (Y) and white eyed (W) female with brown bodied (Y+) red eyed (W+) male produced F1 to be brown bodied red eyed. On intercrossing of F1 progeny, Morgan observed that the two genes did not segregate independently of each other and, therefore, the F2 ratio deviated significantly from expected 9: 3: 3: 1 ratio. He found 98.7% to be parental and only 1.3% recombinants. (ii) In a second cross in Drosophila between white eyed and miniature winged (wwmm) female with wild type or red eyed normal winged (w+w+m+m+) males, all the F1 were found to be of wild type, i.e., red eyed and normal winged. An F1 female fly was then test crossed with white eyed and miniature winged male. 62.8% of the progeny was of parental types while 37.2% were recombinants.