Question

Following are the observations of Morgan’s dihybrid cross. Fill in the blanks with most appropriate findings.
1) The genes responsible for white eye color and that for yellow body were very ____i____ linked. _____ii____ of the F2 generation had parental phenotype while __iii_______ of F2 offsprings had the recombinant phenotype.
2) The genes responsible for white eye color and that controlling miniature wings were ___iv_____ linked. Percentage of F2 progenies showing parental and recombinant phenotypes were ____v_____ and ____vi_____ respectively.

• A. i-loosely, ii-1.3%, iii-98.7%, iv-loosely, v-62.8%, vi-37.2%
• B. i-loosely, ii-98.7%, iii-37.2%, iv-loosely, v-62.8%, vi-1.3%
• C. i-tightly, ii-1.3%, iii-37.2%, iv-loosely, v-62.8%, vi-98.7%
• D. i- tightly, ii-98.7%, iii-1.3%, iv-loosely, v-62.8%, vi-37.2%

Answer 1

The correct option is D i- tightly, ii-98.7%, iii-1.3%, iv-loosely, v-62.8%, vi-37.2%

T.H. Morgan conducted his experiment on Drosophila melanogaster (commonly known as fruit fly). He carried out several dihybrid crosses using Drosophila to study the pattern of inheritance of certain genes. The crosses were similar to the dihybrid crosses carried out by Mendel in peas, however the outcomes were quite different.
Morgan conducted dihybrid cross for the following pairs of characters in Drosophila:-
A. Flies with wild type and mutant phenotypes for body colour and eye colour.
B. Flies with wild type and mutant phenotypes for eye colour and wing size.
$\begin{array}{ccccc}\text{Character}& \text{Wild type}& \text{Allele}& \text{Mutant type}& \text{Allele}\\ \text{Body color}& \text{Brown}& y^{+}& \text{Yellow}& \text{y}\\ \text{Eye color}& \text{Red}& w^{+}& \text{White}& \text{w}\\ \text{Wing size}& \text{Normal}& m^{+}& \text{Miniature}& \text{m}\end{array}$
The crosses have been shown below:-
In the experiments performed, Morgan observed two types of progenies in the F2 generation- parental and non parental types.
The offsprings whose phenotype is same as the parents were called parental types. Whereas the offsprings whose phenotype is different from the parents were called non parental types.
When two genes in parents are tightly linked and are present in the same chromosome, the resulting offspring will be the parental type. But the non parental types are produced in the cases where the genes are loosely linked and undergo recombination.
In cross A, 98.7% of the F2 progenies were of parental type while the rest 1.3% displayed non-parental combinations. Hence, genes controlling body color and eye color were tightly linked.
In cross B, it was found that 62.8% of the F2 progenies displayed traits similar to parental combination while 37.2% were of non-parental types. Hence, genes controlling eye color and wing size were comparatively loosely linked.