Single Choice

In a plant, three dominant independently assorting genes A, B and C are essential for production of purple pigment. If any of the genes or all three genes are present in recessive condition then flower is colourless. Raw material $$\overset { A }{ \rightarrow} X \overset { B }{ \rightarrow} Y \overset { C }{ \rightarrow} Z$$ pigment A purple plant with genotype AABBCC cossed with a clourless plant with genotype aabbcc gives purple $$F_1$$ hybrid. On selfing of $$F_1$$, what will be the proportion of coloured offspring in $$F_2$$?

A$$\displaystyle \frac{27}{64}$$
Correct Answer
B$$\displaystyle \frac{1}{64}$$
C$$\displaystyle \frac{9}{64}$$
D$$\displaystyle \frac{37}{64}$$

Solution

In the given case, inheritance is controlled by three genes. So, it is a case of polygenic inheritance. In this type of inheritance, the dominant alleles have cumulative effect with each dominate allele expressing a part of the trait, the full being shown only when all the dominant alleles are present.
In this case, during the selfing of $$F_1$$, AaBbCc results in the formation of 8 kinds of gametes in each sex which gives 64 combinations in $$F_2$$ having 7 phenotypes. Out of these 64 combinations, only 27 combinations will show coloured offspring. So, its proportion will be $$\displaystyle \frac{27}{64}$$.

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