Question

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 $\stackrel{A}{\to }X\stackrel{B}{\to }Y\stackrel{C}{\to }Z$ 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. $\frac{27}{64}$
• B. $\frac{9}{64}$
• C. $\frac{37}{64}$
• D. $\frac{1}{64}$

Answer 1

Answer: (A)

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 $\frac{27}{64}$.