Question

In a plant, flower colour is the example of quantitative trait and controlled by one gene pair. How many plants show parental phenotype in F${}_2$ generation?

• A. $\frac{2}{16}$
• B. $\frac{2}{4}$
• C. $\frac{2}{64}$
• D. $\frac{2}{256}$

Answer 1

The correct option is B $\frac{2}{4}$

According to the question, flower colour is controlled by one gene pair and hence is the example of monohybrid cross. Total number of types of gamete produce by an organism is 2${}^n$, where n is the number of heterozygous genes present. As we know that monohybrid plant is heterozygous for one gene pair, thus total possible gametes by it = 2${}^1$=2. During fertilization, any of the 2 types of pollen can fuse with any of the 2 types of ovum to produce 4 possible combinations of zygote. A trait governed by more than one gene where dominant allele of each gene express only a part of trait and the full trait is expressed only in presence of dominant alleles of all multiple genes, is called as polygenic inheritance. Out of total four zygotic combinations; only pure recessive and pure dominant will show the parental type. Hence, the ratio of parental type = (1/4 homozygous dominant + 1/4 homozygous recessive) = 2/4. Thus, option B is correct answer. Total 16 zygotic combinations are produced in a dihybrid cross; 64 are produced in trihybrid cross while 256 $F_2$ combinations are produced in a tetrahybrid cross; but the question is about monohybrid cross which makes options A, C and D wrong.