Question

When a red grain variety of wheat is crossed with another white grain variety, a $F_1$ hybrid is produced. On selfing of this, an $F_2$- generation is produced. How many of these resemble phenotypically to their parents (let grain colour of wheat controlled by three gene pairs)?
A. $\dfrac{2}{6}$
B. $\dfrac{20}{64}$
C. $\dfrac{15}{64}$
D. $\dfrac{2}{64}$

Answer 1

Hint: Basically, Genetics is a branch of biology that deals with the study of transmission of genes and characteristics from generation to generation (parents to offspring). In Mendelian genetics, the offspring of parental generation $P_1$ are the $F_1$ (first filial) generation. $F_2$(second filial) generation is produced by the self fertilization of the $F_1$ generation.

Complete Answer:
- Grain colour is a polygenic characteristic in wheat. A polygenic trait is regulated by more than one gene in which only a portion of the trait is expressed by the dominant allele of each gene and the entire trait is expressed only in the presence of the dominant alleles of all multiple genes.
- Therefore, these genes show various phenotypes depending on the genotype. According to the question, the grain color in wheat is controlled by three gene pairs.
- A cross between red grain (AABBCC) and white grain (aabbcc) varieties produce $F_1$ generation with AaBbCc genotype. Selfing of $F_1$ hybrids produce $F_2$ -generation.
- The fraction of the parental genotype AABBCC is $\dfrac{1}{64}$ and that of aabbcc is $\dfrac{1}{64}$. Thus, the total fraction of $F_2$generation having the parental genotypes and therefore, the parental phenotypes is $\dfrac{2}{64}$.

Therefore, the correct answer is option D, $\dfrac{2}{64}$.

Note: The genes of $F_1$ generation pure lines show limited variation as they have a uniform phenotype. Repeating the cross once the characteristics of $F_1$ cross is known, yields the exact result.