Question

In a dihybrid cross between RRYY and rryy, the number of RrYy ${ F }_{ 2 }$ genotypes will be
(a) 4
(b) 3
(c) 2
(d) 9

Answer 1

A dihybrid cross between two plants would lead to one type of progeny in the ${ F }_{ 1 }$ generation, which on selfing will produce progeny in the ${ F }_{ 2 }$ generation and the number of the particular genotype can be calculated from the Punnett square.

Complete step by step answer:
- In the given dihybrid cross, the plant with yellow-colored round seeds (RRYY) is crossed with green colored wrinkled seeds (rryy) . The first generation or ${ F }_{ 1 }$ would produce yellow and round progeny RrYy. The ${ F }_{ 2 }$ generation would involve selfing or self-pollination of all the ${ F }_{ 1 }$ progeny.
- Here, the alleles of the two loci segregate independently. The resulting progeny show different phenotypes in the ratio 9:3:3:1 ratio. The produced phenotypes were - nine plants with round, yellow seeds; three plants with round, green seeds; three plants with wrinkled, yellow seeds; and one plant with wrinkled, green seeds.
- Since we want to figure the plants with the genotype RrYy, we make use of the Punnett square.

Traits	RY	Ry	rY	ry
RY	RRYY	RRYy	RrYY	RrYy
Ry	RRYy	RRyy	RrYy	Rryy
rY	RrYY	RrYy	rrYY	rrYy
ry	RrYy	Rryy	rrYy	rryy

Here, we can see there are 4 plants with the genotype RrYy.
So, the correct answer is ‘4’.

Additional information:
- A dihybrid cross is a mating experiment between two organisms that are identically hybrid for two traits. This experiment was originally discovered and explained by Mendel.
- It is based on the results of the monohybrid cross. It was hypothesized that the result of two characters segregating simultaneously would be the product of their independent occurrence, leading to a dihybrid cross.
- Mendel’s law of independent assortment states that pairs of traits in the parental generation sort independently from one another, when passing from one generation to the next.

Note:
- The three Mendelian laws that are essential for understanding biological inheritance are as follows.
1. Law of dominance and uniformity - Some alleles in an organism are dominant while some are recessive. An organism with at least one dominant allele will show the phenotype of the dominant allele.
2. Law of segregation - During gametogenesis, the alleles for each gene segregate from each other such that each gamete carries only one allele for each gene.
3. Law of independent assortment - pairs of traits in the parental generation sort independently from one another, when passing from one generation to the next.