Question

What will be the probable phenotype ratio if a white summer squash (Wwyy) is crossed to a yellow summer squash (wwYy)?
(A) 2 white: 2 yellow
(B) 2 white: 2 green
(C) 1 white: 2 yellow: 1 green
(D) 2 white: 1 yellow: 1 green

Answer 1

Hint: A phenotype is an observable characteristic of a person, such as height, color of the eye, and type of blood. The genetic contribution is known as the genotype to the phenotype. The genotype influences certain characteristics to a large degree, whereas others are mainly determined by environmental factors.

Complete answer:
Epistasis is a form of non-Mendelian inheritance in which one gene can interfere with the expression of another gene. This is often found to be associated with gene pathways where the expression of one gene depends directly on the presence or absence of another gene product in the pathway.

Epistasis may also occur when the dominant allele masks the expression of a separate gene. The fruit color of the summer squash is expressed in this way. Homozygous recessive W gene expression (ww) combined with homozygous dominant or heterozygous Y gene expression (YY or Yy) produces yellow fruit, and green fruit is produced by the wwyy genotype.

However, if a dominant copy of the W gene is present in a homozygous or heterozygous form, the summer squash will produce white fruit irrespective of the Y alleles. For both genes (WwYy x WwYy), a cross between white heterozygous cells will produce offspring with a phenotypic ratio of 12 white: 3 yellow: 1 green.

The cross between the two will result in genotypes: WwYy, Wwyy, Wwyy, and Wwyy. The phenotypes will be WwYy and Wwyy will be white because the white gene is dominant and the W gene is epistatic. WwyY is going to be yellow and wwyy is going to be green.

	Wy	Wy	wy	wy
wY	WwYy	WwYy	wwYy	wwYy
wy	Wwyy	Wwyy	wwyy	wwyy
wY	WwYy	WwYy	wwYy	wwYy
wy	Wwyy	Wwyy	wwyy	wwyy

Hence, the correct answer is option (D)
Note: Finally, epistases may be reciprocal in such a way that either gene, present in a dominant (or recessive) form, expresses the same phenotype. Genes can interact at the level of gene products, whether or not they are independently sorted, in such a way that the expression of an allele for one gene masks or modifies the expression of an allele for another gene. It's referred to as epistasis.