The following is the biochemical pathway for purple pigment production in flowers of sweet pea:
Colorless precursor 1 —- Allele A—-> Colorless precursor 2 —--- Allele B—---> Purple pigment
Recessive mutation of either gene A or B leads to the formation of white flowers. A cross is made between two parents with the genotype: AaBb × aabb. Considering that the two genes are not linked, the phenotypes of the expected progenies are:
9 purple : 7 white
3 white : 1 purple
1 purple : 1 white
9 purple : 6 light purple : 1 white
From the Punnett square, we can see that the resulting genotypes are as follows:
AABB: Purple pigment (phenotypically purple flowers)
AABb: Purple pigment (phenotypically purple flowers)
AaBB: Purple pigment (phenotypically purple flowers)
AaBb: Purple pigment (phenotypically purple flowers)
AAbb: No purple pigment (phenotypically white flowers)
Aabb: No purple pigment (phenotypically white flowers)
aaBB: No purple pigment (phenotypically white flowers)
aaBb: No purple pigment (phenotypically white flowers)
Therefore, the expected phenotypes of the progeny from the cross AaBb × aabb would be a combination of purple and white flowers, with a ratio of 4:4, or 1:1, for purple to white flowers. Hence, the correct answer is option 3.
3(white):1(purple)
In case of 'duplicate recessive epistasis homozygous recessive gene masks the other gene i.e., aa is epistatic to A or a || bb is epistatic to B or b