Question

Seed color is controlled by 3 independently assorting bi-allelic genes (P, Q, R), such that homozygote pp exhibits recessive epistasis over the seed color pathway, converting a white pigment into yellow, which then becomes orange in the presence of a Q allele, or red in the presence of R. Individuals with both Q and R alleles show pink seeds. In a cross of PpQqRr individuals with ppqqrr individuals, (a) what is the ratio of white-seeds to red seeds? , and (b) If there are 1000 total progeny, how many would be expected to be white, yellow, orange, red?

Answer 1

Genotype of white seeds = P_Q_R_ or P_Q_rr or P_qqR_ or P_qqrr

Genotype of yellow seeds = ppqqrr

Genotype of orange seeds = ppQ_rr

Genotype of red seeds = ppqqR_

Genotype of pink seeds = ppQ_R_

Parent cross: PpQqRr * ppqqrr

Gametes of the heterozygous parent: PQR, PQr, PqR, Pqr, pqr, pqR, pQr, pQR

Gametes of triple homozygous recessive parent: pqr

Offspring :

PpQqRr (white), PpQqrr (white), PpqqRr (white), Ppqqrr (white),

ppqqrr(yellow), ppqqRr (red), ppQqrr (orange), ppQqRr (pink)

Offspring Phenotypic ratio: 4(white) : 1(yellow) :1(red) :1(orange) :1(pink)

a)

Probability of offspring having white seeds = 4/8

Probability of offspring having red seeds = 1/8

Ratio of white seeds to red seeds = 4(white):1(red)

Therefore, the ratio of white seeds to red seeds is 4:1

b)

Total progeny = 1000

Expected number of offspring with white seeds = Probability% * total offspring = (4/8) * 1000 = 500

Expected number of offspring with yellow seeds = (1/8) * 1000 = 125

Expected number of offspring with orang seeds = (1/8) * 1000 = 125

Expected number offspring with red seeds = (1/8) * 1000 = 125

Expected number of offspring with pink seeds = (1/8) * 1000 = 125