Question

PART 1:

You are studying pea plants where genes A and B determine petal color according to a pathway where B is needed in order to activate C, and an active C converts yellow to blue:

B -> C -> yellow to blue

BbCc plant is crossed to an bbcc plant. If genes B and C segregate independently, what is the predicted phenotypic ratio among the progeny?

UPPERCASE = dominant & LOWERCASE = recessive loss-of-function

Choose from the group of answer choices below

A. 1 yellow : 3 blue

B. 3 yellow : 1 blue

C. 9 yellow : 7 blue

D. 7 yellow : 9 blue

E. 1 yellow: 1 blue

PART 2:

320 progeny from part 1 are made. Out of the 320, 40 are blue and 280 are yellow. How should we represent the BbCc parent's genotype? Indicate phase.

A. BC / bc

B. Bc / bC

C. BC / bc or Bc / bC

D. Bb / Cc

E. Genes B and C are unlinked, so we cannot describe phase

Answer 1

1. The parents in the cross are BbCc and bbcc. The gametes formed by them are BC,Bc,bC&bc and bc respectively. Hence a cross between them can be interpreted in the following table:

	BC	Bc	bC	bc
bc	BbCc Blue	Bbcc Yellow	bbCc Yellow	bbcc Yellow

Hence the phenotypic ratio is : 3yellow : 1blue.

2. If out of 320 offsprings, 280 are yellow and 40 are blue then the B and C are on different locus and hence only on recombination, the blue color is formed. Hence the genotype can be written as Bc/bC.

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