Question

Consider three unlinked loci, each with a dominant and a recessive allele. Start by crossing two pure- breeding lines, one with all recessive alleles at these three loci, the other with all dominant alleles. Self fertilize the F1 to create an F2 population. Among the F2, what fraction do you expect to show the dominant phenotype for all three characters? Show your work. Among the F2 that show the dominant phenotype for all three characters, what fraction are pure breeding? Show your work.

Answer 1

ANSWER :-

Lets consider the three gene loci to be represented as A, B and C and all the three are unlinked. The alleles will be A/a, B/b and C/c for the loci.

First cross for the pure bred recessive parents

	a	a
A	Aa	Aa
A	Aa	Aa

	b	b
B	Bb	Bb
B	Bb	Bb

	c	c
C	Cc	Cc
C	Cc	Cc

Second cross is for the pure bred dominant parents

	A	a
A	AA	Aa
a	Aa	aa

	B	b
B	BB	Bb
b	Bb	bb

	C	C
C	CC	Cc
c	Cc	cc

Around 3/4 th of the individuals from the self-cross tend to carry the dominant phenotype. This indicates that 75% of the individuals will show the dominant phenotype. Among the F2, only 1/4 th of the individuals will carry the homozygous dominant phenotype and 1/4 th of the individuals will show the homozygous recessive phenotype thus accounting to 1/2 of the individuals will be pure breeding and the remaining half will be heterozygotes.

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