The parents of offspring exhibiting a 3:1 phenotypic ratio are?

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C

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A. two homozygous individuals

B. heterozygous for two traits

C. heterozygous for one trait

D. one homozygous and the other heterozygous
 
C. heterozygous for one trait.
This is a standard monohybrid F2 ratio.
Eg. Tt x Tt gives TT,Tt,Tt,tt.
 
The answer is C: Heterozygous for one trait.

(A) would give all one phenotype.
(B) is a dihybrid cross, which gives a phenotypic ratio of 9:3:3:1.
(D) would give a phenotypic ratio of 1:1.
 
C, heterozygous for one trait, since in the first filial 1/4 of offspring are genotypically homozygous dominant, 1/4 homozygous recessive, and the other 1/2 heterzygous, so 3/4 contain at least one copy of the dominant allele and hence are phenotypically dominant and the other 1/4 display the recessive trait.
 
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