500
For a recessive phenotype of an autosomal gene which is lethal in utero, what proportion of viable offspring will be carriers of the gene if both parents are carriers?
A. 1/3
B. 3/4
C. 1/2
D. 2/3
D. 2/3
The key to this question is that the recessive phenotype (homozygous recessive) is lethal, meaning such offspring will not exist. The only possible offspring are carriers and non-carriers. Since both parents are heterozygotes (carriers), the offspring will proportionally be 1:2 AA:Aa, so 2/3 will be carriers.