In a mating between two individuals that are heterozygous for a recessive lethal allele that is expressed in utero, what genotypic ratio (homozygous dominant: heterozygous: homozygous recessive) would you expect to observe in the offspring?

1:2:0

Explanation:

If two recessive lethal alleles come together, they will be expressed and lead to the offspring's death. Let the dominant allele be S and the recessive allele be s. Here, the two individuals are heterozygous for it. So when they mate:

    S     s

S  SS  Ss

s   Ss   ss

1/4 have homozygous fominant genotype (SS), 2/4 have heterozygous genotype (Ss) and 1/4 have homozygous recessive (ss) genotype. Since 1/4 offspring have ss genotype, they will die. Hence, the usual 1:2:1 ratio has been modified here to only 1:2:0

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