Question

Which genotypic ratio indicates a recessive lethal allele when two heterozygotes are mated?

- 1:1
- 3:1
- 1:2:1
- 2:1
- 4:1

Answer 1

The genotypic ratio indicating a recessive lethal allele when two heterozygotes are mated is 2:1 because the homozygous recessive individuals do not survive.

Step-by-step explanation:

When two heterozygotes mate, and there is a recessive lethal allele present that causes death before birth when homozygous, you would expect to observe a genotypic ratio of homozygous dominant to heterozygous to homozygous recessive in the offspring to be 2:1. This is because the homozygous recessive genotype is lethal, and these individuals will not survive, thus altering the expected Mendelian 3:1 ratio seen in typical heterozygous crosses. In other situations where the recessive lethal allele might also exhibit a dominant (but not lethal) phenotype in the heterozygote, such as the Curly allele in Drosophila, only wild-type homozygotes and heterozygotes are observed among the survivors.