A recessive lethal allele causes dwarfism in Aa and fatality in AA. What is the phenotypic
ratio of an Aa x Aa cross?
A. 1:2:1
B. 2:2
C. 2:1
D. 1
In this cross, the genotypes will be AA:Aa:aa at a 1:2:1 ratio. The recessive lethal allele can be
thought of as incompletely dominant, as being homozygous for the allele is fatal, being
heterozygous causes dwarfism, and being homozygous for the wildtype allele is considered
normal. Since the homozygous recessive genotype is lethal those offspring will not be viable
and thus will not be produced, the viable offspring produced will only be Aa or aa, which gives a
phenotypic ratio of 2:1.