Michigan Test for Teacher Certification (MTTC) Secondary Integrated Science Practice Test

A monohybrid cross involves the mating of two organisms that are both heterozygous for a single trait. For example, consider a cross between two pea plants where both parents have the genotype Tt (where T represents the dominant allele and t represents the recessive allele). The possible offspring genotypes from this cross would be TT, Tt, Tt, and tt, which results in a phenotypic ratio of 3:1 (three tall plants to one short plant) for the dominant and recessive traits. This is based on Mendelian inheritance, where the dominant allele expresses itself in the phenotype over the recessive allele.

In contrast, a dihybrid cross examines two traits simultaneously and typically leads to a 9:3:3:1 phenotypic ratio. A homozygous cross involves two parents that are either both dominant or both recessive for a trait, which can lead to offspring showing only one phenotype. A recessive cross involves at least one homozygous recessive parent and, depending on the other parent’s genotype, can lead to variances in ratios that are less predictable than the standard monohybrid ratio.