How Does Inheritance Work? Crosses and Epistasis

Inheritance works through the alleles an organism receives from its parents. A monohybrid cross follows one gene, a dihybrid cross follows two genes, and epistasis occurs when one gene affects how another gene appears in the phenotype (the observable characteristic).

Monohybrid crosses

An allele is a version of a gene. Suppose $A$ is dominant over $a$, and two parents with genotype $Aa$ are crossed. Each parent can pass on either $A$ or $a$ in a gamete (a sex cell). Their offspring have a genotype ratio of $1AA:2Aa:1aa$. Because both $AA$ and $Aa$ show the dominant characteristic, the phenotype ratio is $3:1$. The genotype ratio counts allele combinations; the phenotype ratio counts the characteristics those combinations produce.

Dihybrid crosses

A dihybrid cross tracks alleles of two genes. For $AaBb\times AaBb$, each parent can produce $AB$, $Ab$, $aB$ and $ab$ gametes if the genes assort independently. This means inheritance of an allele of one gene does not determine which allele of the other gene enters the gamete. If each gene shows complete dominance and neither affects the other, the expected phenotype ratio is $9:3:3:1$. This ratio is not automatic: linked genes or interactions between genes can change it.

What changes with epistasis?

In epistasis, an allele at one gene masks or alters the effect of a different gene. Imagine that $A$ allows pigment to be made, while $aa$ prevents pigment production. Where pigment is present, $B$ gives black colour and $bb$ gives brown. In an $AaBb\times AaBb$ cross, offspring with at least one $A$ and one $B$ are black; those with at least one $A$ and $bb$ are brown. All $aa$ offspring are white, regardless of their $B$ alleles. The expected ratio is therefore $9:3:4$: the two groups with $aa$ that would otherwise have different colours combine into one white group.

Exam mistake to avoid: a dominant allele is not necessarily the most common allele. Label parental genotypes, gametes and offspring phenotypes, and check whether the genes interact before choosing a ratio.