Inheritance: genotype, phenotype and simple crosses

A genotype describes genetic makeup at the locus or loci being discussed. A phenotype is an expressed characteristic. Phenotype can depend on the genotype and the environment; the two terms are not interchangeable.

For this lesson, assume one gene with two alleles and complete dominance. Let T be dominant and t recessive. TT and Tt show the dominant phenotype, while tt shows the recessive phenotype. Tt is heterozygous; TT and tt are homozygous.

Worked example: cross Tt with Tt. Each parent can contribute T or t. Combining the possibilities gives TT, Tt, Tt and tt. The expected genotype proportions are one-quarter TT, one-half Tt and one-quarter tt. The expected phenotype proportions are three-quarters dominant and one-quarter recessive.

These are probability expectations, not a promise that every small group of offspring will match the proportions exactly. Each outcome can vary by chance. Also, complete dominance is an assumption of this example; not every trait follows this simple model.

Now cross Tt with tt. The first parent contributes T or t; the second contributes t. The outcomes are Tt and tt in equal expected proportions. Half are expected to show the dominant phenotype under the stated assumptions.

Practice:
1. Which genotype is heterozygous: TT, Tt or tt?
2. What fraction of a Tt x Tt cross is expected to be tt?
3. For TT x tt, what is the expected offspring genotype?
4. Does a dominant allele have to be the most common allele in a population?

Answers: 1 Tt; 2 one-quarter; 3 all Tt in this simple model; 4 no, dominance describes expression in a heterozygote rather than population frequency.

Draw the cross before choosing an option. Label the parental genotypes, possible gametes and combined outcomes. Explain whether the question asks for a genotype or a phenotype, since their ratios can differ.