In pea plants, the appearance of flowers along the main stem is a dominant phenotype called 'axial' and is controlled by an allele T. The recessive phenotype, produced by an allele t, has flowers only at the end of the stem and is called 'terminal.' Pod form displays a dominant phenotype, 'inflated,' controlled by an allele C, and a recessive 'constricted' form, produced by the c allele. A cross is made between a pure-breeding axial, constricted plant and a plant that is pure-breeding terminal, inflated.

If an F₁ plant from the initial cross described above is crossed with a plant that is terminal, constricted, what is the expected distribution among the resulting progeny?

Key Concepts

Dominant and Recessive Alleles

In genetics, alleles are different forms of a gene that can exist at a specific locus on a chromosome. Dominant alleles, represented by uppercase letters (e.g., T for axial flowers), mask the expression of recessive alleles (e.g., t for terminal flowers) when both are present. This means that the phenotype of an organism will reflect the dominant allele if it is present, while the recessive phenotype will only be expressed when two recessive alleles are inherited.

Punnett Square

A Punnett square is a diagram used to predict the genotypic and phenotypic outcomes of a genetic cross. By organizing the alleles of the parent plants along the top and side of a grid, one can visualize all possible combinations of alleles in the offspring. This tool is essential for determining the expected ratios of different phenotypes in the progeny resulting from specific genetic crosses.

Phenotypic Ratios

Phenotypic ratios describe the relative frequencies of different phenotypes in the offspring of a genetic cross. In the context of the question, understanding how to calculate these ratios based on the genotypes of the parent plants is crucial. For example, a typical monohybrid cross can yield a 3:1 ratio of dominant to recessive phenotypes, while a dihybrid cross may yield a 9:3:3:1 ratio, depending on the traits being studied.