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.
The F₁ progeny of this cross are allowed to self-fertilize. What is the expected phenotypic distribution among the F₂ progeny?

Verified step by step guidance

1. Step 1: Identify the genotypes of the parent plants. The pure-breeding axial, constricted plant has the genotype TTcc (homozygous dominant for axial flowers and homozygous recessive for constricted pods). The pure-breeding terminal, inflated plant has the genotype ttCC (homozygous recessive for terminal flowers and homozygous dominant for inflated pods).
2. Step 2: Determine the genotype of the F₁ progeny. Since the parents are homozygous for each trait, the F₁ progeny will inherit one allele from each parent for each gene. This results in a heterozygous genotype TtCc for all F₁ individuals.
3. Step 3: Understand the self-fertilization process. When the F₁ plants self-fertilize, they produce gametes that segregate independently according to Mendel's laws. Each gene (T/t and C/c) will segregate independently, resulting in a dihybrid cross.
4. Step 4: Set up a Punnett square for the dihybrid cross. The gametes produced by the F₁ plants are: TC, Tc, tC, and tc. Use these gametes to create a 4x4 Punnett square to determine the genotypes of the F₂ progeny.
5. Step 5: Analyze the phenotypic ratios. Each genotype corresponds to a specific phenotype: T_C_ (axial, inflated), T_cc (axial, constricted), ttC_ (terminal, inflated), and ttcc (terminal, constricted). Count the occurrences of each phenotype in the Punnett square to determine the expected phenotypic distribution among the F₂ progeny.

Key Concepts

Mendelian Genetics

Mendelian genetics is the study of how traits are inherited through generations based on the principles established by Gregor Mendel. It involves understanding dominant and recessive alleles, where dominant traits mask the expression of recessive ones. This framework is essential for predicting the outcomes of genetic crosses, such as the F₁ and F₂ generations in pea plants.

Punnett Square

A Punnett square is a diagram used to predict the genotypic and phenotypic ratios of offspring from a genetic cross. By organizing the alleles of the parents, it allows for a visual representation of all possible combinations in the offspring. This tool is particularly useful in determining the expected phenotypic distribution among the F₂ progeny in the given cross.

Phenotypic Ratios

Phenotypic ratios refer to the relative frequencies of different phenotypes in the offspring resulting from a genetic cross. In the context of the question, understanding how to calculate these ratios based on the dominant and recessive traits of the alleles involved is crucial for predicting the appearance of the F₂ progeny. The expected ratios can be derived from the genotypes of the F₁ generation and their self-fertilization.