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 the plants with terminal flowers produced by the cross in part (c) are saved and allowed to self-fertilize, what is the expected phenotypic distribution among the progeny?

Verified step by step guidance

1. Step 1: Identify the genotypes of the parent plants in the initial cross. A pure-breeding axial, constricted plant has the genotype TTcc, while a pure-breeding terminal, inflated plant has the genotype ttCC.
2. Step 2: Determine the genotype of the F1 generation. Since the parents are homozygous for each trait, the F1 offspring will inherit one allele from each parent for each gene, resulting in the genotype TtCc. This means all F1 plants will have the dominant phenotypes: axial flowers and inflated pods.
3. Step 3: Focus on the plants with terminal flowers (tt) from the F2 generation. To produce terminal flowers, the plants must inherit the recessive allele t from both parents. This means we are specifically analyzing the subset of plants with the genotype tt for the flower position trait.
4. Step 4: Perform a Punnett square for the pod form (C and c alleles) among the tt plants. Since the F1 plants are heterozygous (Cc), self-fertilization will result in a 1:2:1 genotypic ratio for the pod form: 1 CC (inflated), 2 Cc (inflated), and 1 cc (constricted).
5. Step 5: Combine the results to determine the phenotypic distribution among the progeny. Since all plants in this subset are tt (terminal flowers), the phenotypes will depend solely on the pod form: 3/4 of the plants (CC and Cc) will have terminal flowers with inflated pods, and 1/4 (cc) will have terminal flowers with constricted pods.

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 traits. This framework is essential for predicting the outcomes of genetic crosses, such as the phenotypic ratios in offspring.

Punnett Square

A Punnett square is a diagram used to predict the genotypes and phenotypes of offspring from a genetic cross. By organizing the alleles of the parent plants, it allows for a visual representation of possible combinations in the progeny. This tool is crucial for determining the expected phenotypic distribution, especially when dealing with multiple traits.

Phenotypic Ratios

Phenotypic ratios refer to the relative frequencies of different phenotypes in the offspring resulting from a genetic cross. In this case, the expected ratios can be calculated based on the dominant and recessive traits of the parent plants. Understanding these ratios helps in predicting the appearance of traits in the progeny, which is vital for answering the question posed.