Colored aleurone in the kernels of corn is due to the dominant allele R. The recessive allele r, when homozygous, produces colorless aleurone. The plant color (not the kernel color) is controlled by another gene with two alleles, Y and y. The dominant Y allele results in green color, whereas the homozygous presence of the recessive y allele causes the plant to appear yellow. In a testcross between a plant of unknown genotype and phenotype and a plant that is homozygous recessive for both traits, the following progeny were obtained:
colored, green: 88
colored, yellow: 12
colorless, green: 8
colorless, yellow: 92
Explain how these results were obtained by determining the exact genotype and phenotype of the unknown plant, including the precise arrangement of the alleles on the homologs.

Verified step by step guidance

1. Identify the phenotypes and genotypes of the testcross parent: since it is homozygous recessive for both traits, its genotype is rr yy, producing colorless aleurone and yellow plant color.
2. Assign symbols to the unknown plant's genotype: since it shows dominant phenotypes, it must have at least one R allele and one Y allele. Let the unknown genotype be Rr Yy, but we need to determine the arrangement (coupling or repulsion) of these alleles on homologous chromosomes.
3. Analyze the progeny phenotypes and their numbers: the four classes are colored green (88), colored yellow (12), colorless green (8), and colorless yellow (92). The large numbers correspond to parental types, and the small numbers correspond to recombinant types, indicating linkage between the R and Y genes.
4. Determine which phenotypes correspond to parental and recombinant types: the two most frequent classes (colored green and colorless yellow) represent parental combinations, so the unknown plant's alleles are arranged in coupling phase as R Y / r y or in repulsion phase as R y / r Y. The data suggest coupling because colored green and colorless yellow are most frequent.
5. Conclude the exact genotype and allele arrangement of the unknown plant: it is heterozygous for both genes with alleles in coupling phase, meaning one homolog carries R and Y together, and the other homolog carries r and y together, i.e., genotype R Y / r y.

Key Concepts

Testcross and Its Purpose

A testcross involves crossing an individual with an unknown genotype to a homozygous recessive individual to reveal the unknown genotype based on progeny phenotypes. It helps determine whether the unknown parent is homozygous or heterozygous for the traits by analyzing the ratio of offspring phenotypes.

Linked Genes and Recombination

Genes located close together on the same chromosome tend to be inherited together, a phenomenon called linkage. Recombination during meiosis can separate linked alleles, producing recombinant offspring. The frequency of recombinant phenotypes helps map gene order and determine allele arrangement on homologous chromosomes.

Dominance and Phenotypic Expression

Dominant alleles mask the expression of recessive alleles in heterozygotes, determining the phenotype. In this question, the dominant R allele produces colored aleurone, and dominant Y produces green plants, while recessive homozygotes (rr or yy) show colorless aleurone or yellow plants, respectively.