Question

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

Now determine the precise arrangement of the alleles on the homologs in the unknown plant. Choose the correct arrangement. Choose the correct arrangement.

The genes are unlinked (e.g., on different chromosomes).

R Y

r y

The genes are linked, and the arrangement is not coupled (each homolog has a dominant and a recessive allele).

R y

r Y

The genes are linked, and the arrangement is coupled (one homolog has both dominant alleles; the other homolog has both recessive alleles).

R Y

r y

Answer 1

The unknown parental plant in the testcross displays a pattern consistent with linked genes in a coupled arrangement, where dominant alleles (R and Y) are together on one chromosome and recessive alleles (r and y) on the other, explaining the high number of colored green and colorless yellow progeny and low number of colored yellow and colorless green progeny observed.

Step-by-step explanation:

The testcross between a plant of unknown genotype and a plant that is homozygous recessive (rryy) for both colored aleurone and plant color has produced a specific ratio of offspring showing the different traits. To find out the genotype of the unknown parent, we can deduce it from observing the testcross results. Considering we have both colored green and colorless yellow in relatively high numbers, and colored yellow and colorless green in low numbers, the most likely arrangement of alleles in the unknown parent is where the two genes are linked and the arrangement is coupled. This means that the dominant alleles are together on one chromosome and the recessive alleles are on the other. This configuration explains the high number of colored green and colorless yellow progeny observed (they represent the parental genotype devoid of crossover), and the lowest numbers of colored yellow and colorless green represent the recombinant genotypes (resulting from crossover). Thus, the correct arrangement is:

• R Y
• r y