Colored aleurone in the kernels of corn is due to the dominant allele R. The recessive allele r, when homozygous, produces color
less 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).
The genes are linked, and the arrangement is coupled (one homolog has both dominant alleles; the other homolog has both recessive alleles).
RY
/
ry
Explanation:
R_= colored aleurone
rr= colorless aeurone
Y_= green plant
yy= yellow plant
They do a testcross between a plant of unknown genotype and phenotype and a plant that is homozygous recessive for both traits (yr/yr) the following F1 is obtained:
88 Colored, green (RY/ry)
92 colorless, yellow (ry/ry)
8 colorless, green (rY/ry)
12 colored, yellow (Ry/ry)
If the genes assorted independently, all of the possible offspring phenotypes would appear in the same frequency. However, they obtained 2 phenotypes much more abundant that the other two. Since recombination is a rare event during crossing over, when two genes are linked the recombinant gametes are produced less frequently than the parentals.
We can propose that the phenotypes Colored, green (RY) and colorless yellow (ry) are the parental gametes, and the unknown plant has the arrangement RY/ry, with both genes linked.
