In a diploid species of plant, the genes for height and fruit shape are linked a
ID: 34203 • Letter: I
Question
In a diploid species of plant, the genes for height and fruit shape are linked and located 18 map units apart. Allele D produces tall plants and is dominant to the d allele which produces short plants. Allele R produces round fruit, which is dominant to allele r for oval fruit.
iii. A pure breeding tall plant producing oval fruit is crossed to a pure breeding short plant producing round fruit.
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What will be the genotype of the F1 offspring produced from this cross? In what linkage phase (coupling or repulsion) will the alleles be?
If one of the F1 offspring is testcrossed, what proportion of the resulting F2 offspring will be short and produce round fruit? If one of the F1 offspring is testcrossed, what proportion of the resulting F2 offspring will be tall and produce round fruit?
Explanation / Answer
a) The genotype of F1 offspring produced from this cross will be DdRr.
The alleles will be in repulsion phase because for repulsion phase crosses, gametes containing one dominant and one recessive allele will be most abundant.
b) Test cross is when a cross is done with the recessive parent. In this case a dihybrid test cross involving height and seed shape will be done. The recessive parent is dr.
The proportion of the resulting F2 to be short and round is 1/4.
If one of the F1 offspring is testcrossed, the proportion of the resulting F2 offspring to be tall and produce round fruit is 1/4.
The reason is that the F1 individual is heterozygous for both characters and is crossed with homozygous recessive. Hence, the phenotypic ratio will be 1:1:1:1, giving the proportion as 1/4 for each.