Question

In the fruit fly Drosophila, the gene 'T' for tan body color is dominant to its allele 't' for ebony body color. At another gene locus on the same chromosome the gene 'B' for normal bristles is dominant to its allele 'b' for short bristles. Two flies are crossed that both have the same genotype: the alleles 'T' and 'B' are on one chromosome and the alleles 't' and 'b' are on the homologous chromosome. If we assume that no crossing over occurs between these two gene loci, what is the expected phenotypic ratio among the offspring?

Answer 1

The expected phenotypic ratio among the offspring of two fruit flies with linked alleles TB and tb, assuming no crossing over, is 3:1, with three offspring having tan body and normal bristles and one having ebony body and short bristles.

Step-by-step explanation:

In the scenario where both fruit flies have a genotype with T (tan body color) and B (normal bristles) on one chromosome and t (ebony body color) and b (short bristles) on the homologous chromosome, and assuming no crossing over between these loci, the expected phenotypic ratio among the offspring would be simple. The offspring can only inherit TB or tb from each parent since the alleles are linked on the same chromosomes without recombination. When each parent provides either TB or tb alleles, the possible combinations are TB/TB, TB/tb, tb/TB, and tb/tb. Due to the dominance of T over t and B over b, this results in three possible phenotypes: tan body with normal bristles (TB/TB, TB/tb, tb/TB) and one phenotype of ebony body with short bristles (tb/tb). Therefore, the expected phenotypic ratio would be 3:1, with three offspring displaying tan body and normal bristles and one displaying ebony body and short bristles.