Dominant mutation (Sb) in the Sb+ gene results in flies with stubble bristles. A

Question

Dominant mutation (Sb) in the Sb+ gene results in flies with stubble bristles.

A dominant mutation (Ser) in the Ser+ gene results in flies with serrated wings.

Stubble-bristled flies with normal wings are mated to flies with normal bristles, but serrated wings.

Both Sb+ and Ser+ loci are on the third chromosome.

You perform a SELF-CROSS of flies that carry BOTH the dominant mutations and find that all their progeny look like their parents (have stubble-bristles AND serrated wings).

What is the most plausible explanation of this observation? If linkage is involved how are they linked (chromosome cis/trans)?

Explanation / Answer

Based on the given data,

The Sb+ dominant over Sb, thus the genotypes of stubble bristles are: Sb+ Sb+ and Sb+ Sb. The genotype of normal bristles is Sb Sb. The Ser+ dominant over Ser, thus the genotypes of serrated bristles are: Ser+ Ser+ and Ser+ Ser. The genotype of normal bristles is Ser Ser.

The first mating is:

Sb+ Sb+ // Ser Ser× Ser+// Ser+ Sb Sb

The F1 results

Sb+ Sb // Ser+ Ser+ (stubble-bristles and serrated wings)

Both Sb+ and Ser+ are located on the same chromosome, but in test cross only dominant phenotypes appeared due to linkage. The most plausible explanation is that these two genes are separated by slight distance; hence they are genetically linked or coupled (Cis).

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