A strain of Drosophila known as dunce produces one-half the amount of cAMP phosphodiesterase found in wild-type flies, which causes a learning defect. Another strain of Drosophila known as rutabaga produces a defective adenylyl cyclase protein (one of the AC types expressed in most cells) and also causes a learning defect. A dunce - rutabaga double mutant fly has relatively normal learning. Explain these curious observations.

From the given observations we can say that there are 2 genes that are responsible for normal learning. It is also given that the dunce and rutabaga mutations are the mutations of 2 different genes.

Due to complementation which is a phenomenon of restoration of normal phenotype if mutants are crossed, it is found that the flies have normal learning behavior if both mutants are crossed.

Let's assume that the Drosophila having this learning effect and it is due to genes D and R where D is the normal gene and dd is the case of mutation that results in duncy phenotype. Similarly, Gene R is responsible for normal learning, and rr results in rutabaga phenotype.

Cross between ddRR (duncy) x DDrr (rutabaga)

Gametes - dR x Dr

F1 progeny - DdRr    

Thus, in heterozygous cases, a single copy of both wild-type alleles D and R complement each other.

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