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MCAT Question A Day - 4/29/13 - Answer!

Fruit flies of the genotype AaBb were crossed with individuals of the genotype aabb. The first 1000 offspring included 470 AaBb, 485 aabb, 24 Aabb, and 21 aaBb. What can be inferred from this cross?

A.  It is a dihybrid cross.
B.  It involves independent assortment.
C.  Both A and B
D.  Neither A nor B

The correct answer is (A). This is definitely a dihybrid cross, as reflected by the alleles for the A trait and B trait. However, if these traits assorted independently (which happens when traits are on independent chromosomes), one would expect 25% of each genotype produced. The results suggest that the two traits are linked. The heterozygous parent (AaBb) has one chromosome with AB and the other with ab. Thus, most of the offspring are AaBb and aabb. The genotypes Aabb and aaBb are “recombinants” resulting from crossing over in the heterozygous parent.