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Multiple Choice
Suppose that in unicorns, two autosomal loci, A and B, assort independently. If a unicorn with genotype AaBb is crossed with aabb, what proportion of the offspring are expected to have the genotype aabb?
A
3/4
B
1/4
C
1/8
D
1/2
Verified step by step guidance
1
Identify the genotypes of the parents: one parent is AaBb (heterozygous at both loci) and the other is aabb (homozygous recessive at both loci).
Since the loci A and B assort independently, consider the inheritance of each locus separately using a Punnett square or probability rules.
For locus A: the Aa parent can pass on either A or a allele with equal probability (1/2 each), and the aabb parent can only pass on the a allele (probability 1). So, the probability of offspring being aa is the probability that the Aa parent passes a (1/2) times the probability that the other parent passes a (1), which equals 1/2.
For locus B: similarly, the Bb parent can pass B or b with equal probability (1/2 each), and the aabb parent can only pass b (probability 1). So, the probability of offspring being bb is the probability that the Bb parent passes b (1/2) times the probability that the other parent passes b (1), which equals 1/2.
Since the loci assort independently, multiply the probabilities for each locus to find the probability of offspring being aabb: $\frac{1}{2} \times \frac{1}{2} = \frac{1}{4}$.