Achondroplasia is a form of dwarfism inherited as a completely dominant trait. Two achondroplastic dwarfs have a dwarf child; later, they have a second child who is normal. What are the genotypes of the two parents in this mating? What is the probability that their next child will be normal? A dwarf?

There's 25% probability that their next child will be normal, and 75% of it being a dwarf.

Explanation:

Achondroplasia (A) is a dominant trait.

Two achondroplastic dwarfs (A_ x A_) have a dwarf child (A_) and a normal child (aa). The second child must be aa because if it had a dominant allele it would also be dwarf.

The only way for two achondroplastic parents to have a normal child is that they are both heterozygous (Aa) for this gene.

The cross Aa x Aa would produce the following offspring probabilities:

So there's 1/4=25% probability that their next child will be normal, and 3/4=75% of it being a dwarf.

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