Question

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?

Answer 1

Achondroplastic dwarf parents with genotype Aa can have a normal child with a 25% probability and a child with dwarfism with a 75% probability in subsequent pregnancies.

Step-by-step explanation:

Achondroplasia is a form of dwarfism that is inherited in an autosomal dominant pattern. In this case, two achondroplastic dwarfs having a normal child suggests that both parents are heterozygous for achondroplasia, meaning their genotypes are Aa, where 'A' represents the achondroplastic allele and 'a' the normal allele.

The probability that their next child will be normal (aa) is 25%, because there is a 50% chance each parent will pass on the normal allele. The probability that their next child will be a dwarf is 75%, with a 50% chance of being heterozygous (Aa) and 25% chance of homozygous dominant (AA), which is typically lethal and results in the non-viability of the offspring.

Answer 2

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

Step-by-step 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:

• 1/4 AA - dwarf
• 2/4 Aa - dwarf
• 1/4 aa - normal

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