Alkaptonuria is an infrequent autosomal recessive condition. It is first noticed in newborns when the urine in their diapers turns black upon exposure to air. The condition is caused by the defective transport of the amino acid phenylalanine through the intestinal walls during digestion. About 4 people per 1000 are carriers of alkaptonuria.
Sara and James had never heard of alkaptonuria and were shocked to discover that their first child had the condition. Sara's sister Mary and her husband, Frank, are planning to have a family and are concerned about the possibility of alkaptonuria in one of their children.
The four adults (Sara, James, Mary, and Frank) seek information from a neighbor who is a retired physician. After discussing their family histories, the neighbor says, 'I never took genetics, but I know from my many years in practice that Sara and James are both carriers of this recessive condition. Since their first child had the condition, there is a very low chance that the next child will also have it, because the odds of having two children with a recessive condition are very low. Mary and Frank have no chance of having a child with alkaptonuria because Frank has no family history of the condition.' The two couples each have babies and both babies have alkaptonuria.

What is the chance that the third child of Sara and James will be free of the condition?

Key Concepts

Autosomal Recessive Inheritance

Autosomal recessive inheritance occurs when a trait or condition is expressed only when an individual has two copies of the recessive allele, one inherited from each parent. In the case of alkaptonuria, both parents must be carriers of the defective gene for their child to express the condition. If both parents are carriers, there is a 25% chance with each pregnancy that their child will inherit the condition.

Carrier Status

A carrier is an individual who possesses one copy of a recessive allele for a genetic condition but does not exhibit symptoms of the condition themselves. In the context of alkaptonuria, both Sara and James are carriers, meaning they each have one normal allele and one defective allele. This carrier status is crucial for understanding the genetic risks for their future children.

Punnett Square

A Punnett square is a diagram used to predict the genetic outcomes of a cross between two individuals. It helps visualize the possible combinations of alleles from the parents. For Sara and James, a Punnett square would show that there is a 25% chance their next child will inherit two normal alleles (free of the condition), a 50% chance of being a carrier, and a 25% chance of having alkaptonuria.