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 was incorrect about the information given to Sara and James? What is incorrect about the information given to Mary and Frank?

Key Concepts

Autosomal Recessive Inheritance

Autosomal recessive inheritance occurs when a trait or disorder is expressed only when an individual has two copies of the recessive allele, one inherited from each parent. Carriers, who possess one recessive allele and one dominant allele, do not exhibit symptoms but can pass the recessive allele to their offspring. In the case of alkaptonuria, both parents must be carriers for a child to be affected, which means there is a 25% chance with each pregnancy.

Carrier Probability

The probability of being a carrier for a recessive condition like alkaptonuria can be influenced by family history and population genetics. In this scenario, since Sara and James both have a child with alkaptonuria, it confirms they are both carriers. The neighbor's assertion that the chance of having another affected child is low is misleading; the probability remains 25% for each subsequent child, regardless of previous outcomes.

Family History and Genetic Risk

Family history plays a crucial role in assessing genetic risks for conditions like alkaptonuria. The neighbor incorrectly stated that Mary and Frank have no chance of having a child with the condition based solely on Frank's lack of family history. If Mary is a carrier, there is still a possibility for them to have an affected child, as the inheritance of recessive traits can occur even if one parent does not have a known family history of the condition.