Galactosemia is an autosomal recessive disorder caused by the inability to metabolize galactose, a component of the lactose found in mammalian milk. Galactosemia can be partially managed by eliminating dietary intake of lactose and galactose. Amanda is healthy, as are her parents, but her brother Alonzo has galactosemia. Brice has a similar family history. He and his parents are healthy, but his sister Brianna has galactosemia. Amanda and Brice are planning a family and seek genetic counseling. Based on the information provided, complete the following activities and answer the questions.

What is the probability that the first child of Amanda and Brice will have galactosemia? Show your work.

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. In the case of galactosemia, both parents must be carriers of the mutated gene for their child to be affected. If both parents are healthy but carry one copy of the recessive allele, there is a 25% chance their child will inherit the disorder.

Carrier Status

A carrier is an individual who has one copy of a recessive allele for a genetic disorder but does not exhibit symptoms of the disorder. In this scenario, Amanda and Brice are likely carriers since their siblings have galactosemia. Understanding carrier status is crucial for predicting the likelihood of their offspring inheriting the disorder, as carriers can pass the recessive allele to their 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 Amanda and Brice, constructing a Punnett square with the genotypes of both parents (assuming they are both carriers) will reveal the probabilities of their children being affected by galactosemia, unaffected, or carriers themselves.