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.

If the first child has galactosemia, what is the probability that the second child will have galactosemia? Explain the reasoning for your answer.

Key Concepts

Autosomal Recessive Inheritance

Autosomal recessive inheritance occurs when a disorder is expressed only when an individual inherits two copies of a mutated gene, one from each parent. In the case of galactosemia, both parents must be carriers of the recessive allele for their child to be affected. If both parents are healthy but carry one copy of the mutated gene, there is a 25% chance with each pregnancy that their child will inherit both recessive alleles and express the disorder.

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 autosomal recessive traits like galactosemia, a Punnett square can show the probabilities of offspring being affected, carriers, or unaffected based on the genotypes of the parents.

Probability of Independent Events

In genetics, the probability of independent events refers to the likelihood of one event occurring not affecting the likelihood of another. In the context of Amanda and Brice's potential children, the probability of having a child with galactosemia remains constant at 25% for each pregnancy, regardless of the outcomes of previous pregnancies. This principle is crucial for understanding the genetic risks associated with each child.