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.

Verified step by step guidance

1. <span>Step 1: Understand the inheritance pattern. Galactosemia is an autosomal recessive disorder, meaning that an individual must inherit two copies of the recessive allele (one from each parent) to express the disorder.</span>
2. <span>Step 2: Determine the genotypes of Amanda and Brice. Since both Amanda and Brice are healthy but have siblings with galactosemia, they are likely carriers of the recessive allele. This means their genotypes are heterozygous (Gg), where 'G' is the normal allele and 'g' is the allele for galactosemia.</span>
3. <span>Step 3: Use a Punnett square to determine the probability of their children having galactosemia. Cross the genotypes of Amanda (Gg) and Brice (Gg) to find the possible genotypes of their offspring.</span>
4. <span>Step 4: Analyze the Punnett square results. The possible genotypes for their children are GG, Gg, Gg, and gg. Only the 'gg' genotype will result in galactosemia.</span>
5. <span>Step 5: Calculate the probability. Since each child is an independent event, the probability of the second child having galactosemia remains the same as for the first child, which is the probability of the 'gg' genotype occurring.</span>

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.