A couple has a child with an autosomal recessive form of glycogen storage disease, a deficiency of a lysosomal enzyme. An enlarged liver and heart and weak muscles cause the baby to die from heart and lung failure at 20 months. Now the woman is pregnant again. Her doctor offers her and her husband genetic counseling, but they decline, claiming familiarity with the laws of inheritance- since they have already had an affected child, they reason, the next three will be healthy. Why is their reasoning incorrect?
Each pregnancy is an independent event. One does not influence the other. Assuming each parent is heterozygous for the recessive gene (otherwise they would have dies at 20 months), the chances for the disorder is 25% for every pregnancy.
If the events are independent, the probability of both/all events occurring is determined by multiplying the probabilities of the individual events.
For the next three to be healthy = .75^3 = .42.
The couple's reasoning is incorrect because the inheritance of autosomal recessive disorders follows certain genetic principles that are independent of their previous child being affected. In autosomal recessive inheritance, for a child to be affected, both parents must be carriers of the disease-causing gene.
Let's break down the inheritance pattern and explain why the couple's assumption is flawed:
1. Autosomal recessive inheritance: Autosomal means the gene responsible is located on a non-sex (autosome) chromosome and can affect both males and females equally. Recessive means that two copies of the mutated gene are required for the disease to be expressed.
2. Carrier status: Carriers have one copy of the disease-causing gene but do not show symptoms themselves since they have a healthy copy as well. In this case, the parents each carry one copy of the gene responsible for glycogen storage disease.
3. Risk of having an affected child: When two carriers have a child, there is a 25% (1 in 4) chance that the child will inherit two copies of the disease-causing gene, resulting in the disease. There is a 50% chance the child will be a carrier like the parents, and a 25% chance the child will inherit two healthy copies of the gene.
Given that the couple already has one affected child, it means that both parents are carriers of the glycogen storage disease gene. With each pregnancy, there is a 25% chance that the child will inherit the disease-causing gene from both parents and be affected by the disease, regardless of their previous child being affected or not. The probability of the next three children being healthy is not guaranteed.
Therefore, declining genetic counseling based solely on the assumption that subsequent children will be healthy is incorrect. Genetic counseling could provide the couple with a better understanding of the risks and options available, such as prenatal testing or reproductive technologies like in-vitro fertilization with preimplantation genetic diagnosis. These options can help them make informed decisions about their family planning.