A 'wrongful birth' case was recently brought before a court in which a child with Smith–Lemli–Opitz syndrome was born to apparently healthy parents. This syndrome is characterized by a cluster of birth defects including cleft palate, and an array of problems with the reproductive and urinary organs. Originally considered by their physician as having a nongenetic basis, the parents decided to have another child, who was also born with Smith–Lemli–Opitz syndrome. In the role of a genetic counselor, instruct the court about what occurred, including the probability of the parents having two affected offspring, knowing that the disorder is inherited as a recessive trait. Calculate the probability that both the male and female are carriers for TSD.

Verified step by step guidance

1. Step 1: Understand the inheritance pattern of Smith–Lemli–Opitz syndrome. It is stated that the disorder is inherited as a recessive trait. This means that an individual must inherit two copies of the defective allele (one from each parent) to express the disorder. Carriers have one normal allele and one defective allele but do not show symptoms.
2. Step 2: Determine the probability of both parents being carriers. Since the first child is affected, both parents must be carriers of the recessive allele. This is because an affected child (homozygous recessive) can only inherit one defective allele from each parent.
3. Step 3: Use a Punnett square to calculate the probability of the parents having an affected child. If both parents are carriers (heterozygous), the Punnett square will show the following genotypes: 25% chance of homozygous dominant (unaffected), 50% chance of heterozygous (carrier), and 25% chance of homozygous recessive (affected).
4. Step 4: Calculate the probability of having two affected offspring. Since the probability of having one affected child is 25% (or 1/4), the probability of having two affected children in a row is the product of the probabilities for each child: (1/4) × (1/4).
5. Step 5: Explain the significance of this calculation to the court. The fact that both children are affected confirms that both parents are carriers. The calculated probability demonstrates the likelihood of this outcome under the assumption of Mendelian inheritance for a recessive trait.

Key Concepts

Recessive Inheritance

Recessive inheritance occurs when a trait is expressed only when an individual has two copies of the recessive allele, one from each parent. In the case of Smith–Lemli–Opitz syndrome, both parents must be carriers of the recessive allele for their child to be affected. If both parents are carriers, there is a 25% chance with each pregnancy that the child will inherit the disorder.

Carrier Probability

Carrier probability refers to the likelihood that an individual possesses one copy of a recessive allele without showing symptoms of the associated disorder. In this scenario, if both parents have one normal allele and one recessive allele, they are each 50% likely to be carriers. This probability is crucial for calculating the chances of having affected offspring.

Punnett Square

A Punnett square is a diagram used to predict the genetic makeup of offspring from two parents. It helps visualize the combinations of alleles that can result from a cross. For Smith–Lemli–Opitz syndrome, a Punnett square can illustrate the probabilities of the offspring being affected, carriers, or unaffected based on the parents' genotypes.