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. What is the probability that neither of them is a carrier?

Key Concepts

Recessive Inheritance

Recessive inheritance occurs when a trait is expressed only when an individual has two copies of the recessive allele. In the case of Smith–Lemli–Opitz syndrome, which is inherited as a recessive trait, both parents must carry at least one 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

A carrier is an individual who has one copy of a recessive allele but does not express the trait. To determine the probability that neither parent is a carrier, we can use the information about their affected child. If the child has Smith–Lemli–Opitz syndrome, it indicates that both parents must be carriers, making the probability of neither being a carrier effectively zero.

Punnett Square

A Punnett square is a diagram used to predict the genetic makeup of offspring from a cross between two parents. It helps visualize the possible combinations of alleles that can result from the mating of two individuals. In this case, constructing a Punnett square for the recessive trait can clarify the probabilities of having affected or unaffected offspring, aiding in understanding the genetic risks involved.