If a couple has a “one-in-four” risk of having a child with a disease, and their firstborn has the disease, the next three children will have a reduced risk.

The "one-in-four" probability does not necessarily imply that one of every four children a couple has will be diagnosed with a certain illness. Each child has a 25% (or one in four) chance of getting a disease, according to the "one-in-four" risk. An illustration of this is a Punnett square, which displays the likelihood of inheritance. Let's imagine, for instance, that a man and a woman are both carriers of sickle cell anemia, making them heterozygotes, meaning they each have one dominant and one recessive allele (Ss). To calculate the likelihood that the disease will be inherited, a Punnett square can be constructed.

This Punnett square reveals that the child of this pair has a 25% homozygous dominant (SS), a 50% heterozygous (Ss), and a 25% homozygous recessive (HR) likelihood (ss). Being a recessive condition, sickle cell anemia only affects ss offspring. It is crucial to understand that only because one child manifests the disease, subsequent offspring of the same parents do not necessarily have a lower probability of inheriting it. All of the couple's offspring have an equal chance of inheriting.