Which type of inheritance requires only one affected X chromosome for the disease to manifest?

The manifestation of a disease that requires only one affected X chromosome is characteristic of X-linked dominant inheritance. In this type of inheritance, a single copy of the mutated gene on one of the X chromosomes is sufficient to cause the phenotype in both males and females. This is in contrast to X-linked recessive conditions, where two copies of the mutated gene (one on each X chromosome) are required for females to show symptoms, while males only need one copy to exhibit the disease due to their having only one X chromosome.

In X-linked dominant inheritance, affected males pass the affected X chromosome to all of their daughters, resulting in those daughters being affected, while sons will not be affected since they inherit their father's Y chromosome. Affected females, on the other hand, have a 50% chance of passing the affected X chromosome to each child, which means both daughters and sons can manifest the condition.

This distinction is crucial for understanding patterns of inheritance and predicting the risk of passing on genetic disorders. In summary, X-linked dominant inheritance is the only pattern where the presence of a single affected X chromosome leads to the disease’s manifestation.