In which inheritance pattern are only homozygotes affected?

The inheritance pattern in which only homozygotes are affected is known as autosomal recessive inheritance. In this pattern, a trait or disorder manifests when an individual has two copies of the recessive allele, one inherited from each parent. Therefore, only individuals who are homozygous for the recessive allele will express the condition.

In autosomal recessive inheritance, if a person carries one dominant and one recessive allele (heterozygous), they typically do not show signs of the condition but can pass the recessive allele to their offspring. This is contrasted with the autosomal dominant inheritance, where just one copy of the dominant allele is sufficient for the trait to be expressed, affecting both homozygous and heterozygous individuals.

In the case of X-linked dominant and X-linked recessive patterns, the expression of the trait depends more on the sex of the individual and the number of X chromosomes. X-linked recessive traits can express in males (who have only one X chromosome) but typically require two copies for females (who have two X chromosomes) to express the condition. Thus, in the context of the question, only in autosomal recessive inheritance are homozygotes exclusively affected by the trait or disorder.