All children of an affected person and a genotypically normal person will be phenotypically normal heterozygotes. All children of two affected persons will be affected. Males and females are equally likely to be affected. Heterozygotes are phenotypically normal but are carriers of the trait. If the mutation is known, molecular genetic techniques can identify heterozygous phenotypically normal persons.
Related persons are more likely to share the same mutant allele.
A detailed family history may disclose unknown or forgotten consanguinity. A typical pedigree is shown in Fig. Affected homozygous females transmit the trait to all of their children. Twice as many affected females as males will have the disorder unless it is lethal in males. For example, in incontinentia pigmenti, the X-linked gene appears to be lethal in males and causes a peculiar swirling pattern of melanin pigmentation and other patchy anomalies of teeth, eyes, and CNS in females.