Because all of these clinical anomalies involve collagen in various tissues and because specific collagen types have a predictable distribution, many organs and systems are involved. A trait that appears in only one sex is called sex-limited. This is different from X-linked inheritance, which refers to traits carried on the X chromosome. Sex hormones and other physiologic differences between males and females may alter the expressivity of a gene. For example, premature baldness is an autosomal dominant trait, but presumably as a result of female sex hormones, the condition is rarely expressed in the female, and then usually only after menopause.
Mutations are changes in genetic information that arise spontaneously.
Almost all affected persons can transmit the newly mutated gene to their offspring. In achondroplasia, mutations occur at a very specific place in the gene. In many other conditions, each new mutation seems to occur at a different place in the gene. In general, unaffected parents are not at increased risk of having additional affected offspring, but a few normal-appearing parents have had two or even three offspring with typical achondroplastic dwarfism. The cell possessing the new mutation can then contribute many cells to the developing gonad.