GENETIC INFLUENCES

Most of the literature considered here relates to continuous quantitative phenotypic traits that are likely to be influenced by a number of genes acting through hormones and their receptors and through effects on metabolic processes. Most of the genetic control of each of these traits is probably due to a few genes while other genes have only small effects. These genetic influences may differ with sex and age and be subject to environmental interactions. The emphasis in this section is on genetic influences that are associated with normal variation in growth. The importance of genetic influences on growth is shown vividly, however, by some chromosomal variations that result in abnormal growth. In Turner syndrome, females lack one X chromosome and they are very short. In Klinefelter syndrome, males have an additional Y chromosome and are very tall. In each of these conditions, the variations from normal growth are most marked in the limbs. In Turner syndrome, mother–daughter correlations for stature are similar to those for normal children indicating that many genes influencing stature are located on autosomes (Brook et al., 1974).

Intra-familial correlations provide initial information about genetic influences on phenotypic traits, although they reflect both genetic and environmental influences. Phenotypic traits are specific observable characteristics of an individual or an organism. These correlations are used to estimate heritability (h2), which indicates the extent of the genetic control of a trait, usually on a scale from zero to 1.0, but a scale from zero to 100 may be used.