The influence of heredity and environment in child development is very important topic. Disentangling the role of environmental and genetic influences in the causation of disease is a perplexing and precarious problem. In many diseases, the genetic influences are major and environmental influences are trivial, whereas in others the environment plays a major role and the genetic constitution is unimportant.
For example, the genetic constitution is solely responsible for the appearance of erythrocyte antigens, the environment being without detectable influence. Many of the inborn errors of metabolism are determined by a single gene that is necessary for their expression. However, the phenotypic expression of recessive disease such as phenylketonuria and galactosemia can be considerably modified by alteration of the diet.Paradoxically, the greatest difficulty in assessing relative roles of genetic and environmental influences is encountered with the common diseases of man.
In disorders such as hypertension and rheumatic fever several genes probably influence the disease, but environmental factors are also relevant. It is worth stressing that a familial concentration of cases will be expected even when the trait is multifactorial inherited. If the frequency of a multi factorial determined trait is n, it can be calculated that the frequency of the trait in first-degree relatives (parents and sibs) will be approximately Vn. Although in many infections the environmental agent is crucial, the importance of the genotype of the host should not be discounted.
You Must Know The Behaviour of Heredity And Environment In Our Personality
It seems likely that, in the past, infectious diseases were powerful selective agents, and the ability to survive epidemic disasters was probably dependent, at least in part, on the individual’s genotype.Comparative studies of identical and nonidentical twins, although traditionally regarded as being of great pertinence, have not enabled sharp distinctions between hereditary and environmental influences to be drawn. Even when identical twins are reared apart, they have shared a similar environment before birth, and dogma conclusions are treacherous. Nevertheless, with proper regard for their limitations, studies on twins are frequently informative.
More recently, the nature-nurture polemic has. been illuminated from fundamental studies on gene regulation. Now good evidence suggests that, at any given time, as much as 80 per cent of the genetic material in the cells of higher organism; is inactive. This inactivation can be reversed by certain environmental influences, and the previously inactive genes can be restored to activity. Particular steroid hormones possess the capacity to influence gene activity, and estrogens have been shown to directly influence the synthesis of m ENA and RNA. Indeed, the administration of estrogen to a rooster can activate genes in the rooster’s liver cells so that it synthesizes egg yolk proteins for which it clearly has no overwhelming need.
Additional examples indicate the possibility that the environment may influence the activity of the genetic material. Soon after birth the hepatic levels of the enzyme phenylalanine hydroxylase are equally low in patients with phenylketonuria and in normal people. Normally, the gene for the synthesis of this enzyme becomes fully activated during the early weeks of life, and a healthy child develops. In contrast, children with phenylketonuria appear to be incapable of activating for repressing) the structural gene for the synthesis of phenylalanine hydroxylase, with the result that the clinical syndrome of phenylketonuria develops. These observations raise the possibility that some genetically determined diseases may, in the future, be controlled by environmental agents that act directly on the DNA.
