Children tend to resemble their parents in stature, body proportions, body composition and rate of development. It may be assumed that barring the action of obvious environmental influences (such as chronic illness or long term malnutrition) these resemblances reflect the influence of genes that parents contribute to their biological offspring. The term “genetic potential” usually means that every human being has a genetically determined upper limit to adult stature, the ratio of leg length to sitting height and other anthropometric dimensions. An individual may achieve this genetic potential if the environment is free of insults that delay or retard growth. The child’s development may be shunted from one line to another in a situation when a particular environmental stimulus is lacking at a time when it is necessary for the child. It is inferred that the ultimate size and shape that a child attains as an adult is the result of a continuous interaction between genetical and environmental influences during the whole period of growth.
| Discuss the role of genetic factors in regulating human growth. |
Now let us understand the role and importance of genetic factors in regulating human growth. Genetic factors are clearly of immense importance. Factors affecting the rate or tempo of growth must be considered separately from factors affecting the size, shape and body composition of a child. The genetical control of tempo seems to be independent of genetical control of final adult size, and to a large extent of final shape. Environmentally produced changes in tempo do not necessarily seem to be separately controlled by genetical and environmental factors. The genetical control of shape is much more rigorous than that of size, presumably because shape represents chiefly how the cells are distributed, while size represents more the sum of sizes of the various cells.
The most striking similarity in growth is seen in monozygotic twins, who share the same genes and most aspects of the family environment. Siblings share fewer genes and possibly few aspects of family environment also, but resemble each other a great deal more than unrelated children. Family patterns of growth exist, and closer the genetic relationship, the closer in general the growth pattern. This is probably because growth and adult size and shape are controlled by numerous genes, each of small effect, rather than by few major genes. Data on monozygotic twins (MZ) reared together and apart have been reported by Shields (1962). Those reared apart were more different in adult stature than those reared together, but they were more similar than dizygotic (DZ) like- sexed twins. Shield illustrates some individual cases of MZ twins reared apart where one twin was subject to illness or neglect, showed considerable differences in size, showing the overriding effect of a poor environment.
Tempo of growth in height from birth to 4 years has been studied in twins in the longitudinal Louisville Twin Study. The analysis of the growth curves indicated a strong genetic control of the rate of growth and especially, in change in rate. Studies of the resemblance of siblings at the same age have been reported by Garn and Rohmann (1966). In general correlation co-efficient of body length measures between siblings are of the order of 0.3-0.5, though in some measurements sister-sister values are higher than brother-brother ones. Siblings are also highly correlated in birth weight, but this is mainly due to maternal uterine factors.
The resemblance of body measurements between parents and children is also marked, though not before the children are of about 2 years and showing more effect of their own genes than the effect of uterine environment in which they grew. From 3 to 9 years correlation coefficients of height between parents and offspring are slightly under 0.5 and have been made the basis of standards for childhood height allowing for height of the parents. There is little evidence that on average one parent predominates in their effect on size, or that sons resemble fathers and mothers daughters more than conversely. When the parents’ height is known, the range of variation in adult height, represented by ± 2 standard deviation of the mean is from 25 cm. in the general male populations to 17 cm. in a given family, 16 cm. among brothers and 1.6 cm amongst monozygotic twins reared together. At the same time length of limbs and trunk are also under genetic control, while skeletal breadths and of course fat are less so.
Not only is physical size heritable but the timing and tempo of maturation also are significantly controlled by genes. The genetical control of tempo of growth is best shown by the inheritance of age at menarche. Monozygotic twin sisters growing up together under best conditions reach menarche on an average 2 months apart, whereas dizygotic twins differ on average by 12 months. The sister-sister and mother-daughter correlations are close to 0.50, indicating high degree of genetic determination of age at menarche. Thus, a large proportion of the variability in age at menarche under these conditions is due to genetical influence. It is thought that mother and father exert an equal influence on tempo of growth.
There are number of early studies of dental development that show calcification and dental emergence were highly correlated within MZ twins than DZ twin pairs, thus suggesting a heritability of 0.85-0.90. The general pattern of skeletal maturation (i.e. the tendency to be an early or late maturing individual) also suggests that the tempo of development is highly heritable with sib-sib correlations of 0.45. The process of maturation is commonly believed to be controlled, at least partially by genes independent from those controlling final size. Studies have shown that siblings may reach identical adult height even though they differed in the timings of maturational events.
Differences between populations are also due to differences in their gene pools, in their environments and in the interaction. Studies have shown that AfroAmerican children growing up under favourable conditions are a little taller and heavier than Europeans and Euro-Americans living in the same cities. This is partly or wholly because they are a little more advanced in maturity. Asiatics, on the other hand, under equally favourable circumstances are smaller despite being still further advanced in maturity. Even bodily proportions are different among different three major racial groups. The relatively longest legs characterise the Australians Aborigines and the Africans in Ibadan, with the former far exceedingthe latter. Londoners and Hong Kong Chinese both have relatively shorter legs than Africans, but the Chinese pattern of growth seems to be different from the European. Initially Chinese have relatively longer legs than the Londoners, but during growth they consistently gain less in leg length per unit sitting height. Asiatics have their characteristically short legs from about mid-childhood onwards, to a degree which rapidly increases until growth ends than the Londoners.
Racial differences in shape can also be seen in the relation of biacromial to biiliac width. Afro-American boys and girls in Washington have considerably narrower hips relative to shoulders than either Londoners or Hong Kong Chinese. Chinese are not greatly different from Londoners in this respect except that adolescent girls appear to gain more in hips. There are differences in body composition also, Africans having more muscle and heavier bones per unit weight at least in males, together with less fat in the limbs in proportion to fat on the trunk (Eveleth and Tanner, 1976). The African new born is ahead of the European in skeletal maturity and motor development. He maintains this advance for some 2 or 3 years in most areas in Africa after which nutritional disadvantage interrupts. In America and Europe the African stays in advance in bone age and also in dental maturity. The mean age at menarche for African descended was 12.5 years and 12.8 years for European descended. Well off Asiatic groups have as fast a tempo as Africans, in later childhood if not in earlier years. Mean age at menarche in Hong Kong girls from affluent families was found to be 12.5 years.
Inherited differences of body build may arise by either genetic drift or natural selection. If a small population colonizes a remote habitat, this group may by chance have an unusual frequency of genes favouring a particular body form, and because of limited opportunities for mating, these characteristics will persist in subsequent generations. Moreover, there will be fewer heterozygotes than in larger communities, and some gene combinations with a low initial frequency may disappear from the population by mere chance. However, if a particular body form has favoured survival, there will also be selective pressure increasing the frequency of any related gene combinations with in the population. Further, in an isolated population the apparent advantages of a particular body form might be exaggerated by emergence of unusual pattern of diet and lifestyle within the community.