Clinical significance of

GENETICS

Human genetics is one of the elements that can be used in the planning of co-ordinated attacks on disease, since it can sometimes differentiate those groups or individuals who are susceptible from those who are not.

Group susceptibility

Genetic factors often determine group susceptibility or resistance to disease, for example the racial immunity to vivax malaria of West African Negroes now shown to be related to the Duffy antigen. The recent advances in genetics have boosted interest in molecular epidemiology; that is, the study of genetic and environmental risk factors at the molecular level, to the aetiology, distribution and control of diseases in groups of populations.

Individual susceptibility

The role of genetic factors in individual susceptibility may be seen in twins that are sometimes more liable to certain morbid conditions; a comparison is made between monozygotic and dizygotic twins. In addition, immune deficiencies, whether cellular or humoral (e.g. agammaglobulinemia), are consequent on genetic factors, and such genetic failure may be responsible for altered patterns of disease, for example defective cellular immunity in lepro-matous leprosy. The clinical importance of the HLA genes in relation to transplantation and to possible abnormal immune response to some common chronic diseases has been recently emphasized, as has the remarkable association between HLS-B27 and ankylosing spondylitis and the possible association of Schistosoma mansoni hepatosplenomegaly and HLA-Aj and B5.

Genetic counselling

Population genetic studies are a recent important expansion of the field of genetics and the knowledge thus acquired can be of practical value in preventive medicine, in the form commonly referred to as genetic counselling. This is particularly important in the situation, as it is at present in Africa, where sickle-cell anaemia has an incidence oJ^ iieifrJj 2% in some countries and may be responsible for -a childhood mortality of approximately 5 ,it:i 1000.

Implementing a programme

The ascertainment of follow-up of individuals in need of counselling and prenatal diagno^s sir, ild not be left to chance but should be achieved by the creation of a genetic register system.

Genetic counselling is essentially a process of communication and involves far more than the mere discussion of genetic risks. First, the nature of the disease has to be described, its prognosis given and the nature and efficacy of any treatment discussed. Feelings of guilt and recrimination may have to be dealt with. Second, the various options open to a couple will have to be considered: family limitation, sterilization, adoption, artificial insemination and prenatal diagnosis with elective abortion (see below). There is evidence that in coming to a decision, parents are influenced by the psychological, social and economic problems attendant on a serious genetic disorder.

ASSESSING THE RISK

Genetic risks are naturally assessed in terms of probabilities. They range from the big risk of 1 in 2 with a dominant gene (e.g. Huntington's disease) or 1 in 4 with a recessive gene (e.g. sickle-cell anaemia) through a spectrum of decreasing risks which ultimately reach very low values.

INTERVENTION

An increasing number of inherited conditions can be effectively treated if dealt with promptly, and in others treatment can at least reduce the degree of suffering. Where routine neonatal screening is possible but prenatal diagnosis not available, for example SS disease in some developing countries, there is a special obligation to make sure that early detection occurs. In some conditions, it is even possible to obtain a prenatal diagnosis of the foetus, for example spina bifida and (3-thalassaemia. Indeed the recent advances in molecular biology have made prenatal diagnosis available for a greater number of genetically acquired diseases and at an earlier stage of pregnancy, making safer intervention possible if it is so desired (Table 8.8).

Table 8.8: Some genetic disorders in the tropics that can be diagnosed prenatally

Table 8.8: Some genetic disorders in the tropics that can be diagnosed prenatally

Heat disorders

Heat-induced and heat-related disorders have been recorded since biblical time and are of particular importance in tropical countries.

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