Indian Pediatrics 1998; 35:1110-1113 

Inbreeding Effects on the Incidence of Congenital Disorders and Fetal Growth and Development at Birth in North India

Individuals who. are related through one or more common biological ancestors are called consanguineous relatives. In human population marriage largely regulates mating and consanguineous marriage thus leads to inbreeding. Inbreeding is the genetic consequences of consanguineous marriage. Parental consanguinity increases the frequency of homozygotes in the offspring at the expense of hetrozygotes, hidden recessive and additive alleles would thus increase in frequency in the inbred individuals. A large numbers of infant and childhood disorders of rare single lethal and sublethal recessives or polygenic combinations of rare recessives due to inbreeding have been observed by several authors(1-9).

A vast majority of Muslim population of North India still prefer consangumeous marriages(10). The present study was undertaken in a hospital of North India to assess the baseline information' on the prevalence of different genetic disorders and fetal growth and development with the common preference for consanguineous marriages.

Subjects and Methods

A total of 1672 singleton consecutive live births were studied from the Jawaharlal Nehru Medical College Hospital, Aligarh during the year 1991 and 1992. Each live new born was investigated by the authors (1st and 3rd) within 24 hours of birth for recording sex and gestational age, obtaining birth measurements and detection of congenital malformations. Birth weight was determined with a lever balance to an accuracy of 5 g and recumbent length was measured by infantometer of local make with an accuracy of 0.5 cm. Head circumference was measured with a nonstrechable fiber tape. All measurements were taken within six hours of birth except head circumference which was taken after 24 hours of birth. There were no cases of malformation among parents of the children examined in the present study.

The inbreeding coefficient has been de- fined as the probability of two homologous alleles of an individual being identical by descent and is equivalent to, the genetic correlation between uniting gametes(ll). In order to calculate the coefficient of inbreeding (F) through pedigree data we used slightly modified Wright's(12) formula:

                                       F = Σ (1/2)ⁿ (1 +F_a)

Where n is the number of persons along the path through a common ancestor connecting two parental gametes and Fa the inbreeding coefficient of that common ancestor. The sample size in the present study is not as large as would have been necessary to obtain significant effects of increased inbreeding. This is because the expected amount of change in means and variations on inbreeding among humans are small. The samples cannot be enlarged indefinitely because the samples of different levels of inbreeding have to be collected from the same gene pool and similar environmental background and it limits the population size and availability of suitable samples.

Results

As verified from the pedigrees, the in- breeding coefficients of common ancestors of parents (Fa) pointed out to be zero in all present cases. The study samples the off- spring of first cousin (F = 0.0625), first cousin once removed (F = 0.0312); second cousin (F = 0.0156) and unrelated marriage (F = 0). The average coefficient of inbreeding of the study group was F = 0.0477.

Table I indicates that the average, in- breeding coefficient in 'the present study group (F = 0.0477) was higher than any other parts of the India(13). In more than 60% of the consanguineous marriages, the spouses were first cousins.

A total of 14 different congenital disorders were identified in the present study group (Table II). The role of inbreeding in the etiology of the congenital disorders is clearly shown in Table II & III. The incidence of congenital malformations among the consanguineous was significantly higher (p < 0.001) than that among the non- consanguineous marriages.

The mean values of the three anthropometric measurement and gestational period are given in Table IV. The mean birth weight, length, head circumference and gestational period of the babies born to consanguineous couples (C) was significantly lower (P <0.001) than that among the non-consanguineous marriages.

The results of one-way analysis of variance do not show any significant mean difference amongst FC, FOR and SC at 5% level of probability. The variance ratios (F) were 1.50, 1.54, 1.49 and 1.47, respectively for birth weight, length, head circumference and gestational period with df 3 and 1668, whereas in the results of regression analysis, inbreeding showed a significant negative correlation with all parameters. The slopes (b) - 7861.28±241.21,- 48.35±0.47, - 47.98±0.34, - 37.85±0.38, - 605.78±4.37 were significant at p <0.001 for birth weight, length, head circumference, gestational and congenital malformation, respectively.

Discussion

Consanguinity implies greater homozygosity in the offspring for certain alleles. The average inbreeding quotient in the present study was higher than other reports from India(13). The present analysis of the data shows that inbreeding adversely affects the overall traits. However, all these parameters are subject to great variation with genetic and environmental factors. The socio-economic differences in the present sample are negligible, as the people were engaged in similar jobs, earned about the same salaries and resided in uniform type quarters in the adjacent colonies. The etiology of congenital malformation also seems to be multi-factorial genetically determined. In the present study, a high rate of parental consanguinity was observed among the affected persons.

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