Marriage and Genetics of Human

In this article we will discuss about:- 1. Degree of Resemblance between Parent and Offspring 2. Result of Children of First Cousins Marriage 3. Result of Children of Parents who are Recessives 4. Random Expectations of Consanguinity and Others.

Social authorities have prohibited consanguineous marriages (between close relatives who have the blood relations) since ancient times with some exceptions. Marriages between first cousins have always been relatively rare in our society, it is a widespread belief that the children of consanguineous marriages are particularly imperiled and are much more likely to suffer from malformations and genetic diseases.

It has also been accounted that the children from consanguineous marriage are likely to prove less intelligent than the normal.

The nature of consanguinity consists of the fact that relatives, because they have common ancestors, possess a greater number of common genes than the average percen­tage of common genes in the general popu­lations.

The size of this additional fund of common-genes will ultimately depend upon the degree of the relationship which is given in the following table:

From the above table it can easily be concluded that children of consanguineous marriages have a higher chance of receiving the same alleles for a gene locus from both parents – i.e. of being homozygous. The relationship between the frequency of descendants homozygous for a recessive gene from consanguineous marriages and those accidentally homozygous from arbitrary unions is dependent upon the frequency of the recessive gene. The lower the gene frequency the higher the value.

The inbreeding coefficients of the most common consanguineous marriage types are summarised.

Degree of Resemblance between Parent and Offspring:

The relationship between one parent and one offspring is defined as one genetic step and it is clear that the intervention of meiosis has reduced genotype resemblance to 1/2 in this single step. A child receives only 1/2 of his chromosomes and autosomal genes from one parent and a parent can pass on only 1/2 of his alleles by way of one gamete to any one offspring. Thus from grandfather to grandson would be two genetic steps and, consequently, an average 1/4 genotype identity. In this way only 1/8 of a person’s genes reach any one great grandchild because of the three intervening genetic steps each having a meiotic reduction process.

A special genetic step exists between full sibs as a shortcut. Their similarity through their father involves two genetic steps 50 that their total resemblance for paternal genes is only 1/4. In addition, full sibs have a second pathway of two more genetic steps through their mother and, hence, another 1/4 resemblance for maternal genes. The sum total of resemblance between full sibs’ is 1/2 and this fact allows one to take on genetic step horizontally between sibs as in following figure:

Thus the total genetic resemblance is a summation amounting to 1/2 for full sibs, 1/8 for first cousins and 1/32 for second cousins. This reasoning may be applied to other degrees of relationship e.g. between uncle and niece are two genetic steps, as in aunt and nephew or grandparent and grandchild.

Result of Children of First Cousins Marriage:

A child from the marriage of a rare recessive and his or her normal first cousin would be expected to be recessive with probability 1/8 over the sum of many such cousin marriages. This would be composed of recessives expected among 1/2 of the children from those 1/4 of test cross-cousin marriages in which the normal partner was heterozygous and none at all among the 3/4 of phenotypically similar cousin marriages in which the normal cousin did not receive a copy of the rare allele.

Thus the product, 1/2 of 3/4, would be 1/8 of the whole collection of sibs from those first cousin marriages which were in the test-cross category, one normal by one recessive. However, once a couple has had a recessive child, it is thereby, demonstrated that the dominant genotype is not homozygous, which would have prevented recessive offspring, but rather that the overall or average risk of 1/8 in their phenotypic kind of cousin marriage has been 1/2 in their instance.

This 1/2 chance will apply to each and every child of this couple. Similarly, the chance of a child being recessive from a marriage of two normal first cousins where there is reason to assume that one of the cousins is already of the necessary heterozygous genotype is 1 chance in 32. Cousins marriages tend to produce more homozygous offspring and fewer of the heterozygous offspring than do marriages between the unrelated persons.

Some of this effect goes unnoticed because the homozygous dominants are phenotypically like other dominant persons and because common recessive phenotypes are not increased as conspicuously as are rare recessives from cousins marriages.

Result of Children of Parents who are Recessives:

If cousins marriages or other consan­guineous marriages are found more frequently among parents of a certain kind of phenotype than among representatives marriage in the same population, then this is evident that by recent common descent two replicates of the same rare gene have been brought together by marriages between close relatives. Many albinos have parents who are cousins or who are in some other consanguineous union.

A similar problem concerns the parents of congenitally deaf children, it is obvious that the proportion of homozygotes deriving from cousin marriage will be influenced both by the recessive gene frequency, q, and by the frequency of cousin marriages, C.

The ratio of cousin marriages, Cr, among all panmictic marriages producing the same recessive phenotype is:

Cr = (1 + 15 q) /16q

when q and c are small. But if the population is not panmictic but has cousin marriages in excess of the mean proportion of cousin to n> M-cousins among one’s acquaintances, then the divisor should be 16(q + F) i.e.

Cr= (l + 15q)/ 16(q+F)

where F is the coefficient of inbreeding.

Random Expectations of Consanguinity:

The random frequency of cousin marriages to be expected is obviously related to the average number of cousins and to the circle of one’s social acquaintances. In turn, the average number of cousins will depend on average sib ship size. In a stationary population where the mature children per family are reckoned to be two, the average number of aunts and uncles would be two and the total of first cousins would be four, presumably divided equally between the sexes.

This is not the actual situation. Suppose a person who has many cousins and lives in a small community limited either geographically because of language restrictions, for religious reasons or for maintaining the royal blood of monarchy cousin marriages are opt to be more frequent than in other population isolates.

Here population isolates is defined as that group of persons among whom mates are chosen panmictically. But in these days of more travel, more frequent changes of residence and smaller families, genetic isolates are tending to break-down and a person is much more likely to marry a non-relative.

Inference:

Cousins marriages have a specific additional chance of having offspring homozygous for a rare recessive allele over and above the population chance depends on the number of genetic steps which must be successfully taken by the allele in question, because each genetic step reduces the average genotypic resemblance by 1/2.

Children may Receive the Particular Abnormal Gene from Parents but the Effects may not be Detected at the Early Age:

There are so many human genes well enough analyzed that we may reliably describe the late differentiation of certain phenotypes from the normal. One of the best example is several forms of muscle dystrophy which is not usually detected until they boy or girl is between 10 and 17 years of age. Similarly, the late average age of appearance for the phenotype of Huntington’s chorea (a disease caused by the dominant abnormal gene).

The affliction is a nervous disorder at first involving muscle twitching, then the loss of coordinate and later the loss of mental powers followed by invalidism. Very few persons with the Huntington’s chorea gene show any abnormality during youth and early maturity. Hence this type of genes has been spread far and wide in certain kindred’s by persons who did not know they were genetically sick.

Therefore it appears that an individual apparently normal at one age may not turn out to be genetically normal at another age. In other words, all inherited difference in man may not appear congenitally, many become first noticeable in various intervals after birth. Henceforth the age of each normal person is an important part of the geneticist’s record.

Criteria for Recognizing Simple Recessive Inheritance:

The important point about recessive inheritance is that the affected persons must have received the gene for the condition from both parents, for both members of chromosome pair concerned must carry it, and each parent has contributed one chromosome of that pair.

Two persons marry who are normal in appearance, but actually each carries the gene for, say albinism, upon one chromosome of the relevant pair, and so albinos may appear amongst their offspring’s. If, however, these two people had married other persons, the chances are greatly in favour of all the offspring of both unions being normal. Therefore, in recessive inheritance, the result depends upon both parents in their relationship to each other.

Following are the major criteria for recognizing the transmission of simple recessive gene:

(a) The great majority of the affected persons are the offspring of parents who are normal to all outward appearance.

(b) There is a familial incidence, i.e. sib ship frequently occur in which more than one child is affected. Taking a pool of large number of sib ships, it is possible by suitable methods to discover that the proportion of normals to affected is in fact 3:1.

(c) If the abnormality is rare, an undue proportion of consanguineous marriages is found amongst the parents of affected persons. The rare the defect the higher the proportion of consanguineous marriages.

(d) Affected persons who marry normals have only normal offspring in the great majority of cases.

(e) When affected persons married to normals do have affected children, i.e. when they have happened to marry heterozygotes, the proportion of normals to affected is 1: 1.

(f) There is an increased rate of consanguineous marriages amongst the unions of affected and normals which do yield affected offspring.

(g) Affected persons who marry affected persons have affected offspring only — provided that both owe their abnormality to the same gene.

(h) The actual blood relationship of the parents is an important factor for the appearance of recessive abnormalities in the offspring.

(i) Consanguineous marriages reduce the proportion of heterozygotes and natural selection eliminates the abnormal homo- zygotes, leaving just an increase in the proportion of normal homozygotes.