Marriage between close relatives who have one or more common ancestors is called consanguineous marriage. Consanguinity is the term referred to describe the marriages between blood relatives who have one or more common ancestors.
Non-consanguineous marriages are between individuals of opposite sex who do not have a known common ancestor.
When the frequency of marriages between close relatives who have one or more common ancestors exceed the expected frequency under random mating in a population then it is called inbreeding and when it decreases the expected proportion then it is called outbreeding. Consanguinity refers to marriage type and inbreeding refers to the mating pattern of the population.
The most common form of consanguinity in the human population is cousin marriage. Marriage between children of siblings of the same sex (parallel cousins) is prohibited except in some Islamic societies of the Middle East where marriage between a man and
his father’s brother’s daughter is common.
There are in certain areas (South India, Japan, etc.) where marriages are commonly observed between the children of the siblings of opposite sexes (cross cousins). First cousin marriages make up almost 10 per cent.
In southern part of India, especially in the state of Andhra Pradesh, among certain castes, uncle-niece unions also make up about 10 per cent of marriages. Less frequent marriage types also occur in this part of India such as the marriages between first cousins once removed, second cousins, double first cousins and aunt-nephew.
Genetic load is the fraction by which the average population fitness is decreased in comparison with the genotype showing the highest fitness.
The loss of individuals because they carry certain genes, has been termed as genetic load of a species or population. Every human population carries a burden of deleterious mutations which hampers the fitness of the group. The genetic load refers to the proportion by which fitness is reduced in the population due to the operations of a factor such as mutation.
The genetic load of a species is a measure of the number of deleterious traits maintained in a population or of the damage to the population by the factors under study. It may be measured as decreased average fitness as mortality, sterility, or morbidity due to specified causes, usually deleterious alleles. The genetic load of a species may be partially hidden and partially manifested. The genetic load depends on several variables — the occurrence of mutations, the number of detrimental mutations, the number of mutant recessive alleles, and the number of partially lethal mutant dominant alleles.
Effect of Consanguineous Marriages
The main genetic consequence of inbreeding is an increase in the proportion of homozygotes. Through inbreeding recessive genes becomes expressive.
Inbreeding causes deterioration and outbreeding causes improvement of most of the characters. This phenomenon of deterioration on inbreeding is known as inbreeding depression.
eg. fertility tends to decrease , overall general size tends to decrease.
If two independent pure lines are crossed, the hybrids between them mostly show a considerable increase in size, fertility and many other desirable traits. This has been called hybrid vigor or heterosis.
These inbreeding and outbreeding consequences are also seen in man.
It should be emphasised that the increasing homozygosity i.e., the general effect of inbreeding does not predict whether inbreeding is good or bad. It depends on the nature of the homozygotes.