Inbred, consanguineous marriages and their risks

Inbred means crossing between related individuals. Related individuals are those who have at least "one common ancestor", at most at the level of a great-grandparent.

Introduction

If inbreeding occurs, the number of heterozygotes decreases' and the number of homozygotes increases' (applies to the population, as well as to individual families). To investigate inbreeding, an allele of common origin was introduced - ibd allele' (Identical By Descent) - it is such an allele that an individual inherited from a common ancestor.

• Coefficient of inbreeding – F = determines the probability that in an individual both alleles of a given locus are ibd alleles

$F=\left({\frac {1}{2}}\right)^{n+1}$

← where n = number of generations (connecting lines of the family tree diagram)

Inbred between related individuals.

• Kinship coefficient – f = is defined as the probability that a randomly selected allele of a given locus of one individual is ibd with an allele randomly selected from the same locus in another individual
• Relationship coefficient – r = probability that a randomly selected allele in two related individuals is an ibd allele, i.e.:

$r={2\cdot f}$

$r=\left({\frac {1}{2}}\right)^{n}$

Consanguineous marriages

• From a clinical point of view, they represent an increased risk of having a child with autosomal recessive (AR), or a polygenically inherited disease.
• Assuming the disease is rare, then the gene frequency is low and the probability of being homozygous for ibd alleles is relatively high.
• The most common marriages are cousin and second cousin marriages.
• The proportion of AR children born from first-cousin marriages depends on the gene frequency and the amount of marriages of this type in the population. This is the so-called Dahlberg relationship = the dependence of the relative proportion of AR homozygotes born from cousin marriages on the gene frequency and frequency of these marriages in the population.

Inbred in the population - risks

Dependence of risk on inbreeding

The model population with inbred is based on the assumption that a relative part of the population (F) is fully inbred' and a other part of the population (1 – F) then panmictic

• it can be proven that the distribution of genotypes in the population is:

Genotype	AA	Oh	aa
Frequency	p (p + Fq)	2pq (1 – F)	q (q + Fp)

• in the population in which inbreeding takes place, there are no changes in gene frequencies, but there are changes in the frequency of genotypes' (heterozygotes decrease and homozygotes increase)
• inbreeding coefficients for human populations are usually very low
  • the exception may be so-called "isolates" - they may be geographical (islands, mountain valleys) or social (nationality, religious sect)

Example

• Disorders of the development of the locomotor system, malformation of the CNS associated with impairment of mental functions (high risk of mental retardation), albinism, hemophilia.

Links

Related Articles

• Relationship coefficient
• Coefficient of inbreeding
• CNS malformation