Inbreeding and Genetic Disorder

Inbreeding is usually defined as the mating between relatives and the progeny that result of a consanguineous mating between two related individuals is said to be inbred (CavalliSforza & Bodmer, 1971; Hedrick, 2005; Vogel & Motulsky, 1997). As a result of inheriting the same chromosomal segment through both parents, who inherited it from a common ancestor, the individuals born of consanguineous unions have a number of segments of their chromosomes that are homozygous. Therefore, inbreeding increases the amount of homozygosity and, consequently, recessive alleles hidden by heterozygosity with dominant alleles will be expressed through inbreeding. On this basis, it is expected that recessive traits such as many human genetic disorders will occur with increased frequency in the progeny of consanguineous couples. In addition, since many recessive alleles present in natural populations have harmful effects on the organism, inbreeding usually leads to a decrease in size, vigor and reproductive fitness. In a broad sense, it is necessary to consider that inbreeding can occur under two quite different biological situations. There may be inbreeding because of restriction of population number. The degree of relationship between the individuals in a population depends on the size of that population since the individuals are more closely related to each other in a small population than in a large one. Thus, inbreeding is a phenomenon frequently associated with small populations. On the other hand, inbreeding can occur in a large population as a form of nonrandom mating when the frequency of consanguineous matings is higher than that expected by chance. In this case, the population will show a homozygote excess with respect to a random mating population in which genotypic frequencies are expected to be in Hardy-Weinberg equilibrium. The greatest extent of inbreeding is found in plants. A number of plant species are predominantly self-fertilizing which means that most individuals reproduce by selffertilization, the most extreme form of inbreeding. In animals, inbreeding is less prevalent than in plants, even though some invertebrates have brother-sister matings as some Hymenoptera. Inbreeding also plays a very important role in animal and plant breeding because the number of breeding individuals in breeding programs is often not large. In this way, the inbreeding effects associated with small population size must be considered in the context of animal and plant breeding. In humans, consanguineous marriage is frequent in many populations. In fact, it has been recently estimated that consanguineous couples and their progeny suppose about 10.4 % of