Paper No. : 08 Human Population Genetics Module : 16 Inbreeding coefficient and Inbreeding depression Development Team Prof. Anup Kumar Kapoor Principal Investigator Department of Anthropology, University of Delhi Paper Coordinator Prof. Gautam K. Kshatriya Department of Anthropology, University of Delhi Content Writer Dr. Rajesh Kumar Gautam Dr. H.S.G.University, Sagar, MP Prof. A.Paparao Sri Content Reviewer Venkateswara University, Tirupati, Andhra Pradesh 1 Human Population Genetics Anthropology Inbreeding coefficient and Inbreeding depression Description of Module Subject Name Anthropology Paper Name 08 Human Population Genetics Module Name/Title Inbreeding coefficient and Inbreeding depression Module Id 16 2 Human Population Genetics Anthropology Inbreeding coefficient and Inbreeding depression CONTENTS: 1. Learning outcomes: At the end of the module the reader will know Inbreeding Consanguinity Inbreeding co-efficient Inbreeding depression Classical examples of Inbreeding depression Inbreeding coefficient and Inbreeding depression Bi-parental reproduction has its own importance in natural selection and evolution. In which, a limited number of mutations, which are not too injurious, to be carried by the species furnish an almost infinite field of possible variation through which species may work its way under natural selection. But, at the same time, in bi-parental reproduction, there may have breeding or mating between closely related individuals for example: brother-sister mating and father-daughter mating. Such kind of mating practices are called Inbreeding. In another word, Inbreeding is a mating practice among organism of common ancestry; it is contrary to out-breeding, which is the mating practice of unrelated individuals. The inbreeding has various pro and cons. In genetics, it is an important practice, for retention of desirable characteristics or the elimination of undesirable ones. For course of evolution, variation in gene pool is essential; in the same way for perpetuation and existence of any species variation in gene pool should be maintained; and to maintain variation in gene pool inbreeding should be avoided. But, at the same time; to maintain homogeneity in offsprings inbreeding is essential. Sometimes, it is the only way to perpetuate any species or group of organism in the circumstances of limited number of eligible mating population. According to Jacquard (1975) Inbreeding’ is used to describe various related phenomena that all refer to situations in which matings occur among relatives and to an increase in homozygosity associated with such matings. However, they differ in the reference population that is used when calculating inbreeding. Inbreeding is always a relative never an absolute measure. Therefore, inbreeding estimates differ depending on the reference population to which they refer. It is this relativity that is responsible for the different meanings of the term ‘inbreeding’, and for some of the misunderstanding that have resulted. There is a historical precedence for such misunderstandings: Fisher (1965) never accepted Wright’s inbreeding coefficients because they are relative (Keller and Waller, 2002). Three of the most commonly used definitions of inbreeding are. 3 Human Population Genetics Anthropology Inbreeding coefficient and Inbreeding depression 1. Pedigree inbreeding An individual is considered inbred under this definition when the parents share ancestors. The extent of inbreeding is related to the amount of ancestry that is shared by the parents of an inbred individual. Pedigree information is used to determine the inbreeding coefficient. Although originally derived as a correlation by Wright (1922). 2. Inbreeding as non-random mating This refers to the degree of relatedness between mates relative to two mates chosen at random from the population. An individual is considered inbred if its parents were more closely related than two randomly chosen individuals. This type of inbreeding is relative to a random mating population of the same size. Although it can be determined from pedigree data, this type of inbreeding is typically measured by the deviation of the observed heterozygosity of an individual relative to the heterozygosity expected under random mating . 3. Inbreeding because of population sub division When populations are subdivided into more or less isolated groups, inbreeding will also occur purely because population size is restricted and genetic drift results. This occurs even if mating is random within subpopulations (Crow and Kimura, 1970). This third definition of inbreeding corresponds to the mean inbreeding coefficient expected in subpopulations under random mating. Inbreeding is considered a problem in humans because inbreeding increases the chances of receiving a deleterious recessive allele inherited from a common ancestor. Most studies are concerned with close inbreeding, also known as incest, which usually sets a threshold at the level of first-cousin mating (Thorn hill 1993). Consanguinity Two individuals are said to be consanguineous if they have at least one ancestor in common. In the breeding of domestic animals consanguineous mating are frequently made. Occasionally matings are made between very close relatives- father or sire and daughter, brother and sister etc. On the basis of the theory of evolution, all individuals of a species are to some extent consanguineous, since all are descended from a remote common ancestor. Consanguinity, and level of inbreeding, must therefore be defined as applying only to relationships established after some evolutionary time point at which, for convenience everyone is considered to be unrelated. 4 Human Population Genetics Anthropology Inbreeding coefficient and Inbreeding depression In practice, for human consanguinity common ancestors more remote than a great great grandfather are rarely considered. In some human societies, more distant consanguinities may have social significance, but from a genetic point of view the connection between two individuals who have one great great great grandparent in common (fourth half cousins) is very vague (Cavalli-Sforza and Bodmer 1971). The progeny of consanguineous parents is, by definition, inbred. The inbred individuals may carry double dose of a gene that was present in a single dose in common ancestor. A recessive gene carried in a single dose in a common ancestor may remain hidden until it comes to light for the first time in an inbred descendant. Therefore, recessive traits will occur with increased frequency in the progeny of consanguineous mates. The occurrence of consanguinity in a population depends on various factors viz. population structure, migration, cultural practices and so on. Generally very close consanguineous mating are avoided in human population. Practically, in all human societies incest is considered to be taboo. The degree of relationship at which mating is considered incestuous may differ slightly from one society to another, but, in general, parent-offspring and brother-sister mating are forbidden in all societies. Still, incestuous union may be in most of the societies, but, they are negligible in proportion. Adam and Neel (1967) have studied such children and most of them were the result of brother-sister mating and father-daughter mating. Marriage with a sib’s progeny (uncle-niece or aunt-nephew) is incestuous and not permissible in some societies. In others, it is permissible with certain specific dispensation. Among many of Indian tribes cross-cousin marriages are considered as preferential marriages. By definition, an inbred individual is connected through both his father and his mother to the same recent ancestor. He can thus receive two copies of a gene that was carried by common ancestor. Two such copies are said to be identical, unless mutation has taken place in one of the line of descent, which, however, is very rare event. An individual who is homozygous for a given gene carries at a certain locus two homologous genes that are physically identical but not necessarily identical by descent. This physical identity is what we call identity by nature. Inbreeding makes an individual homozygous for genes that are identical by descent. Inbreeding co-efficient Measurement of degree of inbreeding can be done by inbreeding co-efficient. The importance of having a coefficient by means of which the extent of inbreeding can be expressed was brought out by 5 Human Population Genetics Anthropology Inbreeding coefficient and Inbreeding depression Pearl (1913). His coefficient was based on the smaller number of ancestors in each generation back of an inbred individual, as compared with the maximum possible number. A separate coefficient is obtained for each generation by the formula (Wright 1992). Where is the ratio of actual to maximum possible ancestors in the n+1st generation. By finding the ratio of a summation of these coefficients to a similar summation for the maximum possible inbreeding in higher animals, viz., brother-sister mating, he obtains a single coefficient for the whole pedigree. This coefficient has the defect, as Pearl himself pointed out, that it may come out the same for systems of breeding which we know are radically different as far as the effects of inbreeding are concerned. In order to overcome this objection Pearl has devised a partial inbreeding index which is intended to express the percentage of the inbreeding which is due to relationship between the father (sire) and mother (dam), inbreeding being measured as above