EFFECT OF INBREEDING ON GENETIC IMPROVEMENT IN POULTRY
Kashif Dawood Khan1*, Aamir Ahmad Raina1, Mohsin Ahmad Parray1
Ph.d scholar ICAR-NDRI Karnal, 132001
Inbreeding means the mating together of individuals that are related to each other by ancestry. The mated individuals should have one or more common ancestor in their pedigree upto 4-6 generations. This means that inbreeding is the mating of related animals within 4-6 generations. The terms “inbreeding coefficient” and “coefficient of inbreeding” both refer to a mathematical formula devised by the late geneticist Sewell Wright. It is a measure of how close two individuals are genetically related to each another. The coefficient of inbreeding, symbolized by the letter F, is the probability that two genes at any locus in an individual are identical by descent (Falconer and Mackay, 1996).
Line breeding is a form of inbreeding that involves selection of mates on the basis of their relationships to a certain superior ancestor. The backcross (crossing a first-generation hybrid with one of the parental types) is a common method of inbreeding. Line breeding is highly strategized inbreeding. It is breeding to close relatives in order to lock in desirable traits. Line breeding seeks to convey outstanding genetics from one generation to another while minimizing the transfer of undesirable traits. A so-called half-sib mating (half- brother to half-sister) is a popular form of line breeding. The difference between inbreeding and line breeding lies in the degrees of separation between one half of a breeding pair and the other. Inbreeding means mating father to daughter, mother to son, and brother to sister. Line breeding involves mating more-distantly related animals.
In recent years, a number of studies on inbreeding effects in livestock and poultry populations have been performed (Klemetsdal, 1998; Thompson et al., 2000a, 2000b). Inbreeding depression in reproductive and productive traits has been reported by Flock et al. (1991) and Smith et al. (1998). Therefore, from the current perspective, the inbreeding rate is perceived as negative, especially for small, closed populations. Hence, mating designs with constraints of inbreeding level are developed (Oyama and Mukai, 1998; Nomura 1998). Contrary to livestock, laying hens are characterized by some traits (e.g. short generation interval) which lead to an increase in the inbreeding rate. Jeyaruban et al. (1995) reported that the use of best linear unbiased prediction (BLUP) induced larger inbreeding rate compared to selection response, especially for traits of low heritability. Various inbreeding effects on some reproductive traits across chicken lines were estimated by a number of authors (Nordskog and Shen Cheng 1988; Gowe et al., 1993; Sewalem et al., 1999). On the other hand Hagger et al. (1986) concluded that inbreeding was not important for embryonic viability over the extreme range of egg weight. It seems that contrary to mammals birds can be more responsive to inbreeding rate. This also corresponds with a greater response to chromosomal abberations in birds as described by Sysa and Jaszezak , 1997.
The literature regarding the effect of inbreeding on egg production is somewhat extensive and has been reviewed by Shoffner (1948) and Wilson (1948). The effect of inbreeding on rate of egg production, however, has been studied less extensively. Hays (1924) reported the results of inbreeding on egg production in Rhode Island Reds. He found a decreased rate of winter production when inbreeding coefficients exceeded 25 percent. His data did not show whether inbreeding of less than 25 percent had any effect. Jull (1933) studied inbreeding in White leghorns and found that the rate of egg production decreased with increased inbreeding. Wilson (1948) reported the partial regression of egg production rate on inbreeding coefficient of the dam to be .22 ± .06. This estimate was on an intra-year, intra-sire basis. Duzgunes (1950) reported that rate of egg production during the hatching season decreased with inbreeding in lines where no selection had been practiced for egg number, but increased in lines where such selection had been practiced. However his conclusion was based on rather few degrees of freedom and covered a production period of only 28 days.
The consequences of inbreeding in poultry assume greater importance as hybridization becomes more widespread in poultry industry. Hybrid chickens are being sold commercially today and an important phase in the breeding program is the development and maintenance of inbred lines. The primary effect of inbreeding is to produce homozygosis. As a consequence of this increased homozygosis, the individuals may show a decline in performance. This decline in performance is perhaps the most important single limiting factor in the development and maintenance of inbred lines. Relatively little is known regarding the exact nature of these adverse effects. Wilson (1948) pointed out that reproductive performance is the product of four biological components: (1) egg production; (2) fertility; (3) hatchability, (4) viability of offspring. Knowledge of the effect of inbreeding on each of these characteristics is necessary for the poultry breeder to make intelligent decisions regarding the number and size of lines to develop and maintain as well as the ultimate level of inbreeding to be attained. Such knowledge would also help to determine the ultimate commercial use of surviving lines. If the effect of inbreeding on egg production and other reproductive traits is not large, smaller lines could be maintained and more lines could be tested. This would allow for more efficient selection since the variance between lines contains more of the additive genetic variance than does the variance within lines. On the other hand, if the effect of inbreeding is severe, one would need to start larger lines and do some selecting within lines.
Establishment of a selection program for improvement in growth, egg and chick production requires the estimation of genetic parameters for these traits. Using within-line selection on a small poultry population is
expected to increase the rate of inbreeding in the population, and the increase in rate of inbreeding may lead to undesirable effects such as a decline in performance and loss of vigor (Flock et al., 1991). Therefore, estimation of the rate of
inbreeding is required and its influence on production traits needs to be explored before the application of any selection scheme. The use of inbred lines for genetic improvement of breeding stock has been controversial among geneticists because of the tendency for inbreeding depression, extended generation interval, and reduced selection intensity to offset advantages of improved discrimination among genotypes. However as indicated by Dickerson (1972), Abplanalp (1974), Dickerson and Lindhe (1977), and Kress (1977), there is continuing interest in the use of inbred lines to improve the effectiveness of selection.
REFERENCES
Duzgunes, O., 1950. The effect of inbreeding on re-
productive fitness of S.C.W. Leghorns. Poultry
Sci. 29: 227-235.
Jull, M. A., 1933. Inbreeding and intercrossing in
poultry. The effects, on various characters, of
close inbreeding, and of intercrossing closely in-
bred lines of White Leghorns. J. Heredity, 24:
93-101.
Shoffner, R. N., 1948. The reaction of the fowl to
inbreeding. Poultry Sci. 27: 448-452.
Wilson, W. O., 1948. Egg production rate and fer-
tility in inbred chickens. Poultry Sci. 27: 719-
726.
Hays, F. A., 1924. Inbreeding the Rhode Island Red
fowl with special reference to winter egg produc-
tion. Am. Nat. 58: 43-59.
Flock DK, Ameli H, Glodek P. Inbreeding and heterosis effects on quantitative traits in a White Leghorn
population under long-term reciprocal recurrent selection. Br Poult Sci 1991;32(3):451-6.
Nordskog A.W., Cheng S., 1988 – Inbreeding effects on fertility and hatchability associated with the formation of sublines. Poultry Science 67, 859-864.
Hagger C., Steiger-Stafl D., Margurat C., 1986 – Embryonic mortality in chicken eggs as influenced by egg weight and inbreeding. Poultry Science 35, 812-821.
Gowe R.S., Fairfull R.W., Mcmillan I., Schmidt G.S., 1993 – A strategy for maintaining high fertility and hatchability in a multiple-trait egg stock selection program. Poultry Science 72, 1433-1448.
Sewalem A., Johansson K., Wilhelmson M., Lippers K., 1999 – Inbreeding and
inbreeding depression on reproduction and production traits of White Leghorn lines selected for egg production traits. British Poultry Science 40, 203-208.
Sysa P.S., Jaszczak K., 1997 – Wykorzystanie badań cytogenetycznych w hodowli i selekcji zwierząt (The Use of Cytogenetic Investigations in Breeding and Selection of Animals). In: Biotech-nologia Zwierząt (Animal Biotechnology) (L. Zwierzchowski, K. Jaszczak, J.A. Modliński, Eds.). In Polish. Wydawnictwo Naukowe PWN, pp. 89-118.
Abplanalp, H. A., 1974. Inbreeding as a tool for
poultry improvement. Page 897—908 in Proc. 1st
World Cong. Genet. Appl. to Livestock Prod. I.
Plenary Session, Madrid, Spain.
Dickerson, G. E., 1972. Inbreeding and heterosis in
animals. Page 59—77 in Proc. Anim. Breeding and
Genet. Sympos. Amer. Soc. Anim. Sci.,
Champaign. IL.
Dickerson, G. E., and N. H. Lindhe’ 1977. Potential
uses of inbreeding to increase selection response.
Page 323 — 342 in Proc. Int. Conf. on Quantitative
Genet., Iowa State Univ. Press, Ames, IA.
Kress, Don, 1977. Effects of alternate generation
inbreeding on selection response in Japanese
quail. Page 1 0 – 2 3 in Proc. 26th Nat. Breeders
Roundtable, Poultry Breeders Amer., Arlington,
VA. Lagervall.
Falconer, D. S., and Mackay, T. F. C., 1996. Introduction to Quantitative genetics.
JEYARUBAN M.G., GIBSON J.P., GOWE R.S., 1995 – Comparison of index selection and best
linear unbiased prediction for simulated layer poultry data. Poultry Science 74, 1566-1576.
KLEMETSDAL G., 1998 – The effect of inbreeding on racing performance in Norwegian cold-blooded trotters. Genetics, Selection, Evolution 30, 351-366.
THOMPSON J.R., EVERETT R.W., WOLFE C.W., 2000a – Effects of inbreeding on production and survival in Jerseys. Journal of Dairy Science 83, 2131-2138.
THOMPSON J.R., EVERETT R.W., HAMMERSCHMIDT N.L., 2000b – Effects of inbreeding on production and survival in Holsteins. Journal of Dairy Science 83, 1856-1864.
SMITH L.A., CASSELL B.G., PEARSON R.E., 1998 – The effects of inbreeding on the lifetime
performance of dairy cattle. Journal of Dairy Science 81, 2729-2737.
NOMURA T., 1998 – A mating system to reduce inbreeding in selection programmes: theoretical basis and modification of compensatory mating. Journal of Animal Breeding and Genetics 116, 351-362.
OYAMA K., MUKAI F., 1998 – Determination of the optimum mating design with constraints on inbreeding level and mating frequency of sires via a simple genetic algorithm. Animal Science and Technology 69, 333-340.
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