Heterosis Breeding and Inbreeding Depression


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Heterosis and Inbreeding Depression : 

Heterosis and Inbreeding Depression Mr. Kamlesh Kumar Chandel M. Sc. (Agri.) Plant Breeding and Genetics 2011-2012 . Dr. M. A.Vaddoria Assot.Professor Dept. of Genetics & Plant Breeding J.A.U., Junagadh. Submitted To Submitted By DERPARTMENT OF GENETICS AND PLANT BREEDING, COLLEGE OF AGRICULTURE, JAU, JUNAGADH.

Contents : 

Contents 3

Introduction : 

Introduction The term heterosis was first used by Shull in 1914 Definition: Heterosis refers to superiority of f1 hybrids in one and more characters over its parents. The term hybrid vigor is used as synonym for heterosis. 4 • Single cross hybrid or F1-hybrids (A x B) • Three-way hybrids (A x B) x R • Double-cross hybrids (A x B) x (R x S) Types of Hybrid

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5 Heterosis is the superiority of a heterozygous cross (F1) over the mean of its two genetically dissimilar parents (P) Heterosis Inbreeding Depression Heterosis = F1 - P Relative Heterosis = 100(F1 - P)/P 5

Heterosis Hypotheses : 

Heterosis Hypotheses Dominance hypothesis (Davenport 1908, Bruce 1910): Heterosis is attributable to the joint action of several Loci at each of which the favourable allele is partially or completely dominant. Overdominance hypothesis (Hull 1945, Crow 194 8): Heterosis is caused by overdominance at many loci influencing the trait. Epistasis hypothesis (Powell 1944, Williams 1959): Heterosis is mostly attributable to favourable epistatic interactions between non-allelic genes 6

Heterosis Hypotheses : 

Heterosis Hypotheses 7 Interaction between alleles Dominance hypothesis Overdominance hypothesis Epistasis hypothesis dominant: AA = Aa > aa BB = Bb > bb overdominant: AA < Aa > aa BB < Bb > bb additive: AA > Aa > aa BB > Bb > bb

Factors Affecting Heterosis : 

Factors Affecting Heterosis 1.Mode of pollination The magnitude of heterosis differs depending upon the mode of pollination of a species. The level of heterosis is generally higher in cross pollinated species than in self pollinated species. Genetic diversity of parents The expression of heterosis is influenced by genetic diversity of parents. For example, in wheat higher heterosis is associated with crosses of more distantly related parents. Genetic base of parents The manifestion of heterosis is affected by the genetic base of the parents. For example* in cotton higher heterosis is associated with broad genetic base of the parents. . Adaptability of parents The magnitude of heterosis is also affected by the adaptability of the parents. In cotton and many other crops, heterosis is associated with wider adaptability of the parents, because there is close association between adaptability and genetic base. 8

Estimation of Heterosis : 

Estimation of Heterosis 9 Average heterosis:- When the heterosis is estimated over the mid parent, i.e, mean value or average of the two parents, it is known as average heterosis, which is estimated as follows : Average heterosis = [{Fl - MP)/MP] x 100 where, F, is the mean value of/7, and MP is the mean value of two parents involved in the cross.

Estimation of Heterosis : 

Estimation of Heterosis 10 Heterobeltiosis When the heterosis is estimated over the superior or better parent, it is referred to as heterobeltiosis. It is worked out as follows : Heterobeltiosis = [(F, - BP)/BP] x 100 where, BP is the mean value (over replications) of the better parent of the particular cross.

Estimation of Heterosis : 

Estimation of Heterosis Useful heterosis The term useful heterosis was used by Meredith and Bridge (1972). It refers to the superiority of F1 over the standard commercial check variety. It is also called as economic heterosis. This type of heterosis is of direct practical value in plant breeding. It is estimated as follows : Useful heterosis = [(Fl - CQ/CC] x 100 where, CC is the mean value over replications of the local commercial cultivar. 11

Inbreeding : 

Inbreeding Inbreeding = mating of related individuals “Mating of individuals more closely resulted by ancestry than would be expected under mating.” mating among relatives Which increase homozygosity Inbreeding is intentionally practiced to: -create genetic uniformity of laboratory stocks -produce stocks for crossing (animal and plant breeding) Inbreeding is unintentionally generated: by keeping small populations (such as is found at zoos) during selection 12

Inbreeding Depression : 

Inbreeding Depression “Loss or decrease in vigor due to inbreeding.” Inbreeding may or may not lead to inbreeding depression, which is a reduction in the fitness in the offspring produced by inbred matings. Inbreeding depression results due to fixation of unfavourable recessive genes in F2 13

Fitness and Vigour Relationship : 

Fitness and Vigour Relationship 14 Inbreeding depression is the decline in fitness and vigour with decreased heterozygosity, whereas heterosis is the increase in fitness and vigour with increased heterozygosity.

Inbreeding Depression and Fitness traits : 

15 Inbreeding Depression and Fitness traits

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