Applications of Mating Systems in Plant Breeding: An In-Depth Analysis. Gene Frequencies, Genotype Frequencies, and the Hardy-Weinberg Law: Key Concepts and Plant Breeding Implications

This presentation explores the practical applications of various mating systems in plant breeding, highlighting their impact on genetic diversity and trait enhancement. It covers random mating for maintaining genetic stability, genetic assortative mating for reinforcing desirable traits, genetic disassortative mating for promoting genetic diversity and adaptability, phenotypic assortative mating for focusing on specific observable traits, and phenotypic disassortative mating for maximizing diversity and robustness. Ideal for plant breeders, geneticists, and agricultural professionals, this comprehensive overview demonstrates how these strategies are employed to develop improved and resilient plant varieties.
This presentation covers basic concepts in population genetics, including gene frequency (the proportion of different alleles in a population) and genotype frequency (the proportions of various genotypes). It explains the Hardy-Weinberg Law, which describes how allele and genotype frequencies stay constant in a population with random mating. The presentation also includes a table showing how random mating affects genotype frequencies and discusses factors that can disrupt Hardy-Weinberg equilibrium. Additionally, it explores how understanding these concepts can benefit plant breeders by guiding strategies for improving crop varieties. Ideal for students and those interested in genetics and plant breeding, this resource provides clear insights into genetic principles and their practical applications.

1. System of Mating  Sony Bhattarai B.Sc. (Hons)Ag 7 th semester Roll no. : 83 Bharatpur-11, Chitwan 1

2. System of Mating Mating system refers to the method by which individuals are paired (assorted) for crossing ; in other words, various schemes which are used for crossing or mating of individuals are referred to as systems of mating Sewall Wright (1921) has given five different systems of mating, viz.: (1) Random mating, (2) Genetic assortative mating, (3) Genetic disassortative mating, (4) Phenotypic assortative mating, and (5) Phenotypic disassortative mating 2

3. 1. Random Mating In random mating each female gamete is equally likely to unite with any male gamete and the rate of reproduction of each genotype is equal; there is no selection ;this is a form of outbreeding  In such a situation, gene frequencies remain constant , variance for the character remains constant and the correlation between relatives or prepotency does not change; it is useful in plant breeding in several ways Moreover, fertilization is never completely random due to differences in pollen shed and stigma receptivity, self-incompatibility and direction of wind   Eg : progeny testing, production and maintenance of synthetic and composite varieties, production of polycross progeny, etc 3

4. 2.Genetic Assortative Mating In this scheme, mating is done between individuals that are more closely related by ancestry than in random mating; This mating system is more commonly known as inbreeding This mating increases homozygosity and reduces the heterozygosity  The prepotency of individuals increases under inbreeding; Prepotency is the property of an individual to produce progeny that are similar to each other and to the parent; This mating is useful in the development of inbreeds, both partial and complete 4

5. 3.Genetic Disassortative Mating In this system of mating, individuals are mated that are less closely related by ancestry than would be under random mating The individuals often belong to different populations Intervarietal and interspecific crosses are the examples of this mating system It may be expected that this mating system would reduce homozygosity and increase heterozygosity 5

6. 4. Phenotypic Assortative Mating Mating between individuals that are phenotypically more similar that would be expected under random mating is called phenotypic assortative mating It leads: Division of population into 2 extreme phenotypes Increase in homozygosity and genetic variability provided both the extreme phenotypes are kept in the population Increase in prepotency of the individual due to increase in their homozygosity This system of mating is useful in the isolation of extreme phenotypes; it is used in some breeding scheme as in recurrent selection 6

7. 5. Phenotypic Disassortative Mating: Mating between phenotypically dissimilar individuals belonging to the same population is referred to as phenotypic disassortative mating It leads: ➢ Maintenance of or even some increase in heterozygosity ➢ Some reduction in population variance ➢ Reduction in correlation between relatives This system of mating is useful in making a population stable, i.e., in maintaining variability 7

8. Mating systems have four useful applications in plant breeding given as follows: Random mating is useful for the development and maintenance of synthetic and composite varieties and also in progeny tests Genetic assortative mating is useful in maintaining the purity of genotypes and in developing inbred lines Phenotypic assortative mating is useful for the development of an extreme phenotype, and Disassortative mating help in the development of source population with greater stability 8

9. Hardy-Weinberg Law 9

10.   Gene frequency The proportions of different alleles of a gene present in a Mendelian population or the proportions of gametes produced by a population carrying the different alleles of a gene are the frequencies of these alleles in the population, i.e., gene frequency  Genotype frequency The proportions of different genotypes for a gene in a population It is often called zygotic frequencies  Population genetics is the study of gene and genotypes in a population and factors that help to change those frequencies in the course of time Population genetics 10

11. Hardy-Weinberg Law Hardy (1908) in England and Weinberg (1909) in Germany independently developed this law; This law states that, “the gene and genotype frequencies in a Mendelian population remain constant generation after generation if there is no selection, mutation, migration or random drift” The frequencies of the 3 genotypes for a locus with two alleles, say a and a, would be p2 AA, 2pq Aa, and q 2aa; where p represents the frequency of A and q represents the frequency of a allele in the population, and the sum of p and q is one, that is p + q = 1 Such a population would be at equilibrium since the genotypic frequencies would be stable, that is, would not change, from one generation to the next; This equilibrium is known as Hardy-Weinberg equilibrium; A population is said to be at equilibrium when frequencies of the 3 genotypes, AA, Aa and aa are p2, 2pq and q2 respectively; When a population is equilibrium or not can be easily determined using a chi-square test 11

12. Table: Consequences of random mating of genotypes in a Mendelian Population 12 Mating Frequency of Mating Frequency of progeny from the Mating AA Aa aa AA × AA p 2 × p 2 = p 4 p 4 AA × Aa 2 (p 2 × 2pq) = 4p 3 q 2p 3 q 2p 3 q AA × aa 2 ( p 2 ×q 2 ) = 2 p 2 q 2 2 p 2 q 2 Aa × Aa 2pq ×2pq = 4p 2 q 2 p 2 q 2 2p 2 q 2 p 2 q 2 Aa × aa 2 (2pq × q 2 ) = 4pq 3 2 pq 3 2 pq 3 aa × aa q 2 × q 2 = q 4 q 4

13. Factors affecting Hardy Weinberg equilibrium Migration Mutation Random drift Selection Inbreeding 13

14. 1. Migration It is the movement of individuals into a population from a different population Migration introduces new alleles into the population or may change the frequencies of existing alleles HW assumption – Population should be closed  Migration may disrupt HW equi . and influence the evolution 14 Figure: Migration Source: https://cdn.kastatic.org/ka-perseus-images/5e0899ac412c6d4805b79de3bbdf0500eb009b94.png

15. 2. Mutation: It is the sudden and heritable change in an organism and is generally due to a structural change in a gene It may produce a new allele not present in the population or may change the frequencies of existing alleles A mutate to a = Forward mutation = u a mutate to A = Reverse mutation = v 15 Figure: Mutation Source:https://cdn.kastatic.org/ka- perseus -images/2cf6d47d322b51ad0abd703dad930dafdbfc4aa7.png

16. 3. Random drift/genetic drift: It means a random change in gene frequency due to sampling error It occurs in small populations because sampling error is greater in smaller population than in a larger one; ultimately, the frequency of one of the alleles becomes zero and that of the other allele becomes one 16 Figure: Genetic Drift (Sampling error) Source:https://cdn.kastatic.org/ka- perseus -images/9d0a0e90f3657daec575e5098348f8a1bd2ed3c6.png

17. 4. Selection A differential reproduction rate of various genotypes is known as selection . In selection, the desirable genotype is selected  If plants with AA or aa genotypes are selected, the frequency of A allele in the selected population would be 1 or 0 respectively Selection in a random mating population is highly effective in increasing or decreasing the frequency of alleles but it is unable to either fix or eliminate them  In combination with a system of inbreeding, selection is highly efficient in the fixation and elimination of alleles  The fitness of a genotype may be defined as its reproduction rate in relation to that of other genotypes 17 Figure: Natural Selection Source: https://cdn.kastatic.org/ka-perseus-images/8300b3c900b21e5ff2adb82af52492fce5c8cdc2.png

18. 5. Inbreeding Mating between individuals sharing a common parent in their ancestry is known as inbreeding .  Inbreeding increases homozygosity and reduces heterozygosity.  The rate of decrease in heterozygosity per generation in case of monoecious / hermaphrodite is 1/2N and in case of dioecious is (1/2N+1) where N represents the number of parents in the population. 18

19. The implications of the Hardy-Weinberg equilibrium concept for the plant breeder Random mating in natural populations will occur only in cross-pollinated species with population with infinite size. Random mating is reduced by variation in flowering time that prevent early and late plant from mating, variation in vigor of plants so that uneven no of flowers and seeds are produced Breeding populations are never large enough to obtain natural random mating but the breeding population needs to be sufficiently large that sampling errors may disregarded Sampling errors may be reduced if pollinations are made with mixture of pollen (at least 100 plants in corn) 19

20. Contd …. Random mating does not occur in natural populations of self-pollinated crop species and gene frequency follow the Hardy-Weinberg equilibrium and remain constant Selection by breeder for or against a particular allele or group of alleles contributing to the quantitative character will change the frequency of the allele or alleles in the population Selection for a dominant allele in a limited no of generation will not completely eliminate the recessive alleles from the populations, because the homozygous dominant and heterozygous phenotype cannot be distinguished from each other. But selection for homozygous recessive alleles will eliminate the dominant alleles from the population in one generation 20

21. Acknowledgement I’d want to convey my heartfelt appreciation and gratitude to our teacher, Mr. Mohan K. Bista (Asst. Professor, Genetics and Breeding) for providing me with the chance to work on this topic, “System of Mating and Hardy Weinberg Law” which also aided me in conducting extensive study and learning about a bunch of new topics and ideas Second, I’d want to thank my friends for their assistance in completing this project in such a short period of time It was quite useful in terms of expanding my knowledge and abilities 21

22. Thanks for your Attention ! ! ! The floor is now open, if you have any queries feel free to ask ! ! ! 22