Mating systems (population genetics)

Random and non random mating in population genetics

1. Types of Mating : (i) Random Mating and (ii) Non-Random Mating with suitable examples TOPIC

2. Introduction And History Mating may be defined as the method by which individuals are paired for crossing. Or various schemes which are used for crossing or mating of individuals. Five systems of mating was given by Sewall Wright in 1921

3. American geneticist known for his influential work on evolutionary theory. He was a founder of population genetics alongside Ronald Fisher and J.B.S. Haldane, which was a major step in the development of the modern synthesis combining genetics  with evolution. He gave Five systems of mating in 1921 Sewall Wright

4. Variation Of Mating Systems in Plants Plants vary in their mating system from completely selfing to completely outcrossing. Anther – stigma distance is a useful measure of mating system. Anther stigma distance determine if the mating system differed between the two species

5. Types of mating Systems There are five different types of mating systems Random Mating Genetic Assortative Genetic Disassortative Phenotypic Assortative Phenotypic Disassortative

6. Random Mating System Each Female gamete has equal chances to unite with every male gamete. It’s a form of outbreeding In plant breeding some form of selection is practiced such mating system called as random mating with selection. With selection- Increase frequency of alleles for which selection is practiced. Reduce frequency of other alleles

7. 3. Increase variance These changes are more pronounced when the character is highly heritable and is governed by one or a few genes. Random mating in small populations is unable to prevent an increase in homozygosity due to inbreeding and genetic drift.

8. Rate of reproduction of each genotype is equal Without selection- Gene frequency – constant Variation for character – Constant Correlation between relatives or prepotency – constant Degree of homozygosity - Constant

9. Uses of Random mating in Plant Breeding Used for Progeny testing Production and maintenance of synthetic and composite varieties Production of polycross progenies Evolutionary advantages – maintained high level of diversity

10. Genetic Assortative Mating System Mating occurs between individuals that are more closely rated by ancestry than in random mating. More commonly known as inbreeding. Without selection – Increased total variability among lines Decreased total variability within lines due to random fixation of genes in different families.

11. b. With selection – Variability is reduced towards the direction of selection Homozygosity – Increased due to fixation of genes Heterozygosity – Elimination of heterozygotes from a population due to fixation of genes. Population mean – Reduced due to decrease in number of hybrid genotypes which have more number of dominant genes. Genetic correlation – Increased due to increase in prepotency.

12. Uses of Genetic Assortative Mating System Leads to purity of types. Useful tool for development of inbred lines both partial and complete.

13. aa a a aa homozygous gene pairs 1 Homozygous freq. p=q=0.50=D+1/2 H 1 1 2 1 D +R 50.0 0.50 2 4 + 2 4 2+4 D + 1/2H +R 75.0 0.50 3 24 + 4 8 4+24 D + 3/4H +R 87.5 0.50 4 112 + 8 16 8 +112 D+ 7/8 1H+R 93.75 0.50 5 480 + 16 32 16 +480 D +15/16 H+R 96.86 0.50 t 2t-1 :2 2'-l 050 Limit D O R D + R Single locus two alleles case-selfing: Considering the single locus two allelic system and that each plant produced 4 seeds each generation, the relative frequencies of 3 genotypes under continued selfing starting to starting from F 1 hybrid (An) in an ideal population are given below; Generations Genotypes Freq.of Percentage Gene

14. The selfing of both the homozygotes will breed true whereas the heterozygotes under selfing will produced ¼ AA,1/4 aa and ½ Aa. Thus it is obvious that the percentage of homozygous genotype increase in each generation and heterozygous genotype is decreased. However, this will not bring a change in gene frequencies. The heterozygotes are reduced to half, in each generation. The one half of the heterozygous (Aa) reduced are converted into identical homozygous for both alleles (AA and aa-both increased in equal proportion). The relative frequencies of the 3 genotypes in any generation become 2 ' -1 : 2 : 2'-1 from the ratio 1:2:1 in the first generation under random mating. Thus the proportion of heterozygote in any generation (t) under selfing in a population become (1/2) t instead of ½ in the first generation under random mating.

15. The homozygosity is increased at the expense of heterozygotes in each generation and the proportion of homozygotes in t generation become 1-(1/2) t . The change is maximum in the first generation after which the rate of change is decreased, though the proportion of heterozygotes are reduced to half in each generation. The reduced half proportion of heterozygotes in each generation is covered into homozygotes for two alleles (AA and aa) in equal proportion. Therefore, under continued selfing. the heterozygotes ultimately are reduced to zero. Consequently the identical homozygotes are increased at the expense of heterozygotes each generation and become equal to the proportion of initial gene frequencies. This produces two distinct lines, one homozygous for AA and other for aa. This leads to gene fixation which is for different genes in different lines.

16. Conti….. For example, consider the initial gene frequencies as p(A) =0.2 and q(a) =0.8. After many generations of selfing ,the proportion of AA homozygotes will become equal to 0.20 and of aa homozygotes as 0.80. Thus the consequence of selfing is to convert a diploid (p2+ 2pq +q2) population into ( p+q ) diploid population . Conclusively , the recurrence relationship of reduction in heterozygosity in any t generation ( Ht ) is : Ht = ½ Ht-1 = (1/2)t H0 Where H0 is the heterozygosity in base population.

17. Genetic Disassortative mating system Such individuals are mated which are less closely related by ancestry than random mating. Commonly called as outbreeding. Totally unrelated individuals are mated. These individuals belongs to different populations. eg . Intervarietal & Interspecific crosses.

18. Variability – Increased due to combination of two or more genes from two or more different sources. Heterozygosity – Increased due to combination of genes from different lines. Homozygosity – Reduced rapidly because outbreeding favours heterozygotes. E. Population mean – Increased due to combining more dominant genes from different lines F. Genetic correlation – Decrease due to decrease in homozygosity. G. Decrease in prepotency.

19. crosses between genetically contrasting individuals are made in this type mating that intermediate type. AA x aa Aa intermediate type

20. Phenotypic Assortative mating System Mating between individuals which are phenotypically more similar than would be expected under random mating Refers to mating of extreme types, i.e., cross between AA & AA and aa & aa, also Aa & aa Only two extreme phenotypes i.e., lowest and highest remain in the population Variability : Increase since it divides the population into two extreme phenotypes.

21. Homozygosity : Leads to complete homozygosity in single generation Genetic correlation : Perfect genetic correlation between number of progenies is achieved in one generation. Population mean : Divided into two according to variability

22. USES OF PHENOTYPIC ASSORTATIVE MATING SYSTEM In some breeding schemes like recurrent selection Useful in isolation of extreme phenotypes. The changes due to this mating system are disappear randomly when random mating is restored

23. Examples of assortative mating in humans Dwarfs : very high positive assortative mating, individual with achronoplastics dwarfisms pair up much often than would be expected by chance IQ : slight positive assortative mating Height : slight positive assortative mating Red hair : moderate negetive assortative mating- red hair haired individual pair up less often than would expected by chance .

24. PHENOTYPIC DISASSORTATIVE MATING SYSTEM Mating between phenotypic dissimilar individuals belonging to same populations. I.e., mating between individuals having genotypes AA & aa and Aa & aa Variability : Constant, since it reduces inbreeding. Heterozygosity : Remains constant or slight increase

25. Genetic correlation : Decrease due to decease in prepotency. Prepotency : Decrease due to decrease in homozygosity. Gene frequency : remain constant or sometime may be slight increase in the heterozygosity. Mating of dominant x reccessive are included in this type of mating AA or Aa x aa Aa or aa

26. USES OF PHENOTYPIC DISASSORTATIVE MATING SYSTEM In making population stable i.e., Maintaining variability Progeny more desirable than parents. Useful when desirable type is an intermediate one and the available parents have the extreme phenotypes. Most notable – maintaining variability in relatively smaller population.