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
1.Random Mating
2.Genetic Assortative
3.Genetic Disassortative
4.Phenotypic Assortative
5.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-
1. Increase frequency of alleles for which selection is
practiced.
2. Reduce frequency of other alleles

7. 3. Increase variance
4. These changes are more pronounced
when the character is highly heritable
and is governed by one or a few genes.
5. 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-
1. Gene frequency – constant
2. Variation for character – Constant
3. Correlation between relatives or prepotency
– constant
4. 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 –
1. Increased total variability among lines
2. Decreased total variability within lines due to
random fixation of genes in different families.

11. b. With selection –
1. Variability is reduced towards the direction of
selection
2. Homozygosity – Increased due to fixation of genes
3. Heterozygosity – Elimination of heterozygotes from
a population due to fixation of genes.
4. Population mean – Reduced due to decrease in
number of hybrid genotypes which have more
number of dominant genes.
5. 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 Aa 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 F1hybrid (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. A. Variability – Increased due to combination of
two or more genes from two or more different
sources.
B. Heterozygosity – Increased due to combination
of genes from different lines.
C. 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.