Download Topic 4:Forces that change gene and genotype frequencies File

Forces that change Genotype
and Gene Frequencies
Destabilising factors of the HW
equilibrium
Lecture 4
Aim
• To introduce to students factors that
can influence changes in gene and
genotype frequencies in a
population
Objectives
• By the end of the lecture, students
should be able to
• List factors that can change gene and
genotype frequencies in a population
under HW Equilibrium
• Discuss examples of each force that
destabilizes the HW Equilibrium
• Appreciate the need to change gene and
genotype frequencies in livestock
improvement
Forces that change Genotype
and Gene Frequencies
• These are factors that cause changes in
frequencies
• And destabilise the HW equilibrium
• These factors include
• Migration
• Mutation
• Selection
• Random genetic drift
• Non-random mating
Changing gene frequency is only way of improving
breeds genetically
Migration
Migration is the introduction of new genes
from another population
This can be through mixing of populations
by way of importing live animals, semen,
embryos etc into the existing population
This system has been widely explored in
most tropical countries
Through introduction of exotic breeds
Migration
Migration can be a rapid way of
changing gene and genotype
frequencies in a population
As an example, assume there is a large
population of animals, let us call this
population, 1.0
Suppose in this population, there are
migrants, whose proportion is
m
The proportion of the natives (indigenous)
is therefore
1-m
Migration
Summary
 Population
 Migrants
 Natives
1.0
m
1–m
Assume we deal with a gene whose
frequency is q
The gene frequency
 among natives is
among migrants is
In mixed population
qo
qm
q1
Migration
We need to find gene frequency of the
mixed population,
q1
q1 =
q1 =

=
mqm + (1-m)qo (product of two)
mqm + qo – mqo
m(qm – qo) + qo
mqm is the contribution of migrants to the
population
(1-m)qo is the proportion of genes coming
from the natives
Migration
• There is need to find out how much has
arisen due to introduction of new genes
• The change in gene frequency due to
one generation of immigration is the
difference between frequency before
migration and the frequency after
migration
• Δq =
q1 – qo
•
= m(qm – qo) + qo –qo
•
= m(qm – qo)
Migration
• The rate of change of gene frequency in
a population subject to migration will
depend on
– Migration rate
• The proportion of animals brought into the
population
– The difference in gene frequencies between
immigrants and native
• There is need for a large proportion of
immigrating animals and a large
difference in gene frequencies in order to
make an impact with migration
Migration
• Practical situations include introduction of
Holstein Freisian dairy cattle from Canada
to Malawi
• Introduction of BA chickens into local
chicken population
• In both cases,
• check numbers imported whether they are
adequate
• Check difference on performance trait being
targeted between Exotic and local breed
Migration, Example
Assume Malawi goat population has
gene frequency
qo = 0.4
20 % of genes are introduced into the
population from Saanen,
Gene frequency of Saanen immigrants is
qm =
0.7
We need to find the change in gene
frequency
Migration, Example
First find q1
The determine Δq
The purpose of immigrants is to raise gene
frequency of a trait of interest in the
mixed population
Migration, Example
Check scenarios of changing gene
frequency and proportion migrating into
a population ~ In Excel
Note that practical application of
migration is to introduce a few animals
and then start selection within that
population
Gene mutation
Gene mutation refer to sudden heritable
changes in genetic material.
It is a change of gene from one form to
another
This change result from errors in the DNA
replication
Also called Point mutation
Mutation can also be induced due to
mutagens e.g. ionising radiation, some
chemicals
Gene mutation
The basis is a change in base composition
of DNA
There can be addition, deletion or
substitution of one or several base pairs in
the DNA molecule
Mutation can also affect more than one
gene, some affect the whole
chromosome
Mutation is one major element of
evolution
And through mutation, gene variability is
Gene mutation
Changes in base composition
Attributes of mutation
Mutation rates are very slow for any single
locus
These rates fall in the range of 1x10-6 to
1x10-8 or lower
Mutation are regarded as of little impact
on quantitative traits and therefore not
useful in livestock production
Also mutation that takes place tend to be
of no adaptive value since the mutants
tend not to fit the environment
Attributes of mutation
Mutation may occur in any direction
Let us consider genes
B
b
Gene frequency
p
q
Assume B is mutating towards b at the
µ
rate of µ
B
b
Attributes of mutation
After one generation, the contribution
towards b gene will be the rate of
frequency times the frequency B

q + µp
This is non – recurrent mutation ie. In
unidirection like from dominant to
recessive
Attributes of mutation
 Mutation could also occur in another direction
p + vq
v
Attributes of mutation
Mutation can be recurrent, occurring in
both direction
B
B
µ
b
v
b
Frequency of B gene after one generation
f(B) = p - µp + vq
f(b) = q + µp - vq
Attributes of mutation
The difference depends on size of µp and
vq

Δq = µp – vq
Called mutation net change
Non-random mating
• With random mating, each breeding animal has
an equal opportunity to mate with any animal of
opposite sex
• Non-random mating are assortative mating
situations where
• Those of same phenotype are allowed to mate ~ positive
assortative mating
– Alters genotype frequencies but not gene frequencies from
one generation to next
• Only those individuals of different phenotypes are mated
~ negative assortative mating or corrective mating
– Changes both gene and genotype frequencies from one
generation to next
Non-random mating
• Changes in gene and genotype
frequencies occur due to
disproportionate use of males as
compared to random mating
• Non-random mating affects
frequency of matings
Non-random mating ~
selection
• HWE operates under assumption
that all individuals in a population
contribute equally to the next
generation
• Contribution of offspring to next
generation is called ‘fitness’ or
‘adaptive value’ or ‘selective value’
of an individual
Selection
• If differences in fitness are
associated with presence or
absence of a particular gene in the
individual’s genotype,
• Then selection is operating on the gene
• That means parents of different
genotypes pass on their genes
unequally to the next generation
• Leading to frequency of a gene that is
subjected to selection differing between
parent and offspring
Selection
• The process causes a change in
gene frequency
• And consequently change in
genotype frequency
Go through details of selection in Falconer,
page 26, 3rd Edition
Significance of changing gene and
genotype frequencies of traits of
importance in livestock
• Selection increases gene frequency and in turn
increases performance
• Changing gene frequency only way to make
long term improvement
• Changing gene frequency is a permanent
change, and we can continue to make
progress, but we cannot change gene action
• Mating systems (random mating, assortative
mating, inbreeding, crossbreeding) can change
genotype frequencies, but one can lose what
has been gain
Important forces to livestock breeders
• Major forces that livestock breeders use
to change gene frequency are
• Selection
• Migration
• These can be predicted in both
magnitude and direction, unlike others
• To be discussed in detail
• Note that these forces operate on both
single locus and multiple loci traits