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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