Genes In Population 5b Notes

Genes in population

Hardy-Weinberg principle: relative proportions of different gentoypes and phenotypes remain constant from one generation to another

-The assumption is that there are only 2 alleles for a particular gene in the population

-The gene pool= total alleles in the population (2 x population number)

-p= probability of a dominant allele, q= probability of a recessive allele so p+q=1

-p² = probability of Homozygous for the dominant Allele

-q² = probability of Homozygous for the recessive allele

-2pq= probability for being heterozygous

- p² + 2pq + q² = 1

-Use: allows us to predict the incidence of diseased individuals and unaffected carriers is p and q are known

-In order to calculate frequency of carriers you must do 2 x p x q not just p x q

-This theory is true for the ideal population

-Large

-Random mating –selection of a partner regardless of the partner’s genotype

-no new mutations

-no selection for or against any particular genotype

-This is seen for neutral genes such as blood groups, enzyme variants

-However the Hardy-Weinberg equilibrium can be disturbed by influencing the distribution of genes or altering the gene frequencies

a) Non random mating

-leads to an increase in the frequency of affected homozygotes. This changes the frequency of the recessive and dominant alleles. This is either by

-Assortative mating- tendency of human beings to choose partners that share similar characteristics-height, intelligence, racial origin

-Consaguinity- term used to describe marriage between blood relatives who have at least one common ancestor. Widespread consanguinity will lead to relative increase in frequency of homozygotes and decrease in frequency of heterozygotes. In the case of rare recessive disorders, this results in more homozygotes than predicted by Hardy Weinberg, this hastens selective removal of bad recessive alleles and increase of good ones

b)Mutation

-mutations are the source of variation. Most are neutral but some are detrimental or advantageous

-However it is likely that these are quickly eliminated unless there is a compensating advantage

c) Selection

-the vast majority of new mutations have no effect on gene activity or function. Some can reduce/increase fitness of the carrier. It is the changes in fitness that leads to selection

Fitness is the measure of the ability of an individual to survive and reproduce

-some conditions have early and devastating onset-reduce fitness to zero

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