Genes within Populations

What is a population? How are populations characterized? What does it mean to be diploid, haploid, polyploid? How can we characterize populations based on their genes?

What is a population? A group of individuals of the same species that have a high potential of interbreeding Share a common gene pool

Phenotype & Genotype The phenotype is the expression of the genetic material (the genes) of the chromosomes. The genotype relates to the alleles found at loci on the chromosomes

How many alleles can an individual have at a locus? How many alleles can there be in a population at a specific locus?

Determining the genotype for an enzyme in a fish.

IDHP (isocitrate dehydrogenase) from a grasshopper Individuals AA aa Aa

Genotype There are 3 Genotypes  AA and aa (Homozygous), and Aa (Heterozygous) there are 2 alleles A and a

Genotype frequency If there are 100 individuals 60 are AA 30 are Aa 10 are aa Genotype frequency AA= 60/100= 0.6 Aa = 30/100= 0.3 aa= 10/100 = 0.1 Total = 1.0

Allele Frequency in 100 individuals there are 200 alleles 60 indiv. AA = 120 A 30 indiv. Aa = 30 A and 30 a 10 indiv. aa = 20 a Therefore A = 150 A=150/200 = 0.75 a = 50 a=50/200 = 0.25

Phenotype there are 2 phenotypes trait “X” (AA and Aa) and trait “Y” (aa)

Phenotype frequency trait X = 90 indiv. (AA and Aa) trait Y = 10 indiv. (aa) Freq. X = 90/100 = 0.9 Freq. Y =10/100 = 0.1

Hardy-Wienberg Equilibrium If p = proportion of allele A and if q = proportion of allele a Then p+q = 1 Hardy-Wienberg Equilibrium gives the expected frequency of the three genotypes as: (p+q) 2 = p 2 +2pq + q 2 = 1 AA = p 2, aa = q 2 and Aa = 2pq

Hardy-Wienberg Equilibrium Assumptions Population size is very large Random mating is occurring No mutation is taking place No immigration (geneflow)‏ No selection is occurring

If the proportion of genotype aa in a population = 1% or 0.01 aa = 0.01 q 2 = aa Therefore q = square root of 0.01 =0.1 p= = 0.9 AA = p 2 = 0.81 Aa = 2pq =0.18 aa = q 2 = 0.010

How the Hardy-Wienberg Equilibrium can be used!

Extension to Hardy-Wienberg Three alleles

p p 1 p 2 + 2p 1 p 3 + p p 2 p 3 + p 3 2 = 1 Genotype frequency

3 alleles therefore 6 genotypes genotype q 2 =.01, z 2 = 0.04 genotypes = q 2 +z 2 +p 2 +2qz+2zp+2pq alleles p+q+z=1 Genotypes are: q= 0.1, z= 0.2, p= 0.7 p 2 = 0.49 q 2 =0.01 z 2 =0.04 2pz=0.28 2pq=0.14 2zq=0.04

Populations share a common gene pool! What does this mean? At each gene locus a population will be characterized by a particular allele frequency. The combination of allele frequencies is what characterizes a population and potentially makes populations unique.

What will cause deviation from Hardy- Wienberg Equilibrium?

Genetic Drift For small populations random chance may result in the loss of an allele! Results in Fixation Loss of heterozygosity

Genetic drift is a random process

What is inbreeding and why is it bad?

Inbreeding alleles are common by descent

Genes common by descent Loss of heterozygosity No loss of alleles

GENE FLOW – result of dispersal (an individual leaving one population and entering another population). Gene flow counters genetic drift. FOUNDER EFFECT – refers to the chance gene combination in newly founded population (the variation in a new pop. Generally less than variation in the source population). GENETIC BOTTLENECK – when a population is reduced to numbers but then recovers some genetic variation is generally lost. Other Processes

What generalities and patterns are there? Selection is a change in allele frequency which is directional, NOT random.