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

1 2 3 . . .

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 individuals60 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 = p2 +2pq + q2 = 1

AA = p2, aa = q2 and Aa = 2pq

Hardy-Wienberg EquilibriumAssumptions

• 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• q2 = aa

• Therefore q = square root of 0.01 =0.1• p= 1 - 0.1 = 0.9• AA = p2 = 0.81• Aa = 2pq =0.18• aa = q2 = 0.010

How the Hardy-Wienberg Equilibrium can be used!

Extension to Hardy-Wienberg

• Three alleles

p12 + 2p1p2 + 2p1p3 + p2

2 + 2p2p3 + p32 = 1

Genotype frequency

• 3 alleles therefore 6 genotypes• genotype q2 =.01, z2 = 0.04

genotypes = q2+z2+p2+2qz+2zp+2pqalleles p+q+z=1Genotypes are:q= 0.1, z= 0.2, p= 0.7p2= 0.49q2=0.01z2=0.042pz=0.282pq=0.142zq=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 flowcounters genetic drift.

FOUNDER EFFECT – refers to the chance genecombination in newly founded population (the variation in a new pop. Generally less thanvariation in the source population).

GENETIC BOTTLENECK – when a population is reduced tonumbers but then recovers some genetic variation isgenerally lost.

Other Processes

What generalities and patterns are there?

Selection is a change in allele frequencywhich is directional, NOT random.