Variation within a species can be attributed to many thing: Mutations in genes Independent assortment of homologous

chromosomes during meiosis 1 Independent assortment of sister chromatids

during meiosis 2 The crossing over of homologous

chromosomes during meiosis

A gene pool refers to all of the genes that occur in a population

The frequency of some genes stays consistent over generations. Some frequencies change very rapidly, especially if they are maladaptive to survival or reproduction.

Certain gene frequencies are associated with certain populations.

Smooth peas are either AAor Aa Wrinkled peas are aa

Population biologists and geneticists like to know what the frequency of these alleles are.

A may occur in AA or Aa. Perhaps the A allele has a frequency of 0.78 in a population

a may only occur in aa. Its frequency in the population is 1-0.78=0.22

Many traits have two forms: dominant allele and recessive allele. (ie. Smooth or wrinkled pea)

We call the dominant allele “p”

We call the recessive allele “q”

So “A” becomes “p” and “a” becomes “q” when talking about alleles.

p=frequency of allele “A"q=frequency of allele “a“

p + q= 1

p2 + 2pq + q2= 1

AA + Aa + aa=1

(these are genotype frequencies, not allele frequencies!)

If all other factors remain constant, the gene pool will reach genetic equilibrium (the gene pool will be the same generation after generation).

No evolution occurs, unless something disrupts the equilibrium.

The conditions under which NO CHANGE will occur in a gene pool are: Large population Random mating No mutations of

genes No migration (no

new genes entering!) Equal fertility and

mating ability of all genotypes

p + q = 1p2 + 2pq + q2= 1

AA Aa aa

If the values of p and q are known, you can calculate the freuqncy of the three genotypes “AA” “Aa” “aa”

If the frequencies of the three genotypes are known, you can calculate the frequency of the alleles “p” and “q”

What are the allele frequencies of F and f in the population?

What percent of the population are homozygous dominant, heterozygous, and homozygous recessive for trait “F”?

Suppose a recessive genetic disorder occurs in 9% of the population. How many of the population carry the recessive gene but do not have the disorder (How many are heterozygous??)

Since the disease only occurs in homozygous recessive individuals, the frequency of the aa genotype is 0.09.

p2 + 2pq + q2= 1

q2=aaq2=0.09√0.09=0.3 therefore, frequency of a is 0.31-0.3=0.7 therefore, frequency of A is 0.7

0.72 + 2(0.7)(0.3)+ .32= 10.49 + 0.42 + 0.09=1 42% are

heterozygous

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Disturbs equilibrium Allele frequencies in a population change

randomly over time due to chance This will occur more in smaller

populations, because 1 change will effect most of the population

Also known as immigration or emigration

Movement in or out of a population will change the gene pool.

New species must arise from existing species

Two ways in which a new species can arise are: Geographic isolation Reproductive isoloation

Populations become isolated and must adapt to their new environment.