What do you think of when you hear POPULATION GENETICS?

• Population = a group of individuals of the same species occupying a given area at a certain time

• Genetics = the study of heredity

Population Genetics = the study of heredity within a given population

• Species a group of organisms that actually (or potentially) interbreed in nature and are reproductively isolated from all other groups

• Gene pool all the alleles of all the genes of all the individuals in a population

• Evolution cumulative changes in the gene pool (and therefore changes in characteristics of populations) of organisms from one generation to the next

Natural Selection

• Darwin studied variation in phenotypes

• Natural selection some traits are better equipped for survival in a given environment– They are “naturally selected” for through

survival of the fittest– E.g. short beaked finches vs. long beaked finches

eat seeds eat grubs

– E.g. ??

The Importance of Variation• Variation among

organisms is not only restricted to physical appearance– What other variations can

be seen between organisms?

• Metabolism, fertility, mode of reproduction, behaviour etc. etc.

• In order to have variation amongst organisms, we must have variation in our genes Albatross’ elaborate and unique courtship

behaviour

Variation as a Means for Survival

• Certain genotypes are better equipped than others for survival

• Sexual reproduction variations passed on to offspring

• Leads to natural selection for individuals in given conditions.

Studying Human Genetic Traits

• Studying humans is problematic– WHY??

• Few offspring• Slow reproductive time

• How do we study human populations then?

• POPULATION SAMPLING!!

Population sampling finding the gene frequencies for a particular genetic trait in a small sample of the population and applying them to the whole population

Trends are studied in a small population sample and then applied to the whole population

Trends Gene Frequencies!

Gene pool all of the genes that occur within a specific population

Gene pool size increases as the population size increases!

Sometimes, gene frequencies are associated with certain populations

Swedish people mostly blond haired and blue eyed

- the frequency of blond hair and blue eyes is HIGH in this population

Any other examples?

When studying GENE FREQUENCIES…

Must use…

HARDY WEINBERG EQUILIBRIUM!!!

rrRrr

RrRRR

rR

FINDING THE HARDY-WEINBERG EQUILIBRIUM

Punnett square crossing two heterozygous individuals for tongue rolling ability

R = Tongue rolling

r = non-tongue rolling

rr

(q2)

Rr

(pq)

r

(q)

Rr

(pq)

RR

(p2)

R

(p)

r

(q)

R

(p)

Let p DOMINANT ALLELE frequency

Let q RECESSIVE ALLELE frequency

rr

(q2)

0.16

Rr

(pq)

0.24

r

(q)

0.4

Rr

(pq)

0.24

RR

(p2)

0.36

R

(p)

0.6

r

(q)

0.4

R

(p)

0.6

What do you notice about the numbers? 0.36 + 0.24 + 0.24 + 0.16 = 1

If we express this in algebra form, we get… p2 + 2pq + q2 = 1

Freq. RR 0.36

Freq. Rr 0.48

Freq. rr 0.16

p = Frequency of DOMINANT ALLELE

q = Frequency of RECESSIVE ALLELE

p2 = Frequency of HOMOZYGOUS DOMINANT

GENOTYPE (RR)

2pq = Frequency of HETEROZYGOUS GENOTYPE (Rr)

q2 = Frequency of RECESSIVE GENOTYPE

(rr)

ALLELES

GENOTYPES

HARDY-WEINBERG EQUILIBRIUM

p2 + 2pq + q2 = 1

p + q = 1

Assumptions of the Hardy-Weinberg Equilibrium (Text p. 560)

1. No mutations2. No migration no immigration or emigration

(no gene flow) 3. No selective advantage for any genotype 4. Random mating 5. Very large population size

H-W assumes no evolutionary forces are acting on a population to change the frequencies of different alleles or genotypes. The frequencies of alleles in populations are always changing but H-W equations help us to detect changes.

Hardy-Weinberg Examples

1.In a hypothetical population of birds, the dominant beak color is brown. A recessive allele for blue beak color occurs in a frequency of 0.4.

a) What is the genotypic frequency of the blue beaked trait?b) What is the genotypic frequency of the heterozygous condition?c) What is the genotypic frequency of the brown beaked trait?

2. In a hypothetical population of squirrels, the dominant fur color is black. A recessive allele for grey fur color occurs with a frequency of 0.3. What is the frequency of the heterozygous genotype?

3. Suppose that within a population of garter snakes, 408 of 1700 snakes exhibit a recessive stripe color. In the next generation, 528 out of a population of 2200 exhibit the same trait. Show the steps involved in determining whether evolution has taken place.

4. The frequency of individuals displaying the dominant phenotype is 0.36. Find p and q.

Hardy-Weinberg Examples

Hardy-Weinberg Practice

• H - W Practice sheet – I’ll hand out.• Blue workbook: pages 130 - 131 and 133 – 134• If you don’t have blue booklet today – make sure If you don’t have blue booklet today – make sure

to bring it tomorrow – we need it for an activity to bring it tomorrow – we need it for an activity we’re doing on pages 123 – 125. Bring a pencil we’re doing on pages 123 – 125. Bring a pencil and calculator too. and calculator too.

• H – W Quiz on Thursday.

Try this link for an online quiz for Hardy-Weinberg equilibrium