Chapter 12

When Allele Frequencies Stay the Same..

Population Genetics

• Branch of genetics that considers not individuals or families, but ALL of the alleles in a population.

Gene Pool

Gene Flow

12.1 Importance of Knowing Allele Frequencies

• Frequencies can be calculated for alleles, genotypes, or phenotypes.

Phenotypic Frequencies

• Determined empirically (observing how common a condition is in a population).

• Data supplied to genetic counselors

• Shifting allele frequencies in populations provide the small steps of genetic change = microevolution

• Frequencies change when any of the following occur:– Nonrandom Mating– Migration of individuals– Reproductively isolated subgroups form within a

larger population– Mutation occurs– Natural Selection

Hardy-Weinberg Equilibrium

• Hardy says, “I am reluctant to intrude in a discussion concerning matters of which I have no expert knowledge, and I should have expected the very simple point which I wish to make to have been familiar to biologists.”

• He was hearing biologist assume that dominant traits will tend to increase in populations, while recessive traits will become rarer

The famous equation…

• p² + 2pq + q² = 1.0• P² = homozygous dominant individuals• 2pq = heterozygous individuals• q² = homozygous recessive individuals

• The idea is that if the proportion of genotypes remains the same from gen to gen, then that gene is not evolving!

It’s rare but it is seen…

• Rare for protein encoding genes that affect phentoype.

• Occurs in repeated sequences that do NOT affect phenotypes – therefore not acted upon by Natural Selection

Solving Hardy Weinberg Equilibrium

• Within a population, the easiest to value to determine is q².

• q² q p p² 2pq

• Within a population of butterflies, the color brown (B) is dominant over the color white (b). And, 40% of all butterflies are white. Given this simple information, which is something that is very likely to be on an exam, calculate the five variables:

Practical Application of Hardy-Weinberg Equilibrium

• Truly determining risk for a couple.• What are the chances my child will have cystic

fibrosis?Known: Cystic fibrosis affects 1 in

2,000 Caucasians! 1/2000 = 0.0005 q² q p 2pq!

.0431

??

123 23

1 14

Why?!?

12.4 – DNA Fingerprinting – A practical test of Hardy Weinberg Assumptions

• Polymorphism – a site on a chromosome that varies within at least 1% of the population– Can be as small as one nucleotide (Snp)

• The ability to distinguish people based upon polymorphisms = basis of DNA Fingerprints

• Restriction Enzymes– Spots polymorphisms

DNA Fingerprinting and Hardy Weinberg

Variations of a gene are caused by altered proteins

Different Proteins = Different Sizes

Different sizes travel different lengths

If the frequency of +/+ (p²) = 1/3,000 then a DNA fingerprint will run multiple genes of known statistics!

WHAT ARE THE CHANCES ANOTHER PERSON DID IT??

Chapter 13

Changing Allele Frequencies

Nonrandom Mating

• Surveys show we marry people similar to ourselves 80% of the time.

• Worldwide, 1/3 of all marriages occur between people who were born fewer than 10 miles apart

• Prize bull can fertilize thousands of cows• Chinese Immigrant to South Africa – 7 wives (he

had a dominant disorder – teeth fall out)– 356 living descendents

• 70 have the dental disorder

Nonrandom Mating

• 1 in 200 Hopi Indians are albino. How can the culture of Indians cause the trait to be so prevalent?

• Mutation can cause individuals to be resistant to HIV.– How would this mutation cause a change in Hardy

Weinberg equilibrium over time?• Would you expect blood type frequencies to be

in Hardy Weinberg equilibrium? Why or why not?

Nonrandom Mating

• How can “Cystic Fibrosis Summer Camps” be a detriment to Hardy Weinberg equilibrium?

• In a very religious Jewish sect in New York City, marriages are NOT permitted between carriers of the same inherited disease– Arranged marriages AFTER genetic testing

• How does a consanguineous population affect the equilibrium? – Population of Sardinia (island near Italy) – 86% of the

individuals have the same X chromosome.

Migration

• How can allele frequencies tell a story of the past?

LOCATION PREVALENCE OF Galactokinase deficiency

Bulgaria 1:1600

Austria 1:153,000

Germany 1:157,000

France 1:2,200,000

Geographical Clues

• Allele frequencies that vary between neighboring populations = Cline

• Cline gradients can be gradual• Steep gradients often are a result of

geographical barriers• Evidence: The fertile lands along the Nile River– Mummies

Linguistic Clues

• Italy– Is the changing frequencies due to geography or

language?– Blood groups study across the region of Italy• Language groups were more closely related than

Geography based.

Small Population• Genetic Drift – the random chance of alleles

being passed on to the next generation

Founder Effect• When a small population leaves another and

starts a new colony. The new colony will develop a different allele frequency than the original.

Pennsylvania is a breeding ground…

• Germantown and The Dunkers

• Lancaster County Amish– 11 different disorders are more prevalent amongst

the Amish compared to the US population

Population

Blood Type U.S Dunker European

A 40% 60% 45%

B 15% 5% 15%

Rh- 15% 11% 15%

Haplotypes

• Haplotype – a group of VERY tightly linked genes on a chromosome

• Genetic drift leads to common diseases amongst populations but also other genes that are close to that locus.

Population Bottleneck

Mutation

• Many mutations in homozygous forms are lethal, but since they can be hidden in heterozygous the allele can be passed on.

• Genetic Load – the collection of deleterious alleles in a population.

Natural Selection and Heterozygote Advantage