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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-Weinbery 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
??
1
23 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