Ch.14 The Genetics of Populations Flashcards
Population genetics
Examines the genetic composition of groups of individuals and how this composition changes over time.
Investigates distribution of phenotypes, genotypes, and alleles.
Basic research, conservation of threatened and endangered species.
Applications in Health Sciences and Public Health (infectious diseases and inherited disorders)
Economic impact: DNA ancestry
Inherited Disorders in Human Populations
Carrier frequency of a population: Number of individuals with the heterozygote genotype/Total population = 2pq. (probability of having 1 allele and probability of having 2nd allele.
Occurrence of inherited disorders in a population (monitored by the CDC in the US):
EX: if we know that 1 in 1700 US Caucasian newborns are estimated to be born each year with cystic fibrosis, pharmaceutical companies know how much medication to produce.
When Darwin met Mendel
Actually they didn’t.
Both of them studied heritable traits with variation (and these are genes with alleles).
In early 1900s, geneticists tried to merge Darwin’s ideas with Mendel’s work.
That was the basis for the field of Population Genetics.
Natural Selection explains how allele frequency changes over time.
Charles Darwin
A British biologist who published a book called “On the origin of species by means of natural selection” in 1859.
Darwin’s book explained his ideas about evolution (although he never used this term).
He proposed that populations change over time and the mechanism for this he called Natural Selection.
Darwin made 2 observations
1) Members of a population vary in their inherited traits
2) All species can produce more offspring than the environment can support, and many of these offspring fail to survive and reproduce (mushrooms expel millions of spores, not all will grow into a new mushroom)
Darwin drew 2 inferences
1) Individuals whose inherited traits five them a higher probability of surviving and preproducing in a given environment tend to leave more offspring that other individuals.
2) This unequal ability of individuals to survive and reproduce will lead to the accumulation of favorable traits in population over generations.
Mechanisms of natural selection
1) Population with varied inherited traits (beetles are light and dark)
2) Elimination of individuals with certain traits (light get eaten)
3) Reproduction of survivors (dark reproduce)
4) Increasing frequency of traits that enhance survival and reproductive success (dark coloration increases)
-> population genetics
Environment favors the dark ones bc they can better camouflage against the dark background.
(Mendelian) Populations
A group of individuals of the same species, living in the same place at the same time, interbreeding and sexually reproducing.
Populations may be isolated from one another.
Each population has a gene pool.
A populations genetic composition can be described by?
Its genotype and allelic frequencies.
Gene pool
The sum of all genes within the individuals in this population.
The Darwin and Mendel Problem
If Mendel’s F2 generation always showed a 3:1 ratio… does that not mean that the white flowers should disappear from the population over time??
In other words: recessive alleles would disappear from a population, while dominant alleles would become more common.
Does this mean genetic variation disappears over time?
Does this mean that the recessive allele is less beneficial?
(This puzzled ppl. Not necessarily true - recessive allele not necessarily less common, not less beneficial)
Eye color
6000-10,000 yrs ago, an autosomal recessive mutation in the gene located next to the OCA2 gene (limits melanin deposition in the iris) lead to the blue eye allele.
Eye color punnett square
Ration brown:blue eyes = 3:1 (both parents heterozygous)
Today: 40-60% of Europeans have blue eyes. Doesn’t really change much over the yrs.
The idea that recessive allele disappears from the population bc 3:1, is wrong.
Synthesis of Darwin’s ideas and Mendelian rules
Englishman Reginald Punnett asked his friend Godfrey Hardy for advice (a mathematician).
In 1908 Hardy published an essay solving the problem.
Also in 1908, the German physician Wilhelm Weinberg presented his solution.
Later in 1943 the equations to calculate allele frequencies in a population were named after the 2.
Hardy-Weinberg-equation
Mathematical proof that allele frequencies remain constant over time under certain conditions.
Hardy - Weinberg Principle
Within a sexually reproducing, diploid population, allele and genotype frequencies will remain in equilibrium (remain constant) unless outside forces act to change those frequencies.
Sexual reproduction alone does not change the allele frequencies in a population
Calculating allele and genotype frequencies
If there are 2 alleles at a locus, A and a, their frequencies p (#A/total) and q (#a/total) will add up to 1.
p+q=1
If there are 2 alleles, the probability of
Genotype AA (=frequency): pxp=p^2
Genotype aa (=frequency): qxq=q^2
Genotype frequencies also have to add up to 1:
(p+q)^2=1
Hardy Weinburg Equation
p^2+2pq+q^2=1
p^2 = homo dominant frequency
2pq = probability (of frequency) of the heterozygotes
q^2 = homo recessive frequency
If you know frequency of 1 genotype you can calculate frequencies of other genotype.
Blue footed booby population
2 alleles in a population, W and w
Uppercase W is a dominant allele for a nonwebbed booby foot.
Lowercase w is a recessive allele for a webbed booby foot.
Consider the gene pool of a pop of 500 birds:
320 (64%) homo dominant (WW)
160 (32%) heterozygous (Ww)
20 (4%) are homo recessive (ww)
Allele frequencies of blue footed booby
320/500 = .64 160/500 = .32 20/500 =.04 640W, 160W + 160w (from .32), 40w 640W+160W = 800/1000 = .8W 160w + 40w = 200/1000 = .2w
Next generation of blue footed booby
Gametes reflect allele frequencies of the parent gene pool:
The probability that a sperm or egg carries W=0.8 or 80%.
The probability that a sperm or egg carries w=.2 or 20%.
Gametes reflect allele frequencies of?
Parental gene pool.
Ww x Wx
Allele frequencies = .8W, .2w
The frequency of the recessive allele has remained the same!
The Hardy-Weinberg equation is fulfilled (the allele frequencies don’t change) with the following assumptios:
- No mutations
- Random mating (no preference)
- Extremely large population size (no genetic drift)
- No gene flow (migration)
- No natural selection
If any of these assumptions is not fulfilled, the allele frequency will change, and it can be concluded that evolutionary change is happening in that population, concerning this particular trait.
No mutation
When a mutation introduces a 3rd allele, or causes an allele to disappear, the allele frequencies will change from 1 generation to the next.
Random mating
HWE assumes that potentially all individuals in a population mate with each other without preference.
Ex: If a bird with webbed feet prefers to mate with birds with webbed feet, the allele frequencies would change.
(doesn’t work if there’s preferential mating, mate choice)
Large population size (no genetic drift)
The smaller a sample, the greater the chance of deviation from a predicted result.
Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next through random changes in allele frequencies;
Genetic drift tends to reduce genetic variation through losses of alleles.
Genetic drift: The founder effect
When few individuals become isolated from a larger population, the new population will have a different gene pool than the source population;
EX: When a few individuals inhabit an island, the new island population will have its own unique gene pool.
For ex: some PTC tasters and non tasters get stranded on a remote island; future allele frequencies on this island will be different from the source population.
Polydactyly in the Amish population in Pennsylvania
In the early 1700s a small group of 200 German immigrants reached North America.
The eastern Pennsylvanian Amish ppl are descendants of this group.
One of these immigrants carried the mutation for Ellis-van Creveld syndrome.
This syndrome can result in polydactyly.
Today 5 of 1000 Amish babies in Pennsylvania have this syndrome. Compared to 1 in 60,000 in the general population. (also an ex of nonrandom mating)
Genetic drift: The bottle neck effect:
Can be caused by a severe drop in population size (change of environment, fire, flood). By chance, certain alleles may be over- or underrepresented.
For ex: a wild fire kills all but 1 PTC taster; the surviving population will have different allele frequencies than the original population.
Gene flow
Movement of alleles among populations.
Means that alleles can be removed or added to the gene pool through the movement of fertile individuals or gametes.
Ex: If all but one PTC nontaster leave the population, or if many more tasters migrate into the population, the allele frequencies will change.
(no individual leaves/ joins the group)
Natural Selection
Different success in reproduction results in certain alleles being passed to the next generation in greater proportions.
Ex: tasting or nontasting PTC has neither advantage or disadvantage for the individuals; but if the ability to taste PTC makes you also better in tasting (and avoiding) spoiled food, tasters would have higher survival and reproductive rates than nontasters and the next generation would have a higher frequency of the tasting allele.
What is an allele frequency?
Number of alleles in the population/total number of alleles.
When allele frequencies persist generation after generation in a population, we call this:
Hardy-Weinberg equilibrium
Genetic drift can lead to:
The fixation of one allele at a locus and the loss of all other alleles.
In the formula for determining the population’s genotype frequencies, the 2 in the term 2pq is necessary bc
heterozygous can come about in 2 ways
In the formula for determining the population’s genotype frequencies, the pq in the term 2 pq is necessary because
heterozygotes have 2 alleles
In a hardy weinberg population with 2 alleles, A and a, that are in equilibrium, the frequency of the allele a is 0.3. What is the percentage of the population that is homozygous for this allele?
9%
Over time, the movement of people on Earth has steadily increased. This has altered the course of human evolution by increaseing
gene flow
What term is given to the total genetic information carried by all members of a population?
gene pool
Albinisms is an autosomal recessive trait in humans. Assume that there are 100 albinos (aa) in a population of 1 million. How many individuals would be expected to be homozygous normal (AA) under equilibrium conditions?
980,100
In small isolated populations, gene frequencies can fluctuate considerably. The term that applies to this circumstance is
genetic drift
