Population Genetics
What is the effect of non-random mating in a population?
*Inbreeding* and both positive assortative and negative assortative mating with respect to specific phenotypes are all types of nonrandom mating. The main genetic consequence of inbreeding will be an increase in the frequency of homozygous genotypes in a population and a decrease in the frequency of heterozygous genotypes. *Therefore many inbred human populations suffer from a higher frequency of particular inherited genetic disorders*.
What are the types of natural selection?
1) Directional selection 2) Stabilising selection 3) Disruptive selection
What two predictions does the Hardy-Winberg Law make?
1) The frequency of the alleles in the gene pool does not change over time and contributes equally to the next generation 2) After one generation of random mating, the genotype frequencies for two alleles can be calculated as *p^2 + 2pq + q^2 = 1* where p2 is the frequency of AA individuals 2pq is the frequency of Aa individuals q2 is the frequency of aa individuals
What assumptions is the Hardy-Weinberg Equilibrium based on?
1) The population size is infinitely large 2) Random mating occurs 3) No natural selection 4) No migration 5) No mutation 6) No genetic drift *In nature, no population meets all the criteria for Hardy-Weinberg equilibrium*
What is a population?
A group of individuals belonging to the same species that live in a defined geographic area and interbreed with one another
How can we use DNA sequence data to identify genetic variation?
By comparing the nucleotide sequences of genes carried by individuals in a population
What is directional natural selection?
Directional natural selection will increase the frequency of certain alleles with variable intensity, *depending on the strength of selection*
What are the effects of migration on a population?
Effect of migration can substantially alter allele frequency in population As shown for the "I B allele of the ABO blod group" The I B allele has the highest frequency in central Asia. *The spread of the I B allele across Europe is attributed to Mongol migration after the fall of the Roman Empire.*
What is the founder effect in genetic drifts?
Establishment of a new population by a small number of founding organisms can produce a difference in allele frequencies The founding effect and genetic drift can result in high frequencies of autosomal recessive disorders that are rare outside the affected populations
What are the mathematical concepts behind the Hardy-Weinberg Equilibrium?
For example, a single autosomal allele with two alleles A and a: Frequency of A = 0.7 and a = 0.3 in a population. *Note: A + a = 1.0* So f(AA) = (0.7) x (0.7) = 0.49 (49%) and f(Aa) = (0.7) x (0.3) *x (2)* = 0.42 (42%) and f(aa) = (0.3) x (0.3) = 0.09 (9%) so 0.49+0.42+0.09=1
Example of the founder effect
For example, the Old Order Amish in Pennsylvania were established by a founding population of about 200, between 1720 and 1770. They exhibit high frequencies of a number of autosomal and X-linked recessive disorders that are rare in the populations of origin and the nearby non-Amish populations e.g. Ellis-van Creveld syndrome (form of dwarfism)
What is the effect of genetic drift in a population?
Genetic drift arises in small populations *where significant random fluctuations in allele frequences occur by chance alone* Degree of fluctuation increases as the population size decreasees Drift can also arise through *genetic bottlenecks* where populations undergo drastic, but temporary reductions in numbers due to natural disaster
What is Ellis-van Creveld syndrome?
It is an autosomal recessive skeletal dysplasia characterized by short limbs, short ribs, postaxial polydactyly and dysplastic nails and teeth and in many cases congenital cardiac defects. Ellis-van Creveld syndrome is found in 1 in 1000 individuals in Amish population. Ellis-van Creveld syndrome is estimated at 1 in 60,000 individuals in other human populations.
What is the Hardy-Weinberg equilibrium?
It states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences. It serves as a model to calculate the frequencies of alleles and genotypes in a population N.B. *In nature, no population meets all the criteria for Hardy-Weinberg equilibrium*
What is another example of the Founder effect?
Native Americans living in the southwestern United States have a high frequency of oculocutaneous albinism (OCA). Navajo's in northeast Arizona have a frequency of 1/1500-2000 compared to Europeans (1/36,000) and African Americans (1/10,000). OCA is caused by mutation in the P gene, which is not present in other Native American populations. Heterozygote frequency in the Navajo wasestimated to be 4.5 percent. No carriers were found in the Apache population from which they split between 600 and 1000 years ago so it is likely to be a new mutant allele in a small founder population. Every human carries at least four autosomal recessive alleles for known inherited human diseases. In small populations that are isolated (geographically or culturally) it is more likely one or more of the recessive alleles will become more common within the gene pool and then more individuals with a particular genetic disease will be seen.
What is the principle force that shifts allele frequencies?
Natural selection - it is one of the most important agents of evolutionary change
What is population genetics?
Population genetics studies patterns of genetic variation within and between group of individuals of the same species
What if the Hardy-Weinberg law is followed?
Random mating for one generation produces genotype frequencies that can be predicted from allele frequencies *With random mating and no evolutionary change*, *these frequencies will remain constant from one generation to the next*
What is stabilising natural selection?
Stabilising natural selection favours intermediate type --> both extreme phenotypes are selected against This acts to keep a population well adapted to its current environment
What happens if two populations with different frequencies for Alleles A1 and A2 merge and random mating occurs?
The frequency of both alleles and the frequency of the resulting genotypes would *change in JUSTone generation*
What are the uses of the Hardy-Weinberg equation?
To calculate the frequencies of different genotypes within a population from knowing the frequencies of only one genotype E.g. 1 in 2500 individuals of northern European ancestry has cystic fibrosis (autosomal recessive)
Natural selection against a non-lethal recessive allele
Trend depends on the *strength of selection against it*
How does natural selection change allele frequency?
Wallace-Darwin concept of natural selection states that: 1) Individuals of a species exhibit variations in phenotype 2) Many of these variations are heritable and passed on 3) Organisms tend to reproduce in exponential fashion. More offspring are produced than can survive. Ergo, there is a struggle for survival 4) *Some phenotypes more successful at survival and reproduce at higher rates*
What is genetic variation?
When members of a population all contribute to the populations' gene pool
What can alter the frequency of alleles within a population?
a) *Natural selection* b) *Mutation of a gene* c) *Migration* d) *Genetic drift*
What is disruptive natural selection?
e.g. The peppered moth (Biston betularia) Before and after industrial revolution - the ones that could adapt to the camouflage of the color soot survived
Natural selection against a lethal recessive allele
f a lethal recessive allele a has a frequency of 0.5 in a population its frequency would halve after one generation and be halved again by the sixth generation. Subsequent reduction is slower and is likely to never reach zero *as the lethal a allele survives within the population in heterozygotes*.
