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Name two major kinds of variable traits.
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Discrete traits is to qualitative as continuous traits is to _________.
Quantitative
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What is a polymorphic trait?
A variable, discrete trait that has many forms.
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Which two theories attempt to explain how much polymorphism exists in nature?
- Classical Model
- Balance Hypothesis
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Describe the Classical Model used to explain how much polymorphism exists in nature.
- Polymorphisms are transient
- Individuals tend to be homozygous for the wild type allele
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Who proposed the Classical Model used to explain how much polymorphism exists in nature.
Muller propounded Classical Model, who based it on Morgan's work.
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Describe the Balance Hypothesis used to explain how much polymorphism exists in nature.
- Polymorphism is commonplace and stable
- Heterozygotes are abundant
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Who proposed the Balance Hypothesis used to explain how much polymorphism exists in nature.
Dobzhansky and Ford advocated the Balance Hypothesis.
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Which theory/model better explains how much polymorphism exists in nature. Why?
- The Balance Model is consistent with the Darwinian view that natural selection must act on pre-existing variation
- Empiracle support comes from work on natural popluations, in which differences are subtle, and a "normal" allele cannot be identified
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What is the difference between isozyme and allozyme?
- Isozyme - various bands appearing on a gel grouped by charge and/or size that could originate from several loci
- Allozyme - various bands appearing on a gel grouped by charge and/or size that can be interpreted as alleles of a single locus
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Why would a population be nearly monomorphic for a particular locus with rare heterozygotes?
Because the gene has only one common allele except for some rare mutations or a lethal homozygous allele.
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What is the measure of polymorphism? < P >
- The proportion of genes that are polymorphic in a population
- To set a uniform criterion and allow comparisons between studies, polymorphism may be defined at the 95% or 99% level
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What is the measure of heterozygosity? < H >
- The proportion of heterozygotes over many loci
- Is a better reflection of the exten of genetic diversity in a population
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If a study of 10 human loci revealed 3 were polymorphic, i.e., < P >=3, and the polymorphic loci were heterozygous (Hi) in 0.51, 0.39, and 0.10 of individuals for the three genes in question, then what is < H >?
- 0.51 + 0.39 + 0.19 / 10
- = 0.1 or < H > = 0.1
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< P > and < H > values vary between different kinnds of organisms. Warm-blooded animals have _______ values, and invertebrates have ______ values for both estimates.
Warm-blooded animals have lower values, and invertebrates have higher values for both estimates.
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Why would scientific studies find that as they looked at more and more genes, less and less polymorphism was found?
Likely, the polymorphic genes are the first to be noticed, and so are more readily studied.
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Name some characteristics of microsatellites:
- 2-4 base repeated elements found in certain parts of the genome
- repeated elements are flankedby unique sequences that make them easy to amplify with PCR
- repeats vary in length
- microsatellites evolve quickly
- change is made by slippage of polymerase
- best way to look at neutral evolution because they're introns and are free to change
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How much heterozygosity would one expect in a randomly mating population with six alleles at one locus, present at equal frequencies (1/6 = 0.17)?
The probability that it is a homozygote for a single locus is (1/6)*(1/6) = 1/36, so since there are six homozygotes possible = 6*(1/6)*(1/6) = 1/6, so all other choices are going to produce heterozygotes, (5/6)
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What is a Single-Strand Conformation Polymorphism (SSCP)?
SSCPs are studied with a special electrophoretic apparatus that allows (most of the time) the separation of msall DNA fragments that differ by as little as a single substitution. These can serve as a source of co-dominant alleles in population studies.
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What are Restriction Fragment Length Polymorphisms (RFLP)?
A form of analysis that involves cutting DNA with restriction endonucleases and examining the variation in the size of the DNA fragments in a gel.
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What are Random Amplified Polymorphic DNA?
An approach where short primers are used to amplify fragments with PCR. However, the method has serious problems of reproducibility.
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What are the intrinsic assumptions of HW?
- mating is random (with respect to genotype)
- reproduction is strictly sexual
- population is diploid
- non-overlapping generations
- males and females are equal in numbers and have the same allele frequencies
- population is large
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What are the extrinsic assumptions of HW?
- no selection
- migration is negligible
- mutation is negligible
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When the proportion of an allele is less than 2/3, the majority of those individuals carrying that allele are __________.
Heterozygotes (Aa)
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when an ellele is rare, almost all individuals carrying that allele are ____________.
Heterozygotes (Aa)
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The maximum heterozygosity is ______ and occurs when _______.
max heterozygosity = 0.5 when p = q
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The more alleles are present, the ________ the potential heterozygosity.
greater
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What is assortative mating? What is the conseuqence of it?
- Occurs when individuals are more likely to mate with individuals that share traits.
- Results in a decrease in heterozygosity.
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What is disassortative mating? What is the consequence of it?
- Occurs when individuals are more likely to mate with individuals that do not share traits.
- Results in a decrease in heterozygosity.
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What is the Wahlund effect?
- Non-random mating because of population subdivision.
- Results from the aggregation of data from subpopulations that are, in fact, in HW equilibrium within themselves.
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The predicted heterozygosity of an aggregate population is always _________ than the average of the predicted heterzygosities of the individual demes.
higher
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For each generation that is selfed, heterozygosity is _________.
Reduced by half
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Wahlund effect causes heterozygote ________.
defecit
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Unequal allelic frequencies between sexes causes heterzygote ___________.
excess
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If pmale = 0 and pfemale = 1, then what is the percentage of offspring that will be heterzygotes for p?
100%
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What is the effect of overlapping generations on evolution?
The effect of overlapping generations on evolution is generally considered insignificant.
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Even when p (or q) differs only slightly in the two sexes, the F1 will experience a greater than expected contribution of __ x __ matings. What is the result?
- AA x aa
- results in higher than expected heterzygosity
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A and a are alleles; they are also ________________.
haploid (gamete) genotypes
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ONLY use the HW equation when:
You know for a fact that all HW assumptions are met.
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If all conditions of HW are met, the onset of the equilibrium is ___________.
immediate (one generation of random mating).
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Which letters are sometimes used to denote the frequencies of the three main genotypes?
P, H, and Q
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If you know a population is not in HW, and you wish to calculate p, which formulae can you use?
- p = 2P + H / 2
- q = 2Q + H / 2
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