STUDY QUESTIONS - WEEK TWO
Assumptions: no selection, no mutation, no migration, infinite population size, random mating, no drift.
Predictions: If the population is in equilibrium and you define the allele frequencies as p and q, the population will have genotype frequencies of p*p and q*q of the two homozygotes respectively, and 2pq heterozygotes. The next generation will also have the same frequencies. If the population is not in the equilibrium, but now meets the assumptions, the offspring will have genotype frequencies of p*p, q*q, and 2pq, given the parental allele frequencies of p and q.
2. Verbally describe the essence of mutation selection balance for a deleterious recessive allele (hint: an allele that codes for lower fitness in homozygous individuals).
As
3. What is the ultimate source of genetic variation? Also list some proximate sources of genetic variation.
Mutation is the ultimate source of all variation. New combinations of old genes can also occur through recombination, segregation, and sex.
4. How is variation maintained in natural populations? List all possible mechanisms discussed in class.
Mutation, recombination, sex, segregation, some types of selection, and migration.
5. How does genetic drift interact with selection, mutation, and gene flow? Discuss each pair-wise case.
Drift-Selection: Drift is random, and can accordingly go in the same direction as selection to speed it up, or in the opposite direction to slow it down.
Drift-mutation: neutral mutations could go to fixation through drift, other than that there is little interaction.
Drift-migration: drift may bring a population to fixation, but migration can constantly introduce alleles that have not been fixed (or may be fixed for the other allele in a different pop.) to reintroduce heterozygosity.
6. What does genetic drift do to genetic variation in populations, and how does this affect populations of various size?
Genetic drift can erode allelic variation in a population by random sampling error of the alleles that will contribute to the next generation. In smaller populations it has a greater chance of causing an allele to fix, while in large populations it has little or no effect. (Think of a coin toss: prob. of heads is .5 and tails is .5. If you flip it 4 times you may easily get .75 heads and .25 tails, but if you flip it 1000, even if you have 100 more heads, you will still be at .51 heads, and .49 tails!!)
7. Describe Lamarck's theory of acquired characteristics.
Lamarck suggested that characters passed on to the offspring were due to the fact that they were acquired by the parents due to the "felt need" for it by the parent. So if a parent felt that big muscles were necessary, and they acquired big muscles during their lifetime, then the offspring would have it.
8. Recreate Darwin's theory of Natural Selection as a syllogism.
Fact 1: excess fecundity
Fact 2: population size is relatively stable
Fact 3: limited resources
Fact 4: abundant heritable variation
Therefore there must be differential survival of the offspring.
9. What was Darwin's theory for the process of inheritance? Why did his theory not work? Whose research elucidated the modern theory of genetic inheritance?
Darwin suggested blending inheritance of characters existed, but this was clearly not true (he was desperate!!). Mendel’s work on inheritance showed a "genetic" mechanism applicable to natural selection.
Overdominance occurs when the heterozygote has higher expression for the trait than both of the homozygotes.
Underdominance occurs when the heterozygote has lower expression for the trait than both of the homozygotes (which have similar expression levels)
Codominance occurs when one homozygote has higher gene expression than the heterozygote which in turn has higher expression than the other homozygote.
11. The theoretical disease Eric Cartman syndrome manifests itself in one out of every 20,000 births. Assuming this disease is the result of individuals who are homozygous recessive for one genetic loci, give the allele, genotype, and phenotype frequencies (hint: Barry covered a similar problem in lecture).
We must assume that the population is in HWE. The heterozygotes and homozygous dominates are indistinguishable. The homozygous recessive frequency is 1/20,000 so q*q (freq of aa) = .00005. Therefore q= .007. Thus p = .993, and the homozygous dominate is p*p = .98605 and heterozygotes are 2pq = .0139
12. You have discovered a new population of an endangered daisy near your house. Since you are a world renowned botanist of daisies, you are certain that flower petal color is determined by one gene with two
alleles. You sample your population and find 680 red daisies, 260 pink daisies, and 60 white daisies in total. From this information determine the allele and genotype frequencies for flower color.
The parental population genotype frequency is AA=680/1000 = .68 ; Aa=260/1000 = .26 ; aa=60/1000 = .06. So the allele frequency is A = (2(680) + 1(260))/2000 (total # of alleles) = .81, and by p+q=1 a = .19.
13. You revisit the population a year later and find that the population has doubled. Assuming the requirements for Hardy Weinburg are met, also calculate the new phenotype, genotype, and allele frequencies?
Now of the next generation assuming the pop meets HW assumptions, so AA = p*p = (.81)(.81) = .6561. Aa = 2pq = 2(.81)(.19) = .3078, and aa = q*q = (.19)(.19) = .0361 (both genotype and phenotype frequencies) and the allele frequency is then p = .6561 + .5(.3078) = .81 and q = .0361 + .5(.3078) = .19