BIOEE 1780 Lecture Notes - Lecture 27: Zygosity, Inbreeding, Genotype Frequency

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rosepanda334

7 May 2018

School

Cornell University

Department

Ecology & Evolutionary Biology

Course

BIOEE 1780

Professor

John I.Lovette, Abby Grace Drake

For unlimited access to Class Notes, a Class+ subscription is required.

Genetic Drift: allele frequencies randomly drift away from their starting value

• Occurs when a random, nonrepresentative sample from a population produces the next generation

• Eliminates alleles faster in small populations than in bigger ones

• Reduces genetic variation in populations

• A form of evolution, even though it isn't adaptive (isn't related to selection, purely results from

chance)

• Example

If a set of parents has only one child…

• 25% chance that the child is AA, and a allele is lost from the family

• 25% chance that the child is aa, and A allele is lost from the family

o If the same process occurs in every family in the generation, allele frequencies may change by

chance.

• Coin Flip

o When you flip a coin four times, what is the probability of getting…

• 0 heads

• 0.54 = 0.0625

• 1 head

• 0.25

• 2 heads

• 0.375

• Highest probability outcome, but not guaranteed

• 3 heads

• 0.25

• 4 heads

• 0.0625

• When a dad with Aa genotype has four offspring, how many offspring will receive the A allele from

him?

o 0 offspring = 0.0625

o 1 offspring = 0.25

o 2 offspring = 0.375

o 3 offspring = 0.25

o 4 offspring = 0.0625

• When randomly selecting organisms from a population (i.e. a random draw):

The frequency of an allele corresponds to the probability that it will be fixed in the population.

• For example, an allele with frequency = 0.8 has an 80% chance of reaching fixation.

Reasons Genetic Drift Occurs

• Catastrophes can change allele frequencies

• Bottlenecks or founder events can change allele frequencies

• Random sampling of gametes and/or adults can change allele frequencies

Fixed Allele: an allele that remains in a population when all of the alternative alleles have disappeared

• No variation exists for that gene in the population (all individuals have the same allele)

Genetic Bottleneck: occurs when the number of individuals in a population is reduced drastically

• Type of genetic drift

• Can result in the loss of rare alleles

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Document Summary

For example, an allele with frequency = 0. 8 has an 80% chance of reaching fixation: when randomly selecting organisms from a population (i. e. a random draw): The frequency of an allele corresponds to the probability that it will be fixed in the population. Reasons genetic drift occurs: catastrophes can change allele frequencies, bottlenecks or founder events can change allele frequencies, random sampling of gametes and/or adults can change allele frequencies. Fixed allele: an allele that remains in a population when all of the alternative alleles have disappeared: no variation exists for that gene in the population (all individuals have the same allele) Genetic bottleneck: occurs when the number of individuals in a population is reduced drastically: can result in the loss of rare alleles. On its own, inbreeding doesn"t change the frequency of alleles in a population. Inbreeding isn"t a mechanism of evolution, but it can create the conditions for evolution to occur.

Related Questions

Q1. What is the founder effect?

Sampling error that occurs during the establishment of a newpopulation by a small number of migrants.

Strong natural selection acting on the founders of a newpopulation because the environment they are now living in is sodifferent from the environment they came from.

A phenomenon named after William Founder, who observed that toppredators generally have low genetic diversity.

Rapid population growth following a bottleneck.

Q2. The effects of genetic drift are more pronounced in largerpopulations.

True

False

Q3. Inbreeding tends to increase the proportion of homozygousindividuals in a population (i.e. individuals that have two copiesof the same allele).

True

False

Q4. The effective size of a population is:

The size of an idealized randomly-mating population that is notunder selection and has the same heterozygosity as the actualpopulation.

The size of an idealized randomly-mating population that has thesame heterozygosity as the actual population, but does not loseheterozygosity over time.

The size of an idealized randomly-mating population losinghomozygosity at the same rate as the actual population.

The size of an idealized randomly-mating population losingheterozygosity at the same rate as the actual population.

Q5. Which of the following tends to reduce the effective size ofa population?

Check all that apply:

Increasing the population size.

A biased sex ratio.

Non-random mating.

Q6. If gametes from a gene pool combine randomly to make only asmall number of zygotes, the allele frequencies among the zygotesmay be different than they were in the gene pool because:

The effects of natural selection are more pronounced in smallpopulations.

The effects of genetic drift over several generations are morepronounced with small numbers of gametes.

The effects of differences in frequencies for different allelesare more pronounced with small numbers of zygotes.

The effects of sampling error are more pronounced with smallsamples.

Q7. Genetic drift is different from natural selectionbecause:

In natural selection allele frequencies change because somealleles confer higher fitness, whereas in genetic drift allelefrequencies change because of chance sampling error.

Natural selection acts primarily in large populations, whereasgenetic drift acts primarily in small ones.

Natural selection is a mechanism of evolution, whereas geneticdrift is an outcome of evolution.

Natural selection tends to cause rapid evolution, whereasgenetic drift tends to cause slow evolution.

Q8. Imagine a population evolving by genetic drift in which thefrequency of allele K is 0.65. What is the probability that at somepoint in the future allele K will drift to a frequency of 1?

Q9. A dwindling population of 1000 frogs occupies an isolatedwatershed in Costa Rica. To help preserve the species, scientistscaught 20 frogs to start a new population in a nearby watershed.This species has a gene that affects eye shape. The 1000-memberwild population has two alleles for this gene: R and r, withfrequencies 0.7 and 0.3, respectively. What will be the allelefrequencies of R and r in the 20-member founder population?

The frequencies will be 0.7 for R and 0.3 for r.

The frequencies will be 1.0 for R and 0 for r.

The expected frequencies are 0.7 for R and 0.3 for r. The actualfrequencies could be different.

The founder populations's allele frequencies will necessarily bedifferent than the source population's frequencies.

Q10. For a population containing 90 females and 10 males, whatis the effective population size, N_e ?

chocolateplatypus914

Q49. Which of the following is NOT true?

If a genetic disease reduces fertility and the allele thatcauses the disease offers no other advantage, the allele willlikely eventually disappear, due to natural selection.

Natural selection does not favor individuals who are homozygousfor the sickle-cell allele, because these individuals typically diebefore they are old enough to reproduce.

Individuals who are heterozygous HbA/HbS areprotected from malaria, and this is why sickle-cell diseasepersists in wetter, mosquito-prone regions in Africa.

In regions where malaria does not occur, individuals who areheterozygous HbA/HbS have a fitness advantageover those who are homozygous for the normal hemoglobin allele(HbA).

Q50. AFTER malaria is cured, the frequency of the HbSallele should decrease in regions with lots of mosquitoesbecause:

People will no longer die from sickle-cell disease in theseregions.

Having one copy of the HbS allele will no longer beadvantageous in these regions.

Natural selection will no longer act on the HbS alleleat all in these regions.

All alleles associated with genetic diseases eventuallydisappear.

Q52. If the frequency ofthe HbS allele is 0.4 in a population, what is thefrequency of the HbAallele (assuming this is atwo-allele system)?

Q53. Which of the following would be sufficient for theHardy-Weinberg equation to accurately predict genotype frequenciesfrom allele frequencies?

p + q = 1

The population is not evolving due to natural selection.

The population is not evolving due to any of the conditions thatdisrupt Hardy-Weinberg equilibrium.

The population is infinitely large.

Q54. In a village, if the proportion of individuals who havesickle-cell disease is 0.25, and the population is assumed to be atHardy-Weinberg equilibrium, what is the expected frequency of theHbS allele? (Hint: What is the genotype of people withsickle-cell disease, and how is that genotype represented in theHardy-Weinberg equation?)

0.1250.250.50.75

Q55. In the same village, what proportion of the populationshould be heterozygous HbA/HbS, according to theHardy-Weinberg equation?

Use the following passage to answer the nextthree questions.

It has been hypothesized that people who are heterozygous forthe allele that causes the deadly genetic condition cystic fibrosis(which, among other symptoms, reduces fertility) are more resistantto the deadly disease tuberculosis.

Q56. If the cystic fibrosis allele protects against tuberculosisthe same way the sickle-cell allele protects against malaria, thenwhich of the following should be true of a comparison betweenregions with and without tuberculosis?

Cystic fibrosis deaths should be more common in regions withtuberculosis.

Cystic fibrosis deaths should be less common in regions withtuberculosis.

Cystic fibrosis deaths should be equally common in both types ofregions.

Regional differences in the cystic fibrosis death rate should bepurely random and unpredictable.

Q57. A person who is heterozygous for the cystic fibrosis allelemoves to a small, isolated community where no one previouslycarried the allele. If the cystic fibrosis allele protects againsttuberculosis the same way the sickle-cell allele protects againstmalaria, what should happen to the frequency of the cystic fibrosisallele in the community over time, and why?

The cystic fibrosis allele should disappear from the population,because a single individual with the allele is not enough for it toproliferate.

The cystic fibrosis allele should increase to a relatively highfrequency, because heterozygotes with the allele will be morelikely to survive than others.

The cystic fibrosis allele should become fixed in thepopulation, due to genetic drift.

The cystic fibrosis allele should either disappear or increasein frequency, depending on chance as well as on tuberculosisprevalence and death rate.

Use the following additional passage to answer the nextquestion.

Suppose you travel to the future, to a time when neithercystic fibrosis nor tuberculosis have caused any deaths for manygenerations. In all of these future populations, thecystic fibrosis allele still exists at a low frequency.

Q58. You visit a huge city with millions of people. If you wereto start sampling the cystic fibrosis allele from one generation tothe next, what should happen to its frequency over the next fewgenerations, and why?

The allele frequency should change a lot from one generation tothe next due to random genetic drift.

The allele frequency should not change much from one generationto the next because the population is large.

The allele frequency should steadily increase due to naturalselection.

The allele frequency should steadily decrease due to naturalselection.

amaranthherring501

Need help with evolution questions:

1.) A haplotype is best defined as the ________________.

	haploid genotypes of all the gametes produced by a diploid individual

ABO blood type conferred by an individual gamete

genotype of either the paternal or maternal chromosomal complement

multilocus genotype of a chromosome or gamete

2.) Which of the following statements regarding linkage disequilibrium is true?

Exists when D is less than zero, but not when it is greater than zero.

Is reduced by sexual reproduction.

Is increased by crossing-over during meiosis.

Is increased by any random sampling error that happens to create or destroy certain chromosome genotypes but not others.

Is reduced by selection that favors certain combinations of genotypes.

3.) Selection on multilocus genotypes in random-mating populations leads to linkage disequilibrium when _______________.

it reduces mutation rates

it eliminates all haplotypes from the population

it prevents crossing-over during meiosis

some allele combinations confer greater fitness than do others

4.) When two loci are in linkage equilibrium, the allele carried on a chromosome at one locus is independent of the allele carried on the chromosome at another locus. When this is the case, _________________.

allele frequencies will not change over time

the two loci (or, more specifically, their protein products) do not interact

the frequency of each haplotype can be calculated by multiplying the frequency of its two alleles

all possible haplotypes occur at equal frequency

5.) Which of the following accurately characterize(s) Muller's ratchet model for the selective advantage of sexual reproduction?

Drift establishes linkage disequilibrium because multilocus genotypes with few mutations are eliminated by chance events.

In populations without sexual reproduction, deleterious mutations accumulate, eventually imposing such a burden that the population goes extinct.

Sex is favored selectively because it recreates the zero-mutation genotypes lost due to drift.

All of the above correctly characterize Muller's ratchet model for the evolution of sexual reproduction.

6.) The genetic signature of recent positive selection on a locus is _______________.

high frequency and low linkage disequilibrium

low frequency and low linkage disequilibrium

low frequency and high linkage disequilibrium

high frequency and high linkage disequilibrium

7.) Sexual reproduction is advantageous both in changing environments and in constant environments, compared to asexual reproduction.

True

False

8.) Sexual reproduction causes genetic recombination via crossing-over, which combines genotypes from different parents.

True

False

9.) One reason sex may be maintained in natural populations is proposed by the "Red Queen Hypothesis", which states that.....

genotypes created by sex are created by natural selection over and over

recombination creating novel genotypes that can resist infection by parasites

mutation load is reduced by novel adaptive mutations

the ability of sex to fix multiple

BIOEE 1780 Lecture Notes - Lecture 27: Zygosity, Inbreeding, Genotype Frequency

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