Deck 19: Population Genetics and Human Evolution

Question 1

Define the following terms:
a. population
b. gene pool
c. allele frequency
d. genotype frequency

Answer 1

(a) Population - organization of specific species that share a common gene pool.
(b) Gene pool - a set of genetic information that is carried by members of a sexually reproducing population. Genetic information is information that is passed down to the offspring from the parent.
(c) Allele frequency - the occurrence of alleles of a particular gene in a population.
(d) Genotype frequency - pertains to a population. The genotype frequency describes the amount of genotypes in a population. Genotype defines the genetic make-up of an organism.

Question 2

The MN blood group is a single-gene, two-allele system in which each allele is codominant. Why are such codominant alleles ideal for studies of allele frequencies in a population?

Answer 2

Codominate two-allele systems are ideal for studies of allele frequencies in a population because each allele expression is not masked by the other. Phenotypic expression of an allele is the easiest way to tell if an allele is present. When it is a codominate two-allele system, the alleles in question can be measured by their phenotypic expression.
If the allele is present, it will show, and it is easy to measure its frequency in a population this way. Unlike complete dominant systems, it is harder to tell which alleles are actually present because one allele's presence may be masked by the presence of another more dominate allele.

Question 3

Explain the connection between changes in population allele frequencies and evolution, and relate this to the observations made by Wallace and Darwin concerning natural selection.

Answer 3

Allele frequencies in a population change when the environment changes to favor one phenotype over another. Allele frequencies may rise when a trait bestows an advantage to an organism in terms of survival and replication rate.
When individuals with a specific phenotype are able to produce offspring at a higher rate than their counterparts, then the allele associated with that phenotype is passed along to subsequent generations at a higher frequency, and the next generation will have more individuals with the favored phenotype.
Therefore, the environmentally favored allele's frequency will rise in the population, as seen with evolution over time. s

Question 4

Can populations evolve without changes in allele frequencies?

Question 5

Design an experiment to determine if a population is evolving.

Question 6

What are four assumptions of the Hardy-Weinberg law?

Question 7

Drawing on your newly acquired understanding of the Hardy-Weinberg equilibrium law, point out why the following statement is erroneous: "Because most of the people in Sweden have blond hair and blue eyes, the genes for blond hair and blue eyes must be dominant in that population."

Question 8

In a population where the females have the allelic frequencies A = 0.35 and a = 0.65 and the frequencies for males are A = 0.1 and a = 0.9, how many generations will it take to reach Hardy-Weinberg equilibrium for both the allelic and the genotypic frequencies? Assume random mating and show the allelic and genotypic frequencies for each generation.

Question 9

Suppose you are monitoring the allelic and genotypic frequencies of the MN blood group locus (see Question 2 for a description of the MN blood group) in a small human population. You find that for 1-year-old children, the genotypic frequencies are MM = 0.25, MN = 0.5, and NN = 0.25, whereas the genotypic frequencies for adults are MM = 0.3, MN = 0.4, and NN = 0.3.
a. Compute the M and N allele frequencies for 1-year-olds and adults.
b. Are the allele frequencies in equilibrium in this population?
c. Are the genotypic frequencies in equilibrium?

Question 10

Using Table 1, determine the frequencies of p and q that result in the greatest proportion of heterozygotes in a population.
TablE 1 Heterozygote Frequencies for Recessive Traits

Question 11

In a given population, the frequencies of the four phenotypic classes of the ABO blood groups are found to be A = 0.33, B = 0.33, AB = 0.18, and i = 0.16. What is the frequency of the i allele?

Question 12

If a trait determined by an autosomal recessive allele occurs at a frequency of 0.25 in a population, what are the allelic frequencies? Assume Hardy-Weinberg equilibrium and use A and a to symbolize the dominant and recessive alleles, respectively.

Question 13

Why is it that mutation, acting alone, has little effect on gene frequency?

Question 14

Successful adaptation is defined by:
a. evolving new traits
b. producing many offspring
c. leaving more offspring than others
d. moving to a new location

Question 15

What is the relationship between founder effects and genetic drift?

Question 16

How would a drastic reduction in a population's size affect that population's gene pool?

Question 17

The major factor causing deviations from Hardy- Weinberg equilibrium is:
a. selection
b. nonrandom mating
c. mutation
d. migration
e. early death

Question 18

A specific mutation in the BRCA1 gene has been estimated to be present in approximately 1% of Ashkenazi Jewish women of Eastern European descent. This specific alteration, 185delAG, is found about three times more often in this ethnic group than the combined frequency of the other 125 mutations found to date. It is believed that the mutation is the result of a founder effect from many centuries ago. Explain the founder principle.

Question 19

The theory of natural selection has been summarized popularly as "survival of the fittest." Is this an accurate description of natural selection? Why or why not?

Question 20

Will a recessive allele that is lethal in the homozygous condition ever be completely removed from a large population by natural selection?

Question 21

Do you think that our species is still evolving, or are we shielded from natural selection by civilization? Is it possible that misapplications of technology will end up exposing our species to more rather than less natural selection (consider the history of antibiotics)?

Question 22

Provide a genetic definition of race.
b. Using this definition, can modern humans be divided into races? Why or why not?

Question 23

Briefly describe the two major theories discussed in this chapter about the origin of modern humans.
b. Which of these two theories would predict a closer relationship for the various modern human populations?
c. Which of the two theories is best supported by the genetic evidence?

Question 24

The human and chimpanzee genomes are 98.8% identical. If this is so, why are the phenotypes of chimps and humans so different?

Question 25

Genomics and Human Evolution
The development of language in humans depends in part on expression of the transcription factor gene FOXP2. Research indicates that Neanderthals had a version of the FOXP2 gene identical to that of our species in regions where human and chimpanzee genes differ. Is this enough evidence to conclude that Neanderthals had a complex spoken language? Why or why not?

Question 26

Jane, a healthy woman, was referred for genetic counseling because she had two siblings, a brother Matt and a sister Edna, with cystic fibrosis who died at the ages of 32 and 16, respectively. Jane's husband, John, has no family history of cystic fibrosis. Jane wants to know the probability that she and John will have a child with cystic fibrosis. The genetic counselor used the Hardy-Weinberg model to calculate the probability that this couple will have an affected child.
The counselor explained that there is a two-in-three chance that Jane is a carrier for the mutant CFTR allele; she used a Punnett square to illustrate this. The probability that John is a carrier is equal to the population carrier frequency (2pq). The probability that John and Jane will have a child who has cystic fibrosis equals the probability that Jane is a carrier (2/3), multiplied by the probability that John is a carrier (2pq), multiplied by the probability that they will have an affected child if they are both carriers (1/4).
Using the heterozygote frequency for cystic fibrosis among white Americans to estimate the probability that John is a carrier, what is the likelihood that their child would have the disease?

Question 27

Jane, a healthy woman, was referred for genetic counseling because she had two siblings, a brother Matt and a sister Edna, with cystic fibrosis who died at the ages of 32 and 16, respectively. Jane's husband, John, has no family history of cystic fibrosis. Jane wants to know the probability that she and John will have a child with cystic fibrosis. The genetic counselor used the Hardy-Weinberg model to calculate the probability that this couple will have an affected child.
The counselor explained that there is a two-in-three chance that Jane is a carrier for the mutant CFTR allele; she used a Punnett square to illustrate this. The probability that John is a carrier is equal to the population carrier frequency (2pq). The probability that John and Jane will have a child who has cystic fibrosis equals the probability that Jane is a carrier (2/3), multiplied by the probability that John is a carrier (2pq), multiplied by the probability that they will have an affected child if they are both carriers (1/4).
If you were their genetic counselor, would you recommend that Jane and John be genetically tested before they attempt to have any children?

Question 28

Jane, a healthy woman, was referred for genetic counseling because she had two siblings, a brother Matt and a sister Edna, with cystic fibrosis who died at the ages of 32 and 16, respectively. Jane's husband, John, has no family history of cystic fibrosis. Jane wants to know the probability that she and John will have a child with cystic fibrosis. The genetic counselor used the Hardy-Weinberg model to calculate the probability that this couple will have an affected child.
The counselor explained that there is a two-in-three chance that Jane is a carrier for the mutant CFTR allele; she used a Punnett square to illustrate this. The probability that John is a carrier is equal to the population carrier frequency (2pq). The probability that John and Jane will have a child who has cystic fibrosis equals the probability that Jane is a carrier (2/3), multiplied by the probability that John is a carrier (2pq), multiplied by the probability that they will have an affected child if they are both carriers (1/4).
It is now possible to use preimplantation testing, which involves in vitro fertilization plus genetic testing of the embryo before implantation, to ensure that a heterozygous couple has a child free of cystic fibrosis. Do you see any ethical problems or potential future dangers associated with this technology?

Question 29

Natural selection alters genotypic frequencies by increasing or decreasing fitness (i.e., differential fertility or mortality). There are several examples of selection associated with human genetic disorders. Sickle cell anemia and other abnormal hemoglobins are the best examples of selection in humans. Carriers of the sickle and other hemoglobin mutations are more resistant to malaria than is either homozygous class. Therefore, in areas where malaria is endemic, carriers are less likely to die of malaria and will have proportionally more offspring than will homozygotes, thus passing on more genes. Balancing selection may also have influenced carrier frequencies for more "common" recessive diseases, such as cystic fibrosis in Europeans and Tay-Sachs in the Ashkenazi Jewish population, but the selective agent is not known for certain.
Selection may favor homozygotes over heterozygotes, resulting in an unstable polymorphism. One example is selection against heterozygous fetuses when an Rh? mother carries an Rh1 (heterozygous) fetus. This should result in a gradual elimination of the Rh? allele. However, the high frequency of the Rh? allele in so many populations suggests that other, unknown factors may maintain the Rh? allele in human populations.
If you suspected that heterozygous carriers of a particular disease gene had a selective advantage in resisting a type of infection, how would you go about testing that hypothesis?

Question 30

Natural selection alters genotypic frequencies by increasing or decreasing fitness (i.e., differential fertility or mortality). There are several examples of selection associated with human genetic disorders. Sickle cell anemia and other abnormal hemoglobins are the best examples of selection in humans. Carriers of the sickle and other hemoglobin mutations are more resistant to malaria than is either homozygous class. Therefore, in areas where malaria is endemic, carriers are less likely to die of malaria and will have proportionally more offspring than will homozygotes, thus passing on more genes. Balancing selection may also have influenced carrier frequencies for more "common" recessive diseases, such as cystic fibrosis in Europeans and Tay-Sachs in the Ashkenazi Jewish population, but the selective agent is not known for certain.
Selection may favor homozygotes over heterozygotes, resulting in an unstable polymorphism. One example is selection against heterozygous fetuses when an Rh? mother carries an Rh1 (heterozygous) fetus. This should result in a gradual elimination of the Rh? allele. However, the high frequency of the Rh? allele in so many populations suggests that other, unknown factors may maintain the Rh? allele in human populations.
If allele frequencies in the hemoglobin gene are influenced by sickle cell anemia on the one hand and by resistance to malaria on the other hand, what factors may cause a change in these allele frequencies over time?