Download Chapter 13 Chromosomes

Chapter 15 Changing Allele Frequencies
Answers to Review Questions
1.
a.
Agriculture
b.
Cities, as groups of immigrants arrive and mix in
c.
Endangered species, survivors of massacres or natural disasters and their
descendants
2.
d.
Introducing an inherited disease into a population
a.
Highly virulent TB bacteria are selected against because they kill hosts quickly.
Resistant hosts are selected for because they survive infection and live to reproduce. In
this way, TB evolved from an acute systemic infection into a chronic lung infection. In
the 1980s, antibiotic-resistant TB strains led to re-emergence of the disease.
b.
Bacteria that become resistant to antibiotics by mutation or by acquiring
resistance factors from other bacteria selectively survive in the presence of antibiotics.
c.
Viral diversity is low at the start of infection because the immune system is
vulnerable—viral variants aren't necessary. As symptoms ebb, viral diversity increases,
then it decreases again as the immune system becomes overwhelmed.
3.
Increasing homozygosity increases the chance that homozygous recessives will arise,
who may be too unhealthy to reproduce. The population may decline.
4.
Natural selection
5.
Sickle cell disease protects against malaria.
6.
In the Dunker population, the frequencies of blood type genotypes are quite different
from those of the surrounding U.S. population and the ancestral population in Europe.
7.
The causes of founder effects and population bottlenecks differ. A founder effect reflects
a small group moving to start a new population, whereas a population bottleneck results from
removal of individuals with certain genotypes from the population.
8.
A gradual cline might reflect migration over many years. An abrupt cline could be due to
a cataclysmic geological event that separates two populations, such as an earthquake or flood.
9.
Genetic drift is the chance sampling of some genotypes from a population, and this may
lead to nonrandom mating, in which the most fit individuals reproduce more successfully.
Environmental conditions help determine which genotypes reproduce and are thus selected for.
10.
a.
A founder effect is a type of genetic drift that occurs when a few individuals
found a settlement and their alleles constitute a new gene pool, amplifying the alleles
they introduce and eliminating others.
b.
Linkage disequilibrium is the inheritance of certain combinations of alleles of
different genes at a higher frequency than can be accounted for by their individual
frequencies.
c.
Balanced polymorphism is the persistence of a disease-causing allele in a
population because the heterozygote enjoys a health or reproductive advantage.
d.
11.
Genetic load is the proportion of deleterious alleles in a population.
Shifts in allele frequency should parallel people's movements, which are often described
in historical records, and explained by sociology and anthropology.
Answers to Applied Questions
1.
The changes leave a population of 5 green diamonds, 6 yellow squares, 8 blue triangles,
and 1 red circle
2.
a cline
3.
The falling coconut removes an unusual allele. Another example of genetic drift is people
of an ethnic group having children only among themselves, although they live in a mixed
population.
4.
Founder effect because the trait is tracked to one ancestor.
5.
Within a century, the population of Africa will include more individuals with the
resistance gene because they will preferentially survive.
6.
The gene variant will become less common as people who have it die of AIDS, and the
allele offers no advantage as food poisoning becomes rare.
7.
All but natural selection affect populations of cats and dogs. The Pekingese has eyeballs
that are easily popped out – this would not have been selected for.
8.
Not balanced polymorphism because being homozygous recessive for glycophorin C
deficiency does not cause symptoms.
9.
Tasting bitter is harmful—see whether people who can taste bitter substances are
overrepresented among people with cancer, because they may have avoided protective
vegetables.
Tasting bitter is protective—see whether people who cannot taste bitter are overrepresented
among those who have been poisoned.
10.
Microevolution refers to changes in allele frequencies in populations. Examples:
changing virulence of infectious diseases, antibiotic resistance, infectious diseases that jump
species.
11.
Balanced polymorphism as an explanation requires 12 rare events; drift requires one, the
sequestering of the population.
12.
Natural selection
13.
a.
All modern Afrikaners with porphyria variegata descend from the same person in
whom the disorder originated.
b.
One person, who had the dental disorder, contributed disproportionately to future
generations.
c.
Heterozygotes for cystic fibrosis and sickle cell disease resist certain infectious
diseases, maintaining the disease-causing allele in populations.
d.
The Pima Indian population has a high incidence of an allele that predisposes to
develop type 1 diabetes mellitus, but it wasn't expressed until certain dietary and lifestyle
changes became popular.
e.
The Amish and Pakistani groups have high incidences of certain inherited
diseases because of consanguinity.
f.
Migration patterns are responsible for the different frequencies of the
galactokinase deficiency allele across Europe.
g.
Varying mtDNA sequences along the Nile river valley are due to migration.
h.
The alleles responsible for BRCA1 breast cancer originated in different ancestors
for Ashkenazim and African-Americans.
14.
a.
Founder effect
b.
Geographical barriers and natural selection, acting over time, make two
populations have different variants of inherited characteristics.
15.
c.
Nonrandom mating
d.
Natural selection
e.
Population bottleneck
Treating PKU has increased the proportion of mutant alleles in the population because
without treatment, affected individuals would not have been able to reproduce.
16.
a.
The mutations arose independently.
b.
If other groups with many smart people have high carrier rates for these disorders.
If the smart people are the carriers.
17.
The high incidence is due to extreme consanguinity—nearly everyone is related to nearly
everyone else. Tracking the incidence of this condition is difficult because the symptoms exist
independently, and may be caused by environmental factors rather than genes.
18.
a. Balanced polymorphism
b. Founder effect
c. Migration
19.
West Nile virus-associated illness is evolving. It arrived in the U.S. in New York City in
1999 and has since spread nearly everywhere in the nation.
20.
CF carrier status protects against diarrheal disease by decreasing the number of plasma
membrane receptors through which fluid can leave intestinal lining cells.
21.
European males settled in India. Later, Asian immigrants arrived. The male European
Indians had children with Asian Indian women.
22.
Ability to digest lactose arose independently several times.
23.
The protective allele increased gradually as it became more advantageous as people lived
closer together, where they more readily passed infections.
A SECOND LOOK
1.
When milk was introduced into the diet, people with lactose intolerance, the wild type
condition, may have felt too ill to have many children. Gradually, the wild type allele's frequency
would have diminished due to this selective pressure.
2.
An example of negative selection is any condition that impairs an individual's fertility –
perhaps a blood disorder that makes someone too weak to reproduce.
3.
Lactose intolerance is wild type – that is, most common – when and where inability to
digest milk does not affect an individual because the diet does not include this food.
4.
Geography played a role in the evolution of this gene because allele frequencies
correspond to where agriculture brought dairy to a population.