Human Genetics: Concepts and Applications (Lewis), 9th Edition

Chapter 15: Changing Allele Frequencies

Chapter Outline

CHAPTER OVERVIEW

1. Human mating is nonrandom. People choose their mates for many reasons and frequently they choose people with traits similar to themselves.
2. At the population level, human mating is nonrandom because individuals do not contribute equally to the next generation (i.e., some people have more children than others).
3. For some traits, such as blood types, mating can be considered random and allele frequencies are often in Hardy-Weinberg equilibrium.
4. Populations that practice marriage between related members (consanguinity) experience increases in the proportion of homozygotes at the expense of heterozygotes.

1. Migration disrupts gene frequencies by introducing new alleles into a population, although it may take many generations for different groups to intermarry.
2. We can make inferences about the directions in which ancient people traveled by looking at the distribution of allele frequencies in modern populations.
3. Clines are changes in allele frequencies between neighboring populations.
4. Geographical and language barriers often result in clines with gradual or abrupt changes in allele frequencies.

1. Genetic drift occurs when a subset of a population has different gene frequencies, often reflecting a physical separation.

1. The founder effect occurs when a few individuals leave a community to start a new settlement.
2. The resulting population may, by chance, lack some alleles present in the original population, or have high frequencies of others.

1. In a population bottleneck, many members of a population die, and only a few individuals contribute genetically to the next generation.
2. A population bottleneck may also occur if a small number of individuals colonize an island.

1. Mutations alter allele frequencies by introducing new alleles or copy number variants.
2. Heterozygotes and mutations maintain the frequencies of deleterious alleles in populations, even if homozygotes die.
3. Genetic load is the collection of deleterious alleles in a population.

1. Environmental conditions act on genetic variability through natural selection.
2. Alleles that make individuals more fit - able to survive and reproduce - are passed on to offspring. This is termed positive selection.
3. Those that make individuals less likely to survive and reproduce are less likely to be passed on. This is termed negative selection.
4. Humans use artificial selection to breed plants and animals with particular phenotypes.

1. Differential survival based on phenotype is called natural selection.
2. The bacterium causing tuberculosis (TB) was first identified by Robert Koch in 1882.
3. As a result of natural selection operating on bacterial and human populations, the effect of TB on human health evolved from outbreaks of an acute systemic infection to an increasingly rare chronic lung infection.
4. Starting in the late 1980s, there has been a resurgence of TB as a result of natural selection and mutation. The prevalence of Multidrug resistant TB (MDR-TB) is of great concern.
5. TB is often comorbid with HIV.

1. HIV is a retrovirus and viral mutations accumulate rapidly in its RNA genome.
2. Immunity and treatment create selection and HIV evolves during the course of infection.

1. Balanced polymorphisms exist in populations when heterozygotes carrying a disease-causing allele enjoy a health advantage.
2. The recessive disease causing alleles for sickle cell disease and cystic fibrosis represent balanced polymorphisms since they are associated with conferring resistance to infectious diseases in the heterozygous condition.
3. Heterozygotes for prion protein genes may be resistant to certain transmissible spongiform encephalopathies.

1. Research on allele frequencies in different populations provides information on factors affecting evolution.
2. Mutational analysis in various populations indicates that PKU originated more than once, and that genetic drift, balanced polymorphism, and perhaps mutation have influenced its prevalence.

1. Eugenics is the attempt to control individual reproduction for societal goals.
2. In the early twentieth century, several different eugenic policies were promoted and implemented.
3. Eugenic policies aimed to either maximize the genetic contribution of those deemed acceptable and superior or minimize the contribution of those considered inferior.
4. Some aspects of genetic technology can affect reproductive choices and have been compared to eugenics. However, the goal of genetic screening is to alleviate human suffering rather than to change society.