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

Chapter 14: Constant Allele Frequencies

Chapter Outline

CHAPTER OVERVIEW

1. Population genetics concerns the study of allele frequencies in members of the same species in a specified geographic area.
2. Genes in a population comprise the gene pool.
3. Gene flow is movement of alleles between populations.
4. Microevolution reflects small changes in allele frequencies in populations, which can be traced using the Hardy-Weinberg equation.
5. Allele frequencies are affected by mutation, migration, genetic drift, selection, and non-random mating.
6. Macroevolution refers to the formation of a new species.

1. In Hardy-Weinberg equilibrium, allele frequencies stay relatively constant.
2. Hardy-Weinberg equilibrium occurs in large populations where mating is random and there is no migration, mutation, or natural selection.
3. For a population in Hardy-Weinberg equilibrium where frequencies are known for two alleles, the proportion of homozygous dominants equals the square of the frequency of the dominant allele (p²); the proportion of homozygous recessives equals the square of the frequency of the recessive allele (q²); proportion of heterozygotes equals 2pq.
4. Allele frequencies can be calculated from the proportions of genotypes (either p or q). The frequency of the recessive allele (q) equals the proportion of homozygous recessives plus 1/2 that of the carriers.
5. The frequency of the dominant allele equals the proportion of homozygous dominants plus 1/2 that of the carriers.

1. In real life situations, the value of q² can be derived by knowing the frequency of affected individuals in a population. The Hardy-Weinberg equation can then be used to predict the likelihood of someone being a carrier.
2. For X-linked recessive traits, the frequency of the recessive allele in males is q and in females it is q².
3. For very rare inherited disorders, p approaches 1, so the carrier frequency is essentially 2q (approximately twice the frequency of the disease-causing allele).

1. Hardy-Weinberg equilibrium only applies when natural selection is not occurring. It applies to DNA repeats and other sequences that do not alter evolutionary fitness.
2. DNA repeats are found throughout the genome.
3. Copy number variants can be used for identification.
4. Individuals may be homozygous or heterozygous for the number of copies of a repeat they carry at a particular locus.
5. DNA profiling analyzes differences in repeat copy number. For forensic applications variable number of tandem repeats (VNTRs) and short tandem repeats (STRs) are used.

1. The field of DNA profiling began in the 1980s using restriction enzymes and polyacrylamide gel electrophoresis to identify RFLPs.
2. Mitochondrial DNA (mtDNA) is used when chromosomal DNA is insufficient or too damaged for analysis.
3. Databases and software facilitate sharing and searching of stored DNA profiles for research and forensic applications.

1. Population statistics derived from databases are applied to DNA profiling results to determine the probability that two individuals might share a particular marker pattern.
2. A limitation of the method is that databases may not adequately represent real human populations.
3. Developing narrower databases and considering historical and social factors may help develop more realistic population statistics.

1. STR analysis was used to identify victims in the World Trade Center attack on Sept. 11, 2001.
2. DNA profiling was also implemented to aid identification of victims of the tsunami of 2004, hurricane Katrina and the Pakistan earthquake of 2005.
3. The Shoah project uses DNA technology to link family members separated by the Holocaust of WWII.

1. The success of DNA profiling has created legal concerns over the availability and use of genetic data.
2. Legislation has lagged behind advances in genetic technology in the U.S. and elsewhere.