Download Chapter 14 When Allele Frequencies Stay Constant

Chapter 14
When Allele Frequencies Stay Constant
CHAPTER OVERVIEW
Previously genes were discussed as the genetic source of heritable
traits or as the biochemical blueprint for building proteins. This
chapter considers the dynamics of genes and their associated
alleles in populations and the role of populations as reservoirs of
genetic diversity. Information gained from the study of alleles in
populations is used to determine the frequency of heterozygotes
(carriers) for certain recessive disorders in different populations.
Genetic diversity in the form of DNA polymorphisms is used to
distinguish individuals by DNA profiling. An understanding of
population genetics is required in order to interpret the significance
of DNA marker data. Evolution occurs when changes occur in allele
frequencies as will be presented in chapters 15 and 16.
CHAPTER OUTLINE
14.1 The Importance of Knowing Allele Frequencies
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.
14.2 When Allele Frequencies Stay Constant
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 (p 2); the proportion of homozygous recessives equals the
square of the frequency of the recessive allele (q 2); 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.
14.3 Applying Hardy-Weinberg Equilibrium
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 q 2.
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).
14.4 DNA Profiling is Based on Hardy-Weinberg Assumptions
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.
DNA Profiling Began With Forensics
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.
Population Statistics Are Used to Interpret DNA Profiles
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.
DNA Profiling to Identify Disaster Victims
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.
14.5 Genetic Privacy
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.