Constant Allele Frequencies
Hardy-Weinberg Equilibrium
Population


An interbreeding group of the same species
within a given geographical area
Gene pool


Population genetics


the collection of all alleles in the members of the
population
the study of the genetics of a population and how the
alleles vary with time
Gene Flow

alleles can move between populations when individuals
migrate and mate
Allele Frequencies
Allelic
Frequency


# of particular allele
total # of alleles in the population
Count both chromosomes of each individual
Allele frequencies affect the genotype
frequencies
 The frequency of each type of
homozygote and heterozygote in the
population
Phenotype Frequencies

Frequency of a trait varies in different
populations
Table 14.1
Microevolution and Macroevolution

Microevolution


Genetic change due to changing allelic frequencies in
populations
Macroevolution
 The formation of new species
Allelic frequencies can change when
there is:

Nonrandom mating


Gene flow


Reproductively isolated groups form within or
separate from a larger population
Mutation


e.g. migration
Genetic drift


Individuals of one genotype are more likely to
produce offspring with each other than with those of
other genotypes
Introduces new alleles into the population
Natural selection

Individuals with a particular genotype are more
likely to produce viable offspring
Hardy-Weinberg Equilibrium



Developed by mathematicians
A condition in which allele frequencies
remain constant
Used algebra to explain how allele
frequencies predicts genotype and
phenotype frequencies in equilibrium
Hardy-Weinberg Equilibrium
p+q=1
All of the allele frequencies together equals 1 or the whole
collection of alleles
p = allele frequency of one allele (e.g. dominant)
q = allele frequency of a second allele (e.g. recessive)
p2 + 2pq + q2 = 1
All of the genotype frequencies together equals 1
p2 and q2 =genotype frequencies for each homozygote
2pq =
genotype frequency for heterozygotes
2 possible combinations (p egg + q sperm or vice
versa)
Figure 14.3
Table 14.2
Applying Hardy-Weinberg Equilibrium


Used to determine carrier probability
Homozygous recessive used to determine frequency
of allele in population (phenotype is genotype)
Applying Hardy-Weinberg Equilibrium:
Cystic Fibrosis
Calculating Carrier Frequency for
X-linked Traits
Figure 14.6
DNA Profiling (a.k.a. DNA
Fingerprinting

Hardy-Weinberg equilibrium applies to portions of the
genome that do not affect phenotype





They are not subject to natural selection
Short repeated segments that are not protein encoding,
distributed all over the genome
Detects differences in repeat copy number
Calculates probability that certain combinations can
occur in two sources of DNA
Requires molecular techniques and population studies
Preparing DNA for Profiling –Restriction
Enzymes


Chop up the DNA at specific sequences
using “restriction enzymes”
Creates RFLPs

Restriction fragment length polymorphisms
Preparing DNA for Profiling –Running a
Gel

Run samples on an agarose or
polyacrylamide gel


DNA has a negative charge so it will travel
toward a positive charge
Larger fragments will not move as far through
the gel
DNA Profiling




Developed in 1980s
Identifies individuals
Used in forensics, agriculture, paternity
testing, and historical investigations
DNA can be obtained from many sources
DNA Profiles
Figure 14.9
DNA Profiling

Types of repeats


Variable number tandem repeats
(VNTRs)
Short tandem repeats (STRs)
Shorter than VNTRs
 Useful if DNA from sample is fragmented
or degraded


mtDNA

Useful if nuclear DNA is highly damaged
A Sneeze Identifies Art Thief
Table 14.6
Comparing DNA Sequences
Figure 14.10
Figure 14.10