When Allele Frequencies
Stay Constant
Population
An interbreeding group of the same species within a given
geographical area
Gene pool - the collection of all alleles in all members of the
population
Population genetics - the study of the genetics of a
population and how the alleles vary over time
Phenotypic frequencies
For the most part, we can easily assess the frequency for
various phenotypes
Consider phenylketonuria (PKU)
It is an inborn error of metabolism that can cause mental
retardation unless the person avoids phenylalanine
But the phenotype is related to the genotype
And the genotype is related to the allele frequencies
Microevolution
Genetic changes due to changing allele frequencies in a population
Allele frequencies can change due to:
Nonrandom mating
Gene flow
Genetic drift
Natural selection (unequal reproductive success)
Macroevolution is the formation of new species
Change in allele frequencies is the normal situation
In rare instances where allele frequencies are not changing,
we have Hardy-Weinberg equilibrium
Hardy-Weinberg equilibrium
Consider a gene with only two alleles
p = allele frequency for one allele (A)
q = allele frequency for second allele (a)
p+q=1
p2 + 2pq + q2 = 1
p2 = AA, q2 = aa, 2pq = Aa
Consider a short middle finger phenotype - recessive trait
What if 9% of the population have this?
q2 = 0.09
q = 0.3, p = 0.7
Now if we consider random mating, what will we see in the
future for this population?
By knowing the incidence of a particular disorder, we can
calculate everything else
Population Variation
The frequency of an allele may vary between populations
A disorder may be common in one group but rare in another
There are a number of reasons
Disease alleles arise through mutation
Most early human groups were isolated from each other
If mutations arise independently in different groups, they
will do so at different times
Calculating Risk with X-linked Traits
DNA profiling
Hardy-Weinberg equilibrium also applies to portions of the
genome that do not affect phenotype
Therefore, there is no selective pressure
Short, repeated, non-protein-coding sequences
SNPs (single nucleotide polymorphisms) - single base pair
differences between chromosomes
Repeated sequences - variation in the number of repeats
present
DNA profiling
Repeats are distributed all over the genome
Techniques are used to detect differences in the number of
repeats
The likelihood that certain combinations can occur in two
distinct sources of DNA is calculated
This requires molecular techniques and population studies
DNA fingerprinting
Developed in the 1980s
Used to identify individuals
Many different applications
Forensics
How is the information obtained?
DNA profiling
DNA databases are essential for the statistical analysis
Both nuclear and mitochondrial DNA can be used
Very small amounts of DNA can be used because of PCR