Population Genetics

1
Population Genetics

• Evolution Option D-6

2
Objectives

• Define Population and Gene Pool
• Describe environmental factors which have an
  effect on the frequency of allele in a population
• Describe the Hardy Weinberg Equilibrium
• Describe the conditions that must be maintained
  for the Hardy-Weinberg Equilibrium to exist
  within a population and what changes in allele
  frequencies indicate about the population.
• Describe the results of Founders Effect and
  Bottle Necking Event on allele frequencies within
  a population

3
Populations and Gene Pools

• A population is a group of organisms of the same
  species in a given area at a given time. A
  species is a group of similar organisms which can
  interbreed and produce fertile offspring.
• A population can also be considered as a group of
  alleles. All phenotypes of the organisms are
  determined by the genotype or the alleles of the
  organism. A gene pool is all of the alleles and
  the different gene types for that allele that
  exists within a population.

4
Environmental Factors Which Affect Alleles in a
Population

• Immigration and Emigration Adding or losing
  individuals from a population will add or remove
  alleles from the gene pool
• Mutation Adding new types of genes created by
  mutations will change the frequency of particular
  genes that exist in a population for a particular
  allele in a gene pool.
• Selection Artificial (selective breeding) or
  Natural will increase some allele types frequency
  which are favored while it will cause a decrease
  in the allele types that are not favored or are
  selected against in the gene pool
• Mating If selective mating occurs, some alleles
  may be favored and other not favored by
  individuals within the population which will
  affect a particular alleles frequency in the
  gene pool
• Catastrophic Events or Natural Disasters If a
  catastrophic event occurs it can greatly reduce
  allele frequency in a population which will
  result in an increase in another allele frequency
  in the gene pool.
• Population Size The gene pools of large
  populations are less likely to be affected by the
  previous factors than the gene pools of smaller
  populations.

5
Hardy-Weinberg Equation

• The Hardy-Weinberg equation is a means to
  represent genotype and allele (gene) frequencies
  within a population of a known size. It is as
  follows
• p2 2pq q2 1
• p2 Homozygous Dominant Individuals
• 2pq Heterozygous Dominant Individuals
• q2 Homozygous Recessive Individuals
• 1 100 of population

6
Use of Hardy-Weinberg Equation

• The equation can be used to determine the
  genotype frequency and allele (gene) frequency in
  a population when give the value of p2 or q2.
  Since recessive individuals (q2) are most easily
  identified this is the one which is most commonly
  used. For example in a population of 100
  individuals, 25 of them have blue eyes (bb q2
  ), what is the genotype frequency for the
  homozygous dominant (BB p2 ) and heterozygous
  dominant (Bb 2pq ) individuals and the
  frequency for the dominant (B) and recessive
  alleles (b) in this population?

7
Use of Hardy-Weinberg Equation to Determine
Genotype Frequency

• p2 2pq q2 1 can be written as
• (p q)2 1
• If you take the square root of each
• v (p q)2 v 1
• p q 1
• You know 25 individuals out of 100 have blue eyes
  (bb q2 )
• Solve for q
• q2 .25
• q .5
• Now put in the value of q using the simplified
  equation
• p q 1.00
• p . 5 1.00
• Solve for the value of p
• p .5

8
Use of Hardy-Weinberg Equation to Determine
Genotype Frequency

• Now that you have determined the values of p (.5)
  and q (.5) you now can plug the values into the
  equation and determine genotype and allele
  frequency in the population.
• p2 2pq q2 1
• .52 2(.5)(.5) .52 1
• .25 .50 .25 1
• In other words
• 25 individuals are homozygous dominant
• 50 individuals are heterozygous dominant
• 25 individuals are homozygous recessive

9
Use of Hardy-Weinberg Equation to Determine
Allele Frequency

• Remember the genotype of an individual is
  determined by the interaction between two alleles
  which in turn determine the phenotype of the
  individuals.
• For example from the previous problem
• 25 Individuals were BB for brown eyes which
  means, each of these individuals have two
  dominant (B) alleles giving a total of 50
  dominant alleles to the gene pool.
• 50 Individuals were Bb for brown eyes which
  means half of their genotype consist of a
  dominant allele (B) and half of the recessive
  allele (b). Therefore these individuals add 50
  dominant and 50 recessive alleles to the gene
  pool.
• 25 Individuals were bb for blue eyes which
  means each of these individuals have two
  recessive (b) alleles giving a total of 50
  recessive alleles to the gene pool.
• Therefore the total number of dominant alleles
  for brown eyes in the population would be
• 50 allleles (25 X2) from the BB
  individuals and 50 alleles from the heterozygous
  Bb individuals. Giving a total of 100 dominant
  alleles out of 200 total alleles or 50.
• The total number of the recessive alleles for
  blue eyes in the population would be
• 50 allleles (25 X2) from the bb individuals and
  50 alleles from the heterozygous Bb individuals.
  Giving a total of 100 recessive alleles out of
  200 total alleles or 50.
• 100/200 100/200 100
• BB bb
• 
  .50 .50 100

10
Genetic Equilibrium

• If a genotype or allele frequency does not change
  in a population after repeated sampling and
  calculations over a given period of time the
  population is said to be in genetic equilibrium.
• This is very unlikely however, because any of the
  following events would cause a change and
  equilibrium could not exist. Therefore the
  following must be true if genetic equilibrium is
  to be maintained.
• 1. No mutations
• 2. No selection (natural or artificial)
• 3. Totally random mating
• 4. No immigration or emigration
• 5. No catastrophic events
• 6. Large and stable population size

11
Genetic Drift

• Genetic drift cumulative changes in gene
  frequency over successive generations because of
  chance fluctuations this may eventually result
  in certain alleles being completely lost from a
  population.One aspect of genetic drift is the
  random nature of transmitting alleles from one
  generation to the next given that only a fraction
  of all possible zygotes become mature adults. The
  easiest case to visualize is the one which
  involves binomial sampling error. If a pair of
  diploid sexually reproducing parents (such as
  humans) have only a small number of offspring
  then not all of the parent's alleles will be
  passed on to their progeny due to chance
  assortment of chromosomes at meiosis. In a large
  population this will not have much effect in each
  generation because the random nature of the
  process will tend to average out. But in a small
  population the effect could be rapid and
  significant.

12
Genetic Bottleneck

• This event can occur when a small population is
  affected by some type of random catastrophic
  event which will indiscriminately reduce an
  allele type in the population. This results in a
  great reduction of genetic variation within the
  population. This is commonly observed in
  endangered species.

13
Founder Effect

• Founder Effect occurs when a small population
  becomes isolated and inbreeding occurs. Therefore
  the frequency of an allele expression increases.
  For example Polydactylism or supernumerary digits
  occurs in certain Amish communities in
  Pennsylvania and in certain isolated villages in
  the Andes Mountains. This trait is dominant and
  occurs in about 1/400 births. However due to
  population isolation and inbreeding, you may have
  as many as 75 of a population exhibiting this
  particular trait.

14
Population Genetics and Evolution

• Remember that genotype determines phenotype of
  the organisms. The phenotypes are the physical
  characteristics or adaptations an organism has.
  If an organism does not survive and reproduce or
  does not reproduce due to selective forces of the
  environment its alleles are not passed on into
  the future gene pool of the population.
  Therefore, its particular allele type will
  decrease in the gene pool. If an organism
  survives and reproduces, one half of its alleles
  will be passed on into the future gene pool of
  the population. Therefore, its particular allele
  type will remain the same or increase in the gene
  pool. Therefore genotypes and phenotypes (traits)
  are constantly changing in population over
  extended periods of time, this results in changes
  in organisms in other words, evolution of a
  species. Evolution begins at the population
  level for all species.