BIOL10004 Study Guide - Quiz Guide: Gene Flow, Founder Effect, Panmixia
Genetic diversity arises by mutation and recombination
• Genetic diversity is caused by mutation and recombination
Mutation:
• Radom chemical event causes mutation
- Replace a base by another
- Insertion / deletion of base
- Rearrangement of large region of chromosome
• Not necessarily harmful or beneficial
• Synonymous mutation:
- Change from one codon for an amino acid to another codon for the same amino acid
- Only a little effect on survival or reproduction of the carrier
• Missense mutation:
- A mutation changes the nucleotide sequence of a condon, cause it encodes a
different amino acid
• Lethal mutation:
Result in failure of the organism to survive during the development
- Kill the organism
• Slightly detleterious mutation:
- Mutatio that do’t kill a ogais ut odeatel edue its fitess
• Alleles that arise from advantageous mutations increase in frequency only because they
cause greater fitness to individuals in a given environment.
• Only synonymous mutations or very rare advantageous mutations would produce alleles
that increase in frequency in the population.
Recombination
• is a process by which pieces of DNA are broken and recombined to produce new
combinations of alleles.
• In eukaryotic cell,
- Recombination occurs during meiosis
- During the first phase of meiosis, the homologous pairs of maternal and paternal
chromosomes align.
- During the alignment, the arms of the chromosomes can overlap and temporarily
fuse, causing a crossover
- Crossovers result in recombination and the exchange of genetic material between
the maternal and paternal chromosomes
• recombination rapidly generates new allele combination, then generate new genetic
variants ( genotype )
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• Recombination is limited in some species that are haploid, cos it can only happen when
two different genome are present in a diploid cell
Stability and change in genetic structure of population
• Genetic composition of a population can be presented by a proportion of alleles and
genotype in that population
• Allele can increase or decrease in frequency in a population
• Random mating :
- Each individual has an equal chance of mating with each other individual
- Alleles of each locus are paired up at random to make new genotypes in each
generation
• Hardy–Weinberg principle
- a model for the genetic structure of a population
- states that phenotypic proportions in an ideal population
- tend to remain constant at equilibrium values that can be estimated from allele
proportions.
- 5 properties:
1. no mutation
2. population is infinitely large
3. individuals mate at random
4. no exchange of genetic material with other population ( gene flow)
5. no natural selection
- Hardy-Weinberg equilibrium:
The equation p2 +2pq+q2= 1 predicts an equilibrium
p + q =1
- e.g. a population contain single polymorphic locus with two alleles, A and a.
# population will contain three genotypes ( AA, Aa, aa)
# If the proportion of A allele in population is p, proportion of a allele is q
# after one generation an ideal population will establish an equilibrium
*which proportion of AA=p 2 , Aa=2pq and proportion of
aa = q2
• Evolutionary change may result if there are deviations from the Hardy–Weinberg
equilibrium.
Deviation from the Hardy-Weinberg equilibrium
• Mutation is a week evolutionary force:
- Is a deviation from the Hardy-Weinberg ideal population
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- mutation is essential for the initial generation of genetic variation, its low frequency
of occurrence makes it unlikely to significantly shift a population from the Hardy–
Weinberg equilibrium.
• Gene flow
- The exchange of genetic material between population
- can affect the proportions of alleles if the populations have different alleles, or the
same alleles with different proportions.
- The greater the different on allele proportion between the gene pools, the greater
effect will be
- where gene flow is low, populations are free to diverge genetically from one another
due to evolutionary forces
• Assortative mating
- Non-random mating in which phenotypically alike (positive assortative mating )/
non-alike individuals mate preferentially (negative assortative mating )
- Natural population often exhibit assortative mating
- Positive assortative mating:
the two parents will often contribute the same allele to the offspring, so the
proportion of homozygotes will be higher than expected from random mating.
- Negative assortative mating:
when individuals prefer to mate with genetically dissimilar individuals.
Chance and inheritance: genetic drift
• inheritance of alleles is a chance process, certain alleles may change in proportion from
generation to generation simply by chance.
• Natural populations can undergo change because chance plays a role in inheritance.
Such a process is termed genetic drift.
• The ado hage i a populatio’s allele feue fo oe geeatio to the et
that is attributable to chance, it occurs more quickly in small population
• Genetic drift leads to a gradual loss of genetic variation in populations, as allele
frequencies drift to fixation
• The tendency of small populations to lose alleles by genetic drift may account for the
high levels of homozygosity
• Genetic drift is particularly important in small populations, for example, in small
founding populations (the founder event) or when populations decrease for a period of
time (a population bottleneck).
Founder event
• Loss of genetic diversity when a small group of individuals separates from the larger
population and establishes a colony in a new location;
• genetic drift is common due to the small population size
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Document Summary
Only a little effect on survival or reproduction of the carrier: missense mutation: 5 properties: no mutation, population is infinitely large, individuals mate at random, no exchange of genetic material with other population ( gene flow, no natural selection. Genetic diversity arises by mutation and recombination: genetic diversity is caused by mutation and recombination. Rearrangement of large region of chromosome: not necessarily harmful or beneficial, synonymous mutation: Change from one codon for an amino acid to another codon for the same amino acid. Only a little effect on survival or reproduction of the carrier: missense mutationslimemoulds. A mutation changes the nucleotide sequence of a condon, cause it encodes a different amino acid: lethal mutation: Result in failure of the organism to survive during the development. Kill the organism: slightly detleterious mutation: Recombination is a process by which pieces of dna are broken and recombined to produce new combinations of alleles.
