Week 3 Genetic Basis of Evolution Flashcards
Define a Gene
a DNA segment that contributes to phenotype/function
Define a Locus (pl. Loci)
– The position on a chromosome of a gene or other chromosome marker
– Can also refer to the DNA at that position
– The use of locus is sometimes restricted to mean regions of DNA that are expressed
What procedure can be used to find specific DNA sequences in the genome?
FISH = flourescence in situ hybridisation
MYCN is an oncogene in Neuroblastoma, the FISH procedure can find the locus of this oncogene
Define an Allele
Variant of a gene
Different alleles can lead to different phenotype
How many copies of each gene does a diploid carry?
2 copies of each gene
Alleles present in a homozygote and heterozygote?
- *homozygote:** possesses two copies of the same allele
- *heterozygote:** possesses two different alleles
What is the allele frequency (proportion) calculation?
(2xHomozygote + Heterozygote) / (total)
Define a Genotype
The genetic makeup of an individual (one or more loci)
Genotype Frequency Calculation
When alleles are rare, are they more commonly found in homozygote or heterozygote genotypes?
Heterozygote genotypes
Define a Phenotype
The physical/behavioural (etc.) characteristics of an individual
Interaction of genes and environment. Genetic component of the phenotype is heritable, environmentally acquired component of phenotype is not.
Define a Gamete
Germline cell that is able to unite with another of the opposite sex during sexual reproduction
– Produced by meiosis
– Contains half the chromosomes of the parents
Define a Zygote
The earliest developmental stage of the embryo
– Produced by the fusion of two gametes
Does the terms ‘dominant’ and ‘recessive’ apply to genes or alleles?
Alleles
What is the Hardy-Weinberg Equilibrium?
Developed as a null model, in order to understand how changes in gene frequencies occur. That is, we need to understand the conditions under which NO evolution will occur
In 1908, GH Hardy and W Weinberg independently published a theorem showing how allele frequencies behave given a particular set of assumptions.
Assumptions of Hardy-Weinberg Equilibrium:
• The organism is diploid
• Reproduction is sexual
• Generations are non-overlapping
• Mating is random
• Populations are infinite
• There is no mutation
• There is no migration
• There is no natural selection/drift
HWE: Given the adult population, what is the value of f(p) and f(q)?
f(p) = 137/200 = 0.685
f(q) = 63/200 = 0.315
HWE: Gene and genotype frequencies must sum up to what?
What are the equations for the theorem?
p + q = 1
p2 + 2pq + q2 = 1
1
HWE: If the assumptions of HWE are met, frequecies change or do not change?
do not change
HWE: When observed genotype frequencies are equal to the expected genotype frequencies under the HWT, we say that the population is in ….
HWE
HWE: A population will go to HWE in only …. generation of random mating
in only one generation of random mating
Define Evolution
A change in allele frequencies over time
What are the two factors which are major contributors to genetic differentiation?
Genetic drift & Selection
Define Genetic Drift
(what is it a consequence of & where does it take place)
process by which allele frequencies change over time due to the effects of random sampling
- Takes place as a consequence of finite population size
- Genetic drift takes place in all populations, and any selection must occur against this background of drift
How does genetic drift work?
- the change is due to “sampling error” in selecting the alleles for the next generation from the gene pool of the current generation.
- By pure chance we might sample a particular allele more or less often than expected, causing the allele frequencies to change from one generation to the next
Graph of a particular allele frequency as it changes over time:
what happens?
- allele frequency eventually gets to p=1
Graph of a particular allele frequency as it changes over time: why does the allele frequncy get stuck at p=1?
Fixation - stuck at p=1 because there is only 1 allele left to sample
The allele has become fixed in the population
The other possibility is that the allele gets lost, in which case the other allele must have become fixed (assuming two alleles)
Graph of genetic drift with the process of evolution:
what happens?
- Equal chance of drifting up or down
- Overtime it is certain that one or the other allele will become fixed and the other will become lost.
- The likelyhood of these events depends on the starting allele frequency
- No selection in this model
Is genetic drift stronger in a smaller or larger population?
Genetic drift is stronger in a small population
The effect of random sampling is greater
Genetic Drift: what is the bottleneck effect?
an extreme example of genetic drift that happens when the size of a population is severely reduced
Events like natural disasters can decimate a population, killing most indviduals and leaving behind a small, random assortment of survivors
Why does selection occur?
Because different individuals have different fitness
Define Fitness
The reproductive success of a genotype, usually measured as the number of offspring produced by an individual that survive to reproductive age relative to the average for the population.
How do you measure Absolute Fitness?
calculate the change in abundance of a genotype from one generation to the next (assuming infinite population size, no mutation, and non-overlapping generations)
the most fit genotypes will increase in abundance compared to less fit genotypes.
How do you measure Relative Fitness?
calculated by dividing all fitness values by the largest value
Relative fitness is always measured with respect to the “best” or “most fit” genotype in our sample
meaning the fittest genotype always has a relative fitness of 1
What is selection, in regards to fitness occuring or not?
a process (not really a ‘force’) leading to different expectations of transmitting genes to the next generation
- different fitness amoung a popultion means selection is occuring
- same fitness means no selection, or that the population is evolving neutrally (evolution by genetic drift)
The fitness of different genotypes is often represented by what symbol?
w (omega)
Example: fitness of AB genotype = wAB
The strength of selection is often represented by what symbol?
s
AB not the fittest genotype - strength of selection against heterozygotes can be thought of as the deficit from a relative fitness of 1, so that
wAB =1–s
Selection and Drift: The effect of high fitness is to make an individual …… to be the parent of offspring in the next generation
But..
The effect of high fitness is to make an individual more likely to be the parent of offspring in the next generation
But.. It is still possible that a fit individual will get unlucky and end up having no offspring
Selection and Drift: what is occuring in this graph?
A model in which A is dominant and has high fitness
Allele frequencies still drift around as before, but now there is a systematic change in an upward direction.
There is still one case in which, despite the high fitness of individuals with the A allele, the A allele gets lost due to pure chance
What is stronger, selection or drift?
Selection
