Allele Frequency
Number of genes of the counted allelic type, divided by the total number of gene
- At a given site, the frequency of allele A is the number of genes in the population that fall into the A allelic class divided by the total number of genes in the population -
Can you calculate allele frequencies in simplified examples?
Yes, the number of genes of the counted allelic type, divided by the total number of genes (i.e., twice the number of people)
- In this 2-person population, the frequency of the C allele at the fifth site is 0.25 (1/4). The frequency of the A allele is 0.75 (3/4).
- In this 2-person population, the frequency of the T allele at the ninth site is 0.5 (2/4). The frequency of the C allele is also 0.5 (2/4).
How should allele frequencies at sites (SNPs) affecting a binary trait differ in cases and controls?
.
What is mutation?
The ultimate origin of all genetic differences.
What is natural selection?
Sometimes an allele frequency does not change just by chance
- One allele may consistently tend to become more common.
- Individuals who are better at surviving and reproducing pass on to their offspring the genes partly responsible for their success.
What conditions must be met for natural selection to occur?
- There are individual differences that affect fitness (the ability to survive and produce offspring).
- Some fraction of those individual differences are caused by genetic differences (i.e., the differences are heritable, h2 > 0).
Does schizophrenia show any relationship to fitness (the ability to survive and reproduce)?
Great reduction in fitness compared to the general population
- Males are less than females
Does autism spectrum disorder show any relationship to fitness (the ability to survive and reproduce)?
Great reduction in fitness compared to the general population
- Males are less than females
- SCZ fitness reducing
What does natural selection tend to do to existing genetic variation introduced by long-ago mutations?
later generations become genetically and phenotypically more and more like the fittest members of the earlier generations
Does it matter whether natural selection favors a higher mean, a lower mean, or the current mean?
Stabilizing selection favors the current mean; individuals with extreme values in either direction have lower fitness.
- Our examples so far have focused on directional selection, which favors one extreme of the phenotype (higher or lower).
Natural Selection: why do traits show as much genetic variation as they do today?
Heritability is the fuel of evolution; if a trait is not heritable (i.e., h2 = 0), then it cannot change in response to natural selection.
- Problem: If the h2 of a trait is the fuel of its evolution, we might naively expect a trait that has undergone substantial evolution to have a nearly empty tank today.
What is the effect of paternal age on the number of mutations transmitted from fathers to children?
A 45-year-old father is expected to transmit 50 more mutations than a 20- year-old father.
What did the study by D’Onofrio et al. (2014) find about the within-family relationship between birth order and mental illness?
It appears that 50 more mutations cause a ~25-fold increase in the rate of BIP diagnoses!
- Within-family results of D’Onofrio et a provide some evidence for the notion that there are many potential sites affecting mental illnesses.
What might happen to such genetic differences if the optimal trait value differs in different parts of the range where the species lives?
Keep in mind that certain forms of mental illness (BIP, ASD) show positive genetic correlations with traits that we might naively call “good” (higher IQ, more education).
- Perhaps there are tragic tradeoffs; optimizing one aspect the mind may lead to impairments of other aspects.
How is this possibility illustrated by the study of Trinidad guppies by O’Steen et al. (2002)?
.
What is the Hawk-Dove game-theoretic model?
- Hawks fight each other; a hawk takes the resource from a dove without a fight; doves essentially “flip a coin” to decide who gets the resource.
- If there are a lot of hawks constantly bloodying each other in fights that inflict costs greater than the value of the resource, rare doves who never get hurt can have higher fitness.
- But if there are a lot of doves who never put up a fight, rare hawks will have higher fitness because they can always take what they want!
- Neither the allele encoding HAWK nor the allele encoding DOVE needs to be driven from the population. There can be a balance where both alleles are common.
How does this model explain the preservation of individual differences?
Whether this basic idea can be applied to more realistic situations (e.g., highly polygenic traits such as Agreeableness) remains to be seen.
What feature must be present in order for a situation to count as an example of the Prisoner’s Dilemma?
Whether cooperating or not cooperating is beneficial depends entirely on the composition of the population at that time.
- Collaborating or decanting with the dilemma
- cooperate and not
- There is a cost to play nice
In the multiple-round (repeated) Prisoner’s Dilemma, who does better in a tft vs. alld encounter
TFT
In the multiple-round (repeated) Prisoner’s Dilemma, who does better in an alld vs. allc encounter
ALLD
In the multiple-round (repeated) Prisoner’s Dilemma, who does better in a allc vs. alld encounter
ALLD
What principle does Martin Nowak’s “War and Peace” model use to explain the persistence of individual differences in traits related to cooperation and defection?
high Honesty (H) with an unwillingness to defect first and high Agreeableness (A) with the willingness to forgive at least a few defections - The heritabilities of Honesty and Agreeableness might be maintained because human populations are constantly in the middle of changing from one balance to another.
Why is sexual reproduction a scientific puzzle?
Slower than asexual
- more difficult
- harder to find a species to get them to mate with you
- risky
Sexual Reproduction: what does the “twofold cost of males” refer?
Asexual reproduction vs sexual reproduction
