Populations Flashcards
Def of population
All individuals of same species, that live in same area ata same time, and can interbreed
what do population genetics study
. interactions of alleles + genes in pop
. distribution of allele freq in pops
. change of allele frequency under influence of natural selection, mutation, gene flow + genetic drift
define gene pool, natural selection, genetic drift, bottleneck effect, founder effect, gene flow, allele fixation
- gene pool - all alleles at all loci in pop
- NS - survival + reproduction of individuals in pop sue to differences in traits
- Genetic D - changes in gene pool of small pop due to chance eg tornado, earthquake,
DRIFT CAUSES BOTTLENECK AND FOUNDER - Bottleneck - pop undergoes drastic reduction in size due to chance ( cause of Genetic D)
- Founder - small group of individuals become separated from larger pop. (cause of genetic D)
- Gene flow - movement of genes btw pops. gain/loss of alleles from pop due to migration of individuals or transfer of gametes. gene flow is a result of migr.
- Allele F - when 1 allele of gene becomes the only allele, while alternatives eliminated from pop
Phenotype frequency def
how to calculate
proportion of individuals in pop that has specific phenotype.
. num of individuals wth given genotype ÷ total invidualsin pop
AA+Aa/AA+Aa+aa or aa/AA+Aa+aa
what is the sum of all phenotype freq for a trait.
1
PF is always btw 0-1
Genotype Frequency def
proportion of individuals wth specific genotype in pop.
calculate: num of indiv wth phenotye ÷ total indi in pop
AA/AA+Aa+aa or Aa/…. or aa/…..
how can GF be calculated directly
if genotype has its own phenotype (ie, incomplete dominance/ codominance)
MN blood group system - based on 2 Codominant alleles
Allele frequency
proportion of copies of given gene in pop that is specific allele. shows how abundant particular allele is in pop.
calc: num of copies of given allele in pop ÷ total num copies of all alleles of this gene in pop.
( which is twice the num of individuals cos they diploid.)
F(A) and F(a) calculations
F(A) = F(AA) + 0.5F(Aa)
F(a) = F(aa) + 0.5F(Aa)
what do you do when 2 events are independant of eachother? then when an event occurs in 2 incompatible ways.
independant - probability that both occur is PRODUCT of their probs
Incompatible - probabiluty is SUM of their probs
what is relative frequency of an event
number of times its occured ÷ total num it could hv occured
when a statistical event is repeated many times, its relative frequency approximates its theoretical probability.
What is hardy weinberg law. what are the conditions
p^2 + 2pq + q^2 = 1
. large pop
. no mutations
. no migrations
. random mating
Applicationn of HW
prediciting allele + genotype freq in cases of complete dominance, when its impossible to find directly
what are the forces of evolutions ( changes balance of genotypes + breaks down HW rule)
mutations, migrations, non random mating, isolation, natural selection, genetric drift
Gentetic drift
random fluctuations of allele freq due to chance events. Its an example of sampling error
. important in small pops as can eliminate alleles, and leave only 1.
. produces evolutionary change but not adoptive ( the poplation doesnt become more adapted due to it)
what is bottleneck effect
pop is greatly reduced (ie hurricane kills most in a pop). who lives/dies is unrelated to phenotype
(humans) - when small group move to new teriiroty. the new pop differ from og as they share same genes but the balance of gene varients in each pop differ
what is founder effect
few individuals leave their pop + found a new one (eg colonise an island). chnages in gene pool
. not representative of og pop
. most common = loss of alleles, but also possible for a rare mutant allele to increase its freq
example = amish community was founded by small num of colonists. these colonists had gene for polydactyly. since amish quite isolated and few outsiders join, polydactly much more common in amish.
What is migration/ gene flow
. when individualsmove + breed in a new pop (+ gives their genes in that pop)
. also occurs by HYBRIDIZATION - individuals from 2 separate pops breed, their offspring carry genes from 1 pop to another.
increases variability of gene pool. Also unites pops, making a common gene pool of entire species.
What is natural selection
when idividuals who hv certain genes are more fit to mature and reproduce than those without those genes, the freq of these genes will increase.
- only eolutanary force that can cause adaptive evo
what does NS include
- differential viability
- differential fecundity (ability to produce an abundance of offspring)
eg, vit D synthisized in UV exposed human skin. over the years, this need to make Vit D was evo driver for skin lightening in humans.
how to measure natural selection
. relative fitness (w) + selection coefficient (s)
. They characterize success of indiv with given phenotype
. success measured by avg num surviving offspring left by individual having the genotype. This lifetime reproductive success = absolute fitness
. AF dependant on changes to pop size tho
so we do:
af of each genotype ÷ af of best genotype
gets us relative fitness
. best genotype is w=1, genotype thats lethal/ 100% infertility is w=0
. s = 1 - w: selection coefficient measures how strongly selection acts against given genotype.
. higher s is worse. (s=1) best genotype s= 0
how does selection change allele freq for dom + recces
dominant allele cant be hidden so selection against it is very effiecient + eleiminates fromm pop
selection against reccessive efficient at first, but slower after allele freq drops, due to larger prop of the recces allele is protected i herterozygotes.
this means NS alone cant eliminate R alleles, just reduces.
IF S=1 DOMINANT ALLELE ELIMINATED IN NEXT GEN
IF S=1 RECESSIVE ALLELE NEVER ELIMINATED AS PROTECTED IN HETEROZYGOTES
eg of selection = Duffy null. explain
Duffy gene encodes cytokine receptor used by plasmodium vivax to invade erythrocytes.
duffy null mutations give resistance to vivax malaria. Its fixed in sub Saharan Africans where malaria is common (like in madagasgar) + absent in others.
frequency of duffy null of madagascans is >0.7 usually its 0.5.
its selection = 0.066. protects against the malaria
What is importance of mutations
evolution depends onit as it makes new alleles.
allows for natural selection.
. they rare but occurs each generation. along wth genetic drift, non coding seq evolve faster than coding, and not needed genes accumulate mutations + turn into PSEUDOGENE.
what is mutation rate
μ - freq of norm allele to turn into mutant allele per gamete. (longer genes mutate more often)
. too many mutations can lead to disease/ birth defects
. mutation rate increases when DNA replic or repair is disturbed.
. smaller pops less able to compensate for genetic change that increase μ.
what are some mutation rates of diseases
hemophilia A = 2.5 x 10^4
duchenne muscular dystrophy = 1 x 10^4
. avg rate of nucleotide substitutions is 10^-8 to 10^-9 per generation
what happens when mutation is deleterious
the 2 processes will act against each other. Each gen, selection will reject some mutant alleles, but mutation will produce new ones.
overtime the mutant allele reaches equlibrium freq
what is the cause of ore prevelent deleterious recessive alleles.
selective adv of heterozy over homozy for the normal allele
- eg sickle cell anemia (s = 0.76). but in some parts allele F = 18% (μ >2%).
- heterozygotes turn out to be resistant to falciparum malaria
Non random mating and its types
indivi hv mating prefrences so dont randomly mate in pop. changes genotype frequencies.
- ASSORTATIVE mating - individuals choose partners based on phenotype similarity. can be positive or negative (similar/disimilar). positive increases homozygosity, negative heterozygosity.
- INBREEDING - mating btw individuals related by ancestry (common in small isolated pops)
example of positive assortive mating
Deafness - deaf people communicate by sign lang instead of speech.
deafness phenotype has frequncy higher than q^2 as more homozygotes.
connexin gene cause of hereditary deafness
mating with a relative - what does it do
. affects many loci cos partners share many genes from common ancestors
. extreme - self fertimlisation in hermaphrodites (produces gametes by meiosis). Partner wth SAME GENOTYPE AT EVERY LOCUS. heterozygosity decreases (50% reduct per gen)
. inbreeding increases reccessive alleles in homo. it often diminishes fitness = inbreeding depression
examples of preventing inbreeding
- lions push male cubs out of pride when they mature so they dont breed with their sisters.
- self incompatibility in some plants
Relatedness coefficient ( coeff of relationships)
r - meausres how related 2 individuals are. if unrelated r = 0
. calculated by the fact that the prob for each allele transferred from parent to child = 0.5.
. pedigree diagram used (pg 39)
Inbreeding coefficien
F= 1/2 x rleatedness coeffivient
F- prob that any locus of individual has 2 identical alleles by descent.
F only measured if parents are relatives. if unrelated, F of progeny = 0
in isolated pop, at some point individuals run out of unrelated mating partners so everyone acquires some inbreeding coefficient.
Inbreeding coefficient
F=0.5x2
consanguineous = related
Full sibls hv relatedness r = 1/2, + their offspring hv an inbreeding coefficient F =1/4. This means they are identical by descent at one-quarter of their loci.
If any of these loci involve recessive mutant allele that affects pre- or perinatal health, mortality of children will be higher
reduced health of inbred children = inbreeding depressions
- prevelent in spanish kings habsburg dynasty
what are quantative traits
. determined by interaction + additive effects of multiple
genes + environmental factors. Also called polygenic /
multifactorial.
. doesnt follow mendelian inheritance pattern
. example of this trait = height, weight, skin colour, predisposition to heart diseases.
Often, trait which is quantitative in the normal population has a pathology which is
qualitative (e.g. normal height vs. dwarfism, normal skin color vs. albinism).
Model of inheritance of quanitative traits pg 57
normal distribution wth intermediate phenotype the most common.
. Cos more intermediate the phenotype, the greater the number of genotypes that can cause it.
. The more genes involved, more phenotypic classes occurs.
. The more genes involved, the less the distinction between classes.
Normal distrubution
The spread ND controlled by the standard deviation, σ
(s.d.). The smaller the sd the more concentrated the
data. For some traits, the entire range
of the normal curve is normal. For
others (e.g. weight), one /both
extremes are abnormal (pathological).
For many quantitative traits, the
distribution is “skew-normal”, i.e. more
or less asymmetrical.
