Genetics Chapter 24.1 and 24.2
polymorphism
"many forms"- many traits display variation within a population, due to genes with multiple alleles that influence the phenotype of the organism that inherits them. may be due to various types of changes such as deletion of a significant region of the gene, a duplication of a region, or a change in a single nucleotide
formula for allele frequency
# copies of an allele in the pop/ total # alleles for that gene in a pop
formula for genotype frequency
# individuals with a particular genotype in a pop/total # individuals in a pop
equation for probability of elimination in a pop
1-probability of fixation
what 5 conditions must be met in order for the Hardy weinberg equilibrium to be true?
1. no new mutations in the gene 2. no genetic drift- the population must be large enough that allele frequencies do not change due t random sampling effects 3. no migration 4. no natural selection- all of the genotypes must have the same reproductive success 5. random mating regardless of phenotypes or genotypes in the pop
microevolution is rooted in what 2 phenomena?
1. the introduction of new genetic variation into a pop is one essential aspect of microevolution, which can originate by a variety of molecular mechanisms 2. action of evolutionary mechanisms that alter the prevalence of a given allele or genotype in a pop, including random genetic drift, natural selection, and nonrandom mating. these mechanisms have the potential to promote widespread genetic changes in a pop by favoring an allele or creating a balance.
how does natural selection relate to our knowledge of molecular genetics to the phenotypes of individuals?
1. within a pop, variation arises from random mutations in DNA, creating new alleles that may alter the amino acid and therefore protein (some of which affect survival) function 2. Individuals with beneficial alleles are more likely to survive and contribute to the gene pool of the next gen, which may alter allele frequencies over time and thereby the characteristics of a species, result in in a pop better adapted to its environment
equation for probability of fixation of a mutation in a pop
1/(2N) assuming equal # males and females contribute to next gen. N still = # individuals in the pop
equation for the expected number of mutations in a natural pop
2Nu u= mutation rate N= number individuals in a pop
population
A group of individuals of the same species that occupy the same region and can interbreed with one another (not all of a species are a pop because many species occupy a wide geographic area and are divided into discrete populations). typically dynamic units that change from one generation to the next (in size, geographic location, and genetic composition).
who proposed the theory of evolution by natural selection and when?
Charles Darwin and Alfred Russel Wallace
who derived the hardy weinburg equilibrium and when?
In 1908, Godfrey Hardy and Wilhelm Weinberg derived the mathematical equation that predicts stability of allele and genotype frequencies from one generation to the next
who helped develop the idea of random genetic drift and when?
Sewall Wright in the 1930s
who developed the mathematical foundations as extensions of mendells work by deriving formulas to explain the occurrence of genotypes within pops?
Sir Ronald Fisher, Sewall Wright, and JBS Haldane
what equation can be used to calculate the allele frequencies of the conglomerate?
[delta]P[sub]c = m(P[sub]D - P[sub]R) [delta]P[sub]c = the change in allele frequency in the conglomerate pop P[sub]D = the allele frequency in the donor pop P[sub]R = the allele frequency in the original recipient pop m = proportion of migrants that make up the conglomerate pop (# migrants/total # individuals)
what can be used to determine if observed and expected data are in agreement?
a chi square test, in which the degrees of freedom equals 1
population genetics
a field concerned with changes in genetic variation within a group of individuals over time. Population geneticists examine genetic variation within populations, why it exists, and how it changes over the course of many generations
monomorphic
a gene that exists predominantly as a single allele in a pop (found in at least 99% of cases)
polymorphic
a gene with two or more alleles in a population
bottle neck effect
a pop is dramatically reduced in size due to a natural disaster/destruction of habitat, leaving behind surviving members that may have different allele frequencies than that of the original pop. extreme drift may occur, and alleles may even be eliminated. even when the pop size recovers, it will have less variation than the original pop
what two fundamental calculations are central to population genetics?
allele frequencies and genotype frequencies, each of which total less or equal to one (100%)
how does the pop change when undergoing directional selection?
alleles change in a step by step, generation by generation way. Hardy weinberg equilibrium i modified in the following way: Frequency of dominant homozygote = p^2W[sub]AA frequency of heterozygote = 2pqW[sub]Aa Frequency of dominant homozygote = q^2W[sub]aa these three equations add up to the "mean fitness of the pop" (W): W = p^2W[sub]AA + 2pqW[sub]Aa + q^2W[sub]aa or 1 = (p^2W[sub]AA/W) + (2pqW[sub]Aa/W) + (q^2W[sub]aa/W) therefore Frequency of AA = (p^2W[sub]AA/W) Frequency of Aa = (2pqW[sub]Aa/W) Frequency of aa = (q^2W[sub]aa/W)
negative-frequency dependent selection
another mechanism of balancing selection- fitness of a genotype decreases when its frequency becomes higher (rare individuals have higher fitness than common individuals), favoring rare individuals, creating a balanced polymorphism in which no genotype becomes too rare or too common
why is the hardy weinburg equilibrium useful?
because it is a null hypothesis, provides a framework on which to understand allele frequencies within a pop when such an equilibrium is violated it provides a quantitative relationship between allele and genotype frequencies in a pop, frequency of an allele shares a direct relationship with the phenotype it is responsible for
migration
can occur between two different populations. the introduction of migrants into a pop may change the allele frequencies of that pop
random genetic drift
change in genetic variation from generation to generation due to random sampling error caused by random fluctuations. is greatly affected by pop size and geography. tends to have a smaller effect in a small pop. drift results in loss of allele or its fixation in a pop, and eventually results in homozygosity (fixation or elimination).
what are the consequences of changes in a populations location or size?
changes in the genetic composition of the pop
microevolution
describes changes in a pops gene pool from generation to generation
how do you compute F?
determine the common ancestors, determine the inbreeding paths (shortest path through the pedigree that include both parents and the common ancestor. length of the path = # individuals in the path except for the individual being examined then use the equation: F = sigma (1/2)^n X (1 + F[sub]A) where n = # individuals in the inbreeding path F[sub]A = inbreeding coefficient of the common ancestor sigma= add (1/2)^n X (1 + F[sub]A) for each inbreeding path
how are individuals related to the gene pool?
each individuals receives genes from its parents, who are members of the gene pool. Individuals that reproduce contribute to the gene pool of the next generation
directional selection
favors individuals at one extreme of a phenotypic distribution, may be caused by the introduction of a new (favored) allele, or the pop may be exposed to prolonged change in its environment which favors an extreme phenotype. may lead to fixation of a beneficial allele, but if frequency is low, it may be eliminated by genetic drift
balancing selection
favors the maintenance of 2+ alleles in a more homogeneous environment, results in a genetic polymorphism in a pop
disruptive/diversifying selection
favors the survival of 2 or more genotypes that produce different phenotypes. the fitness values of a particular genotype are higher in one environment and lower in a different one. usually occurs in pops that occupy diverse environments so that some embers of the species survive in each type of environment
what causes changes in the size of a pop?
feast or famine cycles during which environmental factors cause the pop to swell or shrink natural predators or disease may periodically decrease the size of a pop to significantly lower levels
How do geography and pop size affect genetic drift?
geographical isolation of a small, local pop of a species become separated and are more susceptible to drift. these small isolated pops tend to be more genetical disparate in relation to other pops, occurs via bottle neck or founder effects
what may cause balanced polymorphisms?
heterozygote advantage or neg frequency dependent selection
heterozygote advantage
heterozygote has higher fitness than either corresponding homozygote, results in balancing selection. equilibrium is reached in which both alleles are maintained in the pop. can explain the high frequency of alleles that are deleterious in a homozygous condition ex sickle cell anemia and cystic fibrosis
what are the consequences of inbreeding in a pop?
in agriculure, can be used to produce homozygotes of a desired trait, but many genetic diseases are inherited in a recessive manner, and inbreeding increases the likelihood of being homozygous and therefore affected by the disease. also lowers the mean fitness of a natural pop if the homozygote offspring have a lower fitness value
mutations
involve changes in gene sequences, chromosome structure, and or chromosome number, provided new alleles to the pop but doesn't substantially alter allele frequencies
gene pool
made up of all of the alleles of every gene in the population
how do mutations affect the pop?
may be deleterious neutral (more likely), or beneficial to the survival/reproductive potential of the individual that inherits them
what causes a population to change in geographical location?
migration of the pop or individuals to a new site. where they can establish a distinct pop in that location
are mutations a major factor in promoting widespread changes in a pop?
now because of their low rate of occurrence, and they require evolutionary mechanisms to act on them
gene flow
occurs whenever individuals migrate between pops and were able to breed successfully with members of the recipient pop. depends on migration and the ability of the migrants alleles to be passed to subsequent generations
inbreeding/consanguinity
opposite of outbreeding; its the mating of two genetically related individuals. affects the balance of genotypes predicted by hardy weinberg equilibrium
how do you use the fitness values for a genotypes to calculate the allele frequencies at eq?
s= selection coefficient, measures the degree to which a genotype is selected against = 1-W genotype with highest fitness has s = 0, genotypes at disadvantage have s bigger than 0 but less than 1. ex. s[sub]AA = 1 - W[sub]AA equilibrium is at S[sub]AAp = s[sub]aa, can now solve for p and/or q
founder effect
small group of individuals separate from the larger pop and establish a colony in a new location, results in a loss of genetic variation and different allele frequencies than that of the original pop
local populations/demes
smaller populations that usually compose a larger pop. members of the deme are far more likely to breed with themselves than with other members of the general population, are often separated by moderate geographic barriers
equation for the # generations required to achieve fixation (t)
t = 4N N still = # individuals in the pop assuming equal # males and females contribute to next gen.
what are the main consequences of bidirectional migration?
tends to reduce differences in allele frequencies between neighboring pops while enhancing genetic diversity within a pop b/c it introduces new alleles
when did the field of population genetics emerge?
the 1920s to 1930s
natural selection
the environment selects for individuals that possess certain traits that favor reproductive success
what do population geneticists study?
the genetic variation within the gene pool and how such variation changes from one generation to the next, emphasizing allelic variation.
conglomerate
the new population formed by migration
inbreeding coefficient (F)
the probability that 2 alleles in a particular individual will be identical for a given gene because both copies are due to decent from a common ancestor (Fixed in homozygous condition). can be computed by analyzing the degree of relatedness within a pedigree. see pg 688-689 to calculate allele frequencies using F
mutation rare
the probability that a gene will be altered by a new mutation, expressed as the # new mutations in a given gene per generation, usually 1 in 100,000, but varies depending on species, cell types, chromosomal location, and gene size. measured by following the change of a normal/functional allele to a deleterious/nonfuntional allele. see pg 676 for calculations
darwinian fitness
the relative likelihood that one genotype will contribute to the gene pool of the next generation rather than other genotypes, correlates with but is not equal to physics fitness
single nucleotide polymorphism (SNP)
the smallest type of genetic change that can occur within a given gene and are also the most common.
when is a pop said to be in disequilibrium
when a high chi square value is obtained and the hypothesis (that the allele and genotype frequencies are in hardy weinberg equilibrium) is rejected, indicating evolutionary change
assortative mating
when aging is nonrandom in a pop
inbreeding depression
when destruction of a habitat or other severe decreases in pop size forces inbreeding, leading to homozygotes which may be less fit, and thereby decrease the reproductive success of the pop
nonrandom mating
when individuals select mates based on their phenotypes or genetic lineage, which can alter the relative proportion of homo and hetero zygotes predicted by the hardy weinberg equation but doesn't change allele frequencies
positive vs negative assortative mating
when individuals with similar phenotypes choose each other as mates vs when dissimilar phenotypes mate preferntially
what to equations are involved in the hardy weinburg equilibrium?
when p = the dominant allele and q = the recessive allele; p+q = 1 and p^2 + 2pq + q^2 = 1 <- hardy weinerg equation and frequency of homozygote = p^2 x 100, and frequency of heterozygote = 2 x p x q applies to a diploid species that has only two alleles for the gene
stabilizing selection
when the extreme phenotypes for a trait are selected against and those individuals with intermediate phenotypes have highest fitness values. tends to decrease genetic diversity because it eliminates alleles that cause a greater variation in phenotypes