1 GENETICA DELLE POPOLAZIONI Alberto Pallavicini
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1
GENETICA DELLE POPOLAZIONI
Alberto Pallavicini
Allele Frequencies in Population Gene Pools Vary in Space and Time
• A population is a group of individuals
with a common set of genes that lives in the
same geographic area and can or does
interbreed.
• A population's gene pool is all of the
alleles present in that population. Due to
population dynamics, the gene pool can
change over time.
The Hardy-Weinberg Law Describes the Relationship between Allele Frequencies and Genotype Frequencies in an Ideal Population
• The Hardy-Weinberg law makes two
predictions:
(1) the frequency of the alleles in the gene
pool does not change over time; and
(2) after one generation of random
mating, the genotype frequencies for
two alleles can be calculated as
p2 + 2pq + q2 = 1
where p equals the frequency of allele A
and q is the frequency of allele a
• The Hardy-Weinberg model assumes that
there is
– no selection,
– no new alleles arise from mutation,
– no migration into or out of the
population,
– the population is infinitely large,
– random mating occurs.
The Hardy-Weinberg Law Can Be Applied to Human Populations
• An example of how the Hardy-Weinberg
law can be applied to humans is analysis
of susceptibility to HIV-1 infection based
on the genotype for the CCR5 HIV-1
receptor gene.
Verifica dell’equilibrio di H-W
Per la verifica dello stato di equilibrio il test statistico da utilizzare e quello del χ2 (chi-quadro).
In una popolazione di 150 mosche
• 15 hanno gli occhi rossi
• 90 hanno gli occhi di colore normale
• 45 hanno gli occhi rosa
Questa popolazione e in equilibro di HW?
Verifica dell’equilibrio di H-W
1° passo: Determinare le frequenze alleliche della generazione corrente
Verifica dell’equilibrio di H-W
Verifica dell’equilibrio di H-W
2° passo: Determinare le frequenze genotipiche attese nella prossima generazione
• p2= f (R)∗ f ( R)=0.5625
• q2= f (r)∗f (r)=0.0625
• 2 pq=2∗[ f (R)∗ f (r)]=0.375
• 84 mosche con occhi normali (RR)
• 9 mosche con occhi rossi (rr)
• 56 mosche con occhi rosa (Rr)
3° passo: Si confronta la frequenza attesa con i numeri originali della popolazione
Verifica dell’equilibrio di H-W
Gradi di libertà della distribuzione=( Numero delle classi−1)−numero di parametri stimati
Poichè abbiamo 3 genotipi, avremo
no gradi di libertà=(3−1)−1=1
Se consideriamo un livello di significatività pari al 5%, ovvero α = 0.05, otteniamo il valore critico
X2 0.05 =3.84
Verifica dell’equilibrio di H-W
The Hardy-Weinberg Law Can Be Used for Multiple Alleles, X-Linked Traits, and Estimating Heterozygote Frequencies
• Frequencies for multiple alleles can be
calculated using the Hardy-Weinberg
equation by adding more variables…
• For instance, in a situation involving three
alleles
(p + q + r = 1),
the frequencies of the genotypes are given by
(p + q + r)2 = p2 + q2 + r2 + 2pq + 2pr + 2qr = 1.
• An example of genotype frequency
calculations for ABO blood type .
• In using the Hardy-Weinberg equation
to calculate allele and genotype
frequencies for X-linked traits in
mammals, the frequency of the X-linked
allele in the gene pool will equal the
frequency of males expressing the X-
linked trait.
• For females, the frequency of having
the allele in question will be q2 if the
allele frequency is q.
– Daltonismo 8% nei maschi
– Quale è la frequenza delle femmine
daltoniche? E di quelle portatrici?
• The Hardy-Weinberg law also allows
the frequency of heterozygotes in a
population to be estimated. In
general, the frequencies of all three
genotypes can be estimated once the
frequency of either allele is known and
Hardy-Weinberg assumptions are
invoked.
ESERCIZIO
Per il locus PCI, nella popolazione di Daphnia dei bacino di Ojibway, Spitz trovò due alleli, S e S-, e determinò che il numero di individui di ciascun genotipo era 42 SS, 48 SS- e 38 S-S-. Verifica se i genotipi mostrano deviazione dall'HWE.
Natural Selection Is a Major Force Driving Allele Frequency Change
• If individuals of all genotypes are subject
to natural selection and do not have
equal rates of survival and reproductive
success, allele frequencies may change
from one generation to the next.
•Natural selection is the principal force
that shifts allele frequencies within large
populations.
• Hardy-Weinberg analysis allows
fitness w to be examined for each
genotype.
• For a homozygous recessive individual
that dies before producing offspring,
w = 0,
and the frequency of the recessive allele
will decrease in each generation.
Calcolo della fitness
• qg=q
0/(1+gq
0)
a1a1 a1a2 a2a2
W 1 1 1 - s
genot. po2 2poqo qo
2
Gametes po2 2poqo qo
2(1-s)
Total gametes 1 - sqo2
q1=q
0(1-sq0)/(1-sq0)
• The rate at which the frequency of a
deleterious allele declines depends on
the strength of selection applied.
• Selection in natural populations
works as predicted to increase the
frequency of the allele to which
selective pressure is applied.
• No such increase is observed in
populations not subjected to the
selection.
• Selection
acting on
quantitative
traits can be
– directional,
– stabilizing, or
– disruptive.
• In directional selection, the genotype
conferring one phenotypic extreme is
selected, resulting in a change in the
population mean over time.
One of the classic studies in the evolution of natural populations
was conducted by Rosemary and Peter Grant and coworkers
on Darwin's finches. It is among the first–and is certainly the
most elegant–study to document evolution in a wild
population of vertebrates. Rosemary and Peter investigated
Darwin's finches for almost 3 decades and conducted most of
their field work on two small islands, Daphne Major and
Genovesa, in the Galápagos archipelago, Ecuador.
• In stabilizing selection, intermediate
types are favored, and both extreme
phenotypes are selected against. This will
reduce the population variance over time
but not the mean.
• In disruptive selection (divergente), both
phenotypic extremes are selected for, and the
intermediates are selected against. This will
result in a population with an increasingly
bimodal distribution for the trait.
• Mutation is the only process that
creates new alleles in a gene pool.
Because most mutations are recessive,
indirect methods using probability and
statistics are often employed to determine
the mutation rate.
Mutation Creates New Alleles in a Gene Pool
• If the mutation rate is known, the extent
to which mutation can cause allele
frequencies to change from one
generation to the next can be estimated.
• In general, although mutation provides the raw
material for evolution, mutation by itself plays a
relatively insignificant role in changing allele
frequencies.
• CFTR locus
• 2% in European population
• W very low
• Positive selection for heterozygotes?
• When a species divides into populations
that are separated geographically, the
allele frequencies in these new
populations may differ over time due to
migration.
Migration and Gene Flow Can Alter Allele Frequencies
• Migration occurs when individuals
move between the populations and may
have a large effect on allele frequency if
– the rate of migration is large or
– if the allele frequency of the migrant
population differs greatly from that of
the population to which it is moving.
• P'i=(1-m)p
m + mp
i
Genetic Drift Causes Random Changes in Allele Frequency in Small Populations
• Genetic drift occurs when the number of
reproducing individuals in a population is
too small to ensure that all the alleles in the
gene pool will be passed on to the next
generation in their existing frequencies.
• Genetic drift may result in one allele
becoming fixed and one allele disappearing
in a population.
Nonrandom Mating Changes Genotype Frequency but Not Allele Frequency
• Nonrandom mating can take the form of
positive assortive mating in which
similar genotypes are more likely to mate
than dissimilar ones, negative assortive
mating in which dissimilar genotypes are
more likely to mate than similar ones, and
inbreeding in which mating individuals are
related.
• For a given allele, inbreeding increases
the proportion of homozygotes in the
population, and a completely inbred
population theoretically will consist only
of homozygotes.
• Self-fertilization is a form of inbreeding
common in plants. The rate of
homozygotes in a self-fertilizing
population rapidly increases over a few
generations, but the overall allele
frequency does not change.
• One consequence of inbreeding is an
increased chance that an individual
will be homozygous for a recessive
deleterious allele. The significance of
this fact is that inbred populations often
have a lowered mean fitness, called
inbreeding depression.
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