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Heredity, genetics
and evolution
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© Zanichelli editore 2016
What is genetics?
Genetics studies the laws and mechanisms of hereditary
transmission of characteristics in sexually reproductive
species.
It also studies the molecular mechanisms used by cells to
divide and develop according to an internal plan.
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The studies of Gregor Mendel
Gregor Mendel studied the inheritance of biological features
in a model: the pea plant.
He took into consideration single characteristics, or traits, that
could be easily recognized in plants and that occurred in two
variants.
For example, he studied the colour of the seed which can be
yellow or green.
Mendel studied seven different traits.
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The pure lines
A pure line for a particular trait is a plant that always
produces other plants with the same variant when it
reproduces by self-pollination.
Mendel used pure lines as parental generations (P).
self-pollination
self-pollination
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Law of dominance
According to the law of dominance, when two pure lines
for the same trait are crossed, in the first generation (F1) all
hybrids will present only one variant of the parental
generation.
parental generation
first generation F1
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Dominant and recessive traits
The trait that appears in F1 hybrids is called dominant.
The trait that seems to disappear is called recessive.
For each characteristic he studied, Mendel noted that the
dominant trait appeared both in pure lines and in hybrids, and
the recessive trait appeared only in pure lines.
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Law of segregation /1
Mendel discovered that the recessive traits disappear for
one generation, but come back in the following generations.
first generation F1
self-pollination
second generation F2
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Law of segregation /2
According to the law of segregation, each individual has
two inheritance factors (alleles) for each characteristic, but
its gametes contain only one.
Yy
Yy
gametes
Y
y
Y
y
fertilization
Yy
Yy
YY
yy
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Alleles, genes and chromosomes
Each gene is a portion of DNA, located in a specific locus of a
chromosome. It can occur in different versions, called alleles.
In somatic cells, two alleles for the same trait are located in
two homologous chromosomes.
homologous chromosomes
different alleles for one trait
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Meiosis /1
first division
second division
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Meiosis /2
During meiosis, each gamete receives only one chromosome
of a couple of homologous chromosomes: gametes contain
one allele for each trait.
During fertilization, maternal and paternal chromosomes join
together: the zygote contains two alleles for each trait.
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Genotype and phenotype
Genotype
YY
Yy
yy
Phenotype
The genotype is the set of
alleles that determine a trait in
the individual (e.g. YY or Yy).
The phenotype is the
observable trait (e.g. yellow or
green seed).
If the two alleles are the same,
the individual is homozygous.
If the two alleles are different,
the individual is heterozygous.
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The Punnett square
Using the Punnett square, we can predict all the
combinations of alleles resulting from a cross.
yy
y
y
Y
Yy
Yy
Y
Yy
Yy
male gametes
YY
female gametes
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Incomplete dominance
In incomplete dominance,
the phenotype of the
heterozygous individual is
a combination of the
phenotypes of the two
homozygous individuals.
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Codominance
Codominance occurs when the heterozygous individual
presents the traits of both homozygous individuals, because
both alleles determine the phenotype.
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The genetics of blood types
Human blood types are an example of multiple alleles.
There are three alleles (A, B and 0), even if each individual
only has two of them. The combination of alleles leads to 4
possible blood types: AB, A, B and 0.
Blood type
Genotype
A
AA or A0
B
BB or B0
AB
AB
0
00
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Law of independent assortment /1
According to the law of independent assortment, alleles for
two different traits divide in gametes independently, not as a
pair.
YY RR
yy rr
P generation
F1 generation
Yy Rr
Yr
YR
yR
yr
gametes
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Law of independent assortment /2
YR
Yr
yR
yr
YY RR
YY Rr
Yy RR
Yy Rr
YY Rr
YY rr
Yy Rr
Yy rr
YY RR
Yy Rr
yy RR
yy Rr
Yy Rr
Yy rr
yy Rr
yy rr
YR
F2 generation
Yr
yR
yr
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Sexual chromosomes
Human cells have 23 couples of homologous chromosomes.
The first 22 couples are called autosomes and they do not
differ in males and females.
The 23° couple has the sexual chromosomes: XX in
females and XY in males.
Sex linked inheritance refers to the transmission of traits
determined by genes located on the X chromosome.
For these genes, males have only one allele which is always
expressed in the phenotype.
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Color blindness and hemophilia
Color blindness and hemophilia are X linked recessive traits:
the recessive allele is located on the X chromosome.
These conditions are more common in males, because they
have only one X chromosome.
In females, the recessive allele is masked by the presence of
the normal dominant allele located on the other X
chromosome.
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Populations and the genetic pool
A population is a group of individuals of the same species
living in a specific geographic region.
A species includes interfertile populations that cannot
interbreed with other similar groups.
The genetic pool is the set of alleles from all the individuals
in a population.
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Evolution
Evolution is the change in the frequency of one or more
alleles in the genetic pool of a population.
Factors that influence evolution are:
• mutations in gametes (that change the genetic pool);
• genetic drift;
• gene flow;
• natural selection.
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Speciation and reproductive isolation
Sometimes one single population diversifies, creating new
populations whose individuals cannot interbreed anymore:
this is the phenomenon of speciation.
Speciation implies reproductive isolation.
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Natural selection
Natural selection occurs when:
• there is variability in the traits of a population;
• traits are inheritable;
• individuals with a specific phenotype have higher
reproductive success than individuals with a different
phenotype.
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Genetic drift /1
In genetic drift, variation in the allelic frequencies is random
and is a not a consequence of reproductive success.
It has a large impact on small size populations and leads to
smaller variability in the genetic pool.
some
individuals
found a new
population
The founder effect is a type of
genetic drift that occurs when
the founders of a new
population have different allelic
frequencies than the ones in
the original population.
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Genetic drift /2
The population bottleneck is a type of genetic drift that
occurs when an abrupt environmental change affects, with the
same probability, all the individuals in a population thereby
reducing its size.
only some
individuals
survive
Survivors have different allelic
frequencies with respect to the
original population.
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Gene flow
Migration is the transfer of a group of individuals from one
population to another. It determines a gene flow that can alter
the allelic frequencies in the two populations.
population 1
population 1
population 2
population 2
a group of
individuals move
from population 2
to 1
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Fitness and adaptation
Fitness is a measure of the reproductive success of an
individual with a specific phenotype, considering its
contribution to the next generation.
Adaptation is a trait that leads to higher fitness in a specific
environment.
Natural selection does not produce perfect adaptations.
Sometimes the environmental conditions change too fast, or
mutations do not lead to positive results; in some cases there
is not one optimal adaptation to a given environment.
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