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2. Heredity, genetics and evolution
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Heredity, genetics and evolution 2
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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. 3
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4. The studies of Gregor Mendel
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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. 4
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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 5
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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 6
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7. Dominant and recessive traits
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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. 7
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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 8
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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 9
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10. Alleles, genes and chromosomes
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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 10
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11. Meiosis /1 first division second division 11 13/11/11 11
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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. 12
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13. Genotype and phenotype
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Genotype and 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.
Genotype
Phenotype YY Yy yy 13
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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
male gametes Y Yy Yy YY Y Yy Yy
female gametes 14
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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. 15
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Codominance
Codominance occurs when the heterozygous individual presents the traits of both homozygous individuals, because both alleles determine the phenotype. 16
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17. The genetics of blood types
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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 00 17
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18. Law of independent assortment /1
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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
Yy Rr
F1 generation Yr YR yR yr
gametes 18
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19. Law of independent assortment /2
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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 YR
F2 generation Yr yR yr 19
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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. 20
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21. Color blindness and hemophilia
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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. 21
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22. Populations and the genetic pool
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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. 22
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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. 23
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24. Speciation and reproductive isolation
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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. 24
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25. Natural selection Natural selection occurs when:
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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. 25
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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.
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.
some individuals found a new population 26
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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. 27
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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 28
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29. Fitness and adaptation
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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. 29
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