Transcript F 1 generation - Zanichelli online per la scuola

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From Mendel to
modern genetics
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Mendel
and the laws
of inheritance
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Mendel’s experiments
Gregor Mendel performed a
series of observations and
experiments on the inheritance
of characteristics using the
common pea plant (Pisum
sativum).
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Pea plants as a model
Pea plants have some characteristics
that made them a good model for
Mendel’s experiments:
• they are easy to grow and they mature
quickly;
• they have a short generation time and
produce many offspring;
• they perform self-pollination;
• it is easy to control their reproduction.
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The first experiments: seven traits
Mendel noticed that pea plants could assume two different
variants of some traits. He examined seven traits.
Traits
Dominant
trait
Recessive
trait
Seed
shape
Seed
color
Pod shape
Pod
color
Plant
height
Flower
position
Flower
color
Round
Yellow
Inflated
Green
Tall
Axial
Purple
Yellow
Short
Terminal
White
Wrinkled Green Constricted
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The first experiments: pure lines
P generation
Pure lines are plants that show the
same trait generation after
generation. Mendel used pure line
plants in his first experiments.
He crossed two pure lines that
differed for only one trait, for example
seed colour (P generation).
The result was called F1 generation.
F1 generation
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The first experiments: F1 generation
F1 generation
Mendel then crossed the
individuals of the F1 generation.
In the F2 generation, he noticed
that 3 out of 4 offspring showed
one trait.
Phenotypic ratio 3 : 1
F2 generation
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Law of segregation
The law of segregation, or Mendel’s first law, states that
when any individual produce gametes, the two copies of
“factors” segregate, so that offspring receive one factor from
each parent.
Each gamete contains only one factor from each pair.
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Dominant and recessive traits
What Mendel called “factors” are now known as genes.
A gene can occur in alternative variants, called alleles.
The alleles for a gene can be the same (then the organism is
homozygous for the trait) or different (then the organism is
heterozygous for the trait).
When two different alleles are present for one trait, one is
expressed (dominant), while the other is not (recessive).
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Genotype and phenotype
The genotype of an organism is the gene composition
and arrangement.
The phenotype corresponds to the expression of the gene
as a trait.
Genotype
Genotype
Phenotype
YY
Homozygous
dominant
Yellow seeds
Yy
Heterozygous
Yellow seeds
yy
Homozygous
recessive
Green seeds
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Dihybrid cross /1
In a second series of experiments, Mendel selected plants
that differed for two traits: dihybrid cross.
P generation
Round, yellow seeds
YY, RR
Wrinkled, green seeds
yy, rr
?
F1 generation
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Dihybrid cross /2
F1 gametes
YR
F1 gametes
YR
Yr
Yr
yR
yr
A Punnett square is
a diagram that can be
used to predict the
probability of
genotypes and
phenotypes in the
next generation.
yR
yr
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Law of independent assortment
The law of independent assortment, or Mendel’s second
law, states that each pair of factors assort independently:
the inheritance of alleles for one trait does not influence the
inheritance of alleles for another trait.
Each gamete can contain all possible factor combinations.
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Testcross /1
A testcross can be used to determine whether an individual
with a dominant phenotype is homozygous dominant or
heterozygous.
Unknown genotype
(YY or Yy ?)
Homozygous recessive
(yy)
It is the cross between an individual with dominant phenotype
(but unknown genotype) and an individual with a homozygous
recessive genotype.
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Testcross /2
Homozygous dominant
Offspring
Heterozygous
Offspring
If the offspring show only the dominant phenotype, the unknown
individual is homozygous dominant. It half the offspring show
the dominant and the other half the recessive phenotype, the
unknown individual is heterozygous.
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Human genetics
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Humans and genetic pedigrees
The genetic pedigree is a technique that can be used to
reveal the patterns by which a trait is inherited.
It involves the representation of a family tree in which the
presence or absence of a specific trait or disease is indicated
for each family member.
It makes use of standard symbols:
= female
= unaffected
= male
= affected
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Autosomal recessive disorder /1
Carriers
Carrier
Carrier
Carrier
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Autosomal recessive disorder /2
An autosomal recessive disorder is expressed when an
individual has two copies of an altered gene, that is when he
or she is homozygous for the altered gene.
An individual who is heterozygous for the affected gene is
unaffected, but he or she is a carrier for the disorder.
Two affected parents will always have affected children.
Examples: cystic fibrosis, sickle-cell anaemia.
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Autosomal dominant disorder /1
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Autosomal dominant disorder /2
An autosomal dominant disorder can appear also when
an individual has just one copy of an altered gene, that is
when he or she is heterozygous for the altered gene.
Two heterozygous affected parents can have unaffected
children (if they inherit two copies of the unaltered gene).
Two unaffected parents will not have affected children
(unless a new mutation occurs).
Examples: Huntington disease, achondroplasia.
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Incomplete dominance
Incomplete dominance occurs when the heterozygous
phenotype is an intermediate between its homozygous
parents.
P generation
F1 generation
F1 generation
F2 generation
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Multiple alleles and blood types
Most genes exist in a large number of allelic forms (multiple
alleles).
Phenotype
Genotype
A
I AIA , IAi
B
IBIB , IBi
AB
I AIB
0
ii
One example is blood type.
There are three different alleles
for blood type (A, B and 0). A and
B are dominant over 0. A and B
are both co-dominant.
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X-linked inheritance
Genes located on the X chromosome have a special pattern of
inheritance, called X-linked inheritance.
Some human genetic conditions, like color blindness and
hemophilia, follow this pattern of inheritance.
In X-linked recessive disorders, a woman will be unaffected
but a carrier of the condition if she is heterozygous for the
altered gene, whereas a male will be affected if he inherits the
altered gene from the mother.
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