Transcript Introduction and Mendelian Analysis

Prior history
To appreciate Mendels work, one must keep in mind the
prevailing theories of inheritance
Preformationism: the idea that gamete contains an intact
organism, was first proposed in the late 1600s.
Blending inheritance: essences of both sperm and egg mixed to
form offspring intermediate between the parents.
Mendel ignored development, focused solely on transmission of traits
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Darwin and Heredity
Darwin: Among individuals of any species, there are
differences (variations)
Evolution cannot occur unless there are differences
among individuals
Variation is important
Variation must be inherited
Mechanism of inheritance of variations is important
for understanding evolution
Mechanism of inheritance not understood
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Pangenesis
Pangenesis: (1860s)
Whole organism reproduces itself
Gemmules determine characteristics (traits) of
organism
Germ cells contain gemmules of all sort and these are
transmitted to next generation.
Fertilization- gemmules unite and produce new cells of
the types from which they are produced
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Cytology
Cytology:
Organisms are composed of cells
The most distinct structure in a cell is the nucleus.
ALL CELLS have a nucleus
Cells are formed by the division of pre-existing cells
(Mitosis). Mitosis described nuclear and chromosome
dynamics
Sperm and Ovum are cells. This links cells to
heredity!
Fertilization in sea urchin showed that sperm and
ovum fuse. This links parents to offspring
Ploidy?
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Meiosis
Meiosis described Chromosomal changes
-Reductional division of chromosomes keeps number of
chromosomes constant.
Nuclei of embryo and parents are diploids, but nuclei of
germ cells are haploid
Similar phenomenon not seen in any other cellular
organelle.
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Cytology’s contribution prior to Mendel
Heredity is a consequence of genetic continuity of cells
by division
Germ cells are the vehicle of transmission from one
generation to the next
Nucleus is crucial. During division it resolves into long
chromosomes that split lengthwise
Fertilization involves union of sperm and eggs
Fertilization involves union of nuclei
Chromosomes do not lose their individuality. They are
inherited intact.
Germ cells contain half the number of chromosomes
found in body cells.
Diploid embryo descends from maternal/paternal fusion
of haploid chromosome groups
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The origin of genetics:
The study of genetics begins when Gregor Mendel, in 1865,
addressed the question :
"How are characters passed on from one generation to the
next?”
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Model organisms
The genomes of many organisms have been sequenced.
www.genomenewsnetwork.org
A model organism is a species that has been widely studied, usually
because it is easy to maintain and breed in a lab and/or has particular
experimental advantages.
Remember: processes are conserved!!!!!
Model organisms are used to obtain information about other species
– including humans – that are more difficult to study directly.
Genetic model organisms
Experimental model organisms
Genomic model organisms
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Model organisms
Human model organisms
Modern genetics:
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The Pea
Mendel chose the common garden pea to study patterns of
inheritance. This was a excellent choice as a model system
for the following reasons:
1
2
3
4
He identified over 20 traits and studied 7
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True breeding
He identified over 20 traits that bred true and studied 7
The first two years of Mendel's work were devoted to
selecting lines that breed true (pure lines) for a particular
character or trait.
Breeding True:
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Mendel’s first cross
P
F1
F2
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Mendel’s first cross
P
F1
F2
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Keys to success
Mendel crossed pure breeding yellow pea plants to pure
breeding green pea plants. All of the progeny were yellow
pea plants.
Next he selfed these yellow plants allowing the pollen to fall on
its own stigma.
He obtained 6022 yellow pea plants and 2001 green pea plants.
He did these experiments with all seven traits!!!!
The ratios obtained were between 2.82:1 and 3.15:1
A cross involving only one character, seed color, is called a
monohybrid cross.
Keys to success:
1
2
3
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Conclusions
Although others were doing similar experiments at the time,
Mendel work was unique
Results and Conclusions:
1
2
3
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Terms:
Phenotype and genotype: The parental yellow pea plants are a
pure line- they only produce yellow pea plants when selfed.
However the F1 yellow pea plants produce some green pea
plants when selfed. Therefore it is necessary to make the
distinction between the appearance of an organism and its
genetic make-up.
PhenotypeGenotypeWould say the parental yellow pea plants and F1 yellow pea
plants have the same phenotype but a different genotype.
P
F1
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Terms:
Dominant and recessive: All F1 seeds were yellow but when
selfed the F2 produced some green seeds.
Mendel termed the trait that is expressed in the F1 as
dominant
The trait that is hidden but re-expressed in the F2 as
recessive.
The F1 plants must contain factors for green and yellow since
both are found in the F2.
From the fact the reciprocal crosses produce the same result,
Mendel concluded that male and females contribute equally
With these assumptions the simplest model is that the F1
contains two hereditary factors
One for green and another for yellow
Mendel used the Uppercase Y to represent the dominant yellow
factor and the lower case y to represent the recessive green.
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The Principle of Segregation:
Notice that while the parents have two factors, they produce
gametes containing only a single factor
Mendel reasoned that without a mechanism to halve the number
of factors in each generation, that factors would multiple with
each generation and become unmanageable.
Mendel reasoned that during gamete formation the paired
factors separate and each gamete receives one of the two
factors.
Parent
Gamete
F1
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Mendel's assumption of two factors and segregation makes
a strong prediction concerning the genetic make-up of the of
F2 yellow pea plants:
F1
Y
Yy
X
y
Yy
Y
y
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Selfing
Mendel selfed each of the F2 plants
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Test cross
If instead of selfing the F2 plants, they are crossed to
pure breeding green plants, what are the expected outcomes:
F2
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More terms:
Mendel's factors are now known as genes
Alternative forms of a gene that determine different traits are
known as __________
Individuals with two identical alleles are said to be __________
Individuals with two different forms of alleles are said to be
_____________
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The dihybrid cross and the principle of independent
assortment:
In the second set of experiments Mendel investigated the
pattern of inheritance for two sets of characters simultaneously.
A cross involving two sets of characters is called a dihybrid cross.
Pea shape: smooth, wrinkled (Smooth is dominant to wrinkled)
Cotyledons color: yellow, green (Yellow is dominant to green)
P
x
F1
selfed
F2
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The 9:3:3:1 ratio.
The 9:3:3:1 ratio is a lot more complex than the 3:1 ratios of
the monohybrid crosses.
Mendel's insight was to realize the 9:3:3:1 ratio is nothing more
than two 3:1 ratios combined at random. That is if one examined
the traits individually they formed a 3:1 ratio.
To determine the mode of inheritance of the two genes in this
dihybrid cross Mendel examined each of the traits separately:
If we examine seed shape (smooth, wrinkled) and ignore
cotyledon color (yellow, green), in the F2, we expect to find:
3/4 smooth and 1/4 wrinkled:
# Smooth =
#wrinkled =
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In addition, if we only examine cotyledon color,
we expect 3/4 Yellow to 1/4 green.
#Yellow =
#green =
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Monohybrid---dihybrid
3/4 Yellow
3/4x3/4=
9/16
1/4 Green
3/4x1/4=
3/16
3/4 Yellow
1/4x3/4=
3/16
1/4 Green
1/4x1/4=
1/16
3/4 Smooth
1/4 Wrinkled
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Branched Diagram
If you combine the monohybrid ratios for two traits you get:
3/4 yellow
3/4x3/4 = 9/16
1/4 green
3/4 x 1/4 = 3/16
3/4 yellow
1/4x3/4 = 3/16
1/4 green
1/4x1/4 = 1/16
3/4 smooth
1/4 wrinkled
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Each trait behaves as a standard recessive found in a
monohybrid cross.
They do not affect one another
Genes segregate independently!!!!
GeneA in a gamete does not affect the segregation of geneB in
that gamete.
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In a heterozygous individual
SsYy
x
SsYy
Genes line up in two ways during gamete formation
SsYy
SsYy
SY
sy
Gamete
Sy
sY
or
SY
sy
Sy
sY
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Independent assortment
If independent assortment is occurring, four different kinds of
gametes will be produced in equal frequencies.
The only rule is that S and s segregate to separate gametes and
Y and y segregate to separate gametes;
(that is one does not get an Ss gamete or a Yy gamete)
SsYy males and SsYy females can produce four types of gametes
in equal frequencies:
SY, Sy, sY, sy
The male and female gametes randomly combine to restore diploidy
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Punnet diagram of a dihybrid cross
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What is the biological significance of the 9:3:3:1 ratio? This
ratio is only produced if the different genes pairs assort
independently of each other during gamete formation.
That is the presence of one gene in a gamete does not influence
the probability of another gene being found in that gamete
Principle of segregation: for one gene, each individual has two
copies. These two copies segregate from one another during
gamete formation.
Independent assortment: Segregation of one gene pair is
independent of the segregation of any other pair of gene.
Hence the 9:3:3:1 ratio and the biological significance of
Mendel's second lawDifferent gene pairs assort independently during gamete
formation.
The presence of a Y in a gamete does not influence the
probability of a S or s being in that gamete.
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Mendels laws
1.
The principle of segregation: Each individual carries two
copies of a given gene and these segregate from one another
during gamete formation.
2. The principle of independent assortment: The segregation of
one pair of genes is independent of the segregation of any
other pair of genes during gamete formation
(as we will find their are important exceptions to this rule)!!!!!
By applying these rules Mendel concluded that SsYy individuals
produced the following gametes in a 1:1:1:1 ratio
SY Sy sY sy
As described above, he inferred these gamete ratios by selfing
SsYy individuals.
He could also have inferred these gamete ratios by crossing
SsYy individuals to ssyy individuals.
Crossing to the homozygous recessive individuals is known as a
test cross
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Test cross
A test cross is easier that a self cross for the F2
SsYy
x
ssyy
What are the expected genotypic and phenotypic ratios of
the progeny produced from this cross?
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Gene number
Power of Mendel phenotype ratios:
The ratio tells you the number of genes involved in determining a
phenotype
3;1 ratio (selfing) = 1 gene
9:3:3:1 ratio (selfing) = 2 genes
27:9:9:9:3:3:3:1 ratio (selfing) = 3 genes
Say seed color is controlled by two genes- GeneS and GeneT
Green seeds are sstt
All others are yellow ( if either S or T are present, the seed is yellow)
True breeding yellow
SSTT
x
F1
SsTt
(yellow)
SsTt
Self
x
true breeding green
sstt
SsTt
How many green will you get out of this cross:
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These conclusions about gene number become very important when
applied to human traits like size, behavior, temperament etc.
Dog Genome
Dogs have been breed for specific traits for 10,000 years
About 150 breeds have been generated through selective breeding
Diversity in
Physical makeup:
Coat color
height
mass
muscle
Behavior:
herding
tracking
retrieval
Guarding
Intelligence:
The individual breeds can mate with one another and produce viable
fertile offspring
They can also mate with Cayotes and wolves
Border collies do not like water, Newfoundlands love water.
In the cross swimming is dominant!
This complex trait is likely mediated by a small number of genes (~2)
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Hybrid vigor
The first cross between two purebred lines is often healthier than
either parent
Breed1
a-B-C-d-E
a-B-C-d-E
F1
Breed2
A-b-c-d-E
A-b-c-d-E
a-B-C-d-E
A-b-c-d-E
If GeneA causes narcolepsy and geneC causes haemophilia
F1 will be normal.
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Corn
Commercial corn is a F1 hybrid because of hybrid vigor
Two inbred lines are mated to generate a F1 that is sold
AAbb
F1
x
aaBB
AaBb
hybrid vigor
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