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Genetics

Gregor Mendel, Probability, Punnett Squares, Incomplete dominance, Codominance...
by

Miss Schwinge

on 28 January 2014

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Transcript of Genetics

Mendelian Genetics
Genetic Inheritance
Every living thing
, plant or animal, microbe or human,
has a set of characteristics inherited

from its parent or parents.
Gregor Mendel
Mendel
was in charge of the monastery
garden
, and carried out much of his
genetic research
with ordinary
garden peas.
Mendel's Conclusions
Mendel concluded
from these experiments that
the inheritance of biological characteristics is determined by individual units known as genes
, and that
genes are passed from parents to their offspring.
Segregation
But Mendel wanted to answer another question:
Had the recessive alleles disappeared, or were they still present in the F1 plants?
Gregor Mendel,
an Austrian
monk
born in 1822, is responsible for
much of our understanding of biological inheritance.
Pea flowers
are normally
self-pollinating,
which means that
sperm cells in pollen fertilize the egg cells in the same flower
. The
seeds
that are
produced
by
self-pollination

inherit all
of the
characteristics
from the
single plant that bore them.
When
plants
are
truebreeding
, it means that if they are allowed to
self-pollinate
they would
produce offspring identical to themselves (height, color, etc.)
In order to do his
genetic experiments
, Mendel had to
prevent self-pollination
and do the job
himself
by using pollen from other plants through
pollen dusting
. This is known as
cross-pollination
, and produces
seeds
that had
two different plants as parents.
This made it possible for Mendel to
cross-breed plants with different characteristics and then study the results.
Genes and Dominance
Mendel studied several different
plant traits
. A
trait
is a
specific characteristic (such as seed color or plant height)
,
that varies from one individual to another.
Each
trait
that he studied had
two contrasting characters,
for example
green seed color and yellow seed color
Mendel
crossed plants
with the
contrasting traits
and
studied their offspring
. We call each
original pair
of plants the
P (parental) generation
, and the
offspring
are called the
F1 (first filial) generation.
The
offspring
of
crosses between parents with different traits
are called
hybrids.
Each of the
traits
Mendel studied was
controlled by one gene that occurred in two contrasting forms
. These
contrasting forms produced the different characters of each trait.
For example, the
gene
for
plant height
occurs in
one form
that produces
tall plants
and in
another form
that produces
short plants
.
The different forms of a gene are called alleles.
The
second conclusion
he made is called the
principle of dominance.
The
principle of dominance states that some alleles are dominant and others are recessive.
This means that an
organism
with a
dominant allele
for a particular
form of a trait
will
always exhibit that form of the trait
. An organism with a
recessive allele
for a particular
form of a trait
will
exhibit that form only when the dominant allele for the trait is not present.
In Mendel's experiments, the
allele for tall plants was dominant and the allele for short plants was recessive.

The allele for yellow seeds was dominant
, while the
allele for green seeds was recessive.
To answer this question Mendel let all of his
F1 hybrid plants to produce an F2 (second filial) generation by self-pollination.
This means that he
crossed the F1 generation with itself.
When Mendel compared the
results
of the
F1 crosses
(known as
F2 plants because they were the offspring of the F1 crosses
), he discovered that the
traits controlled by the recessive alleles had reappeared.
In fact, roughly
one fourth (1/4) of the F2 plants showed the trait controlled by the recessive allele.
But why did the recessive allele seem to disappear in the F1 generation and then reappear in the F2 generation?
The
reappearance of the recessive allele indicated
that at some point
the allele for shortness had been separated from the allele for tallness.
But how did this
separation, or segregation
, of alleles occur?
Mendel suggested that
the alleles for tallness and shortness in the F1 plants segregated from each other during the formation of the gametes.
In
most sexually reproducing organisms
,
each adult has two copies of each gene, one from each of its parents.
Probability
Whenever Mendel performed a
cross
with pea plants, he
carefully categorized and counted the many offspring.
Every time he
repeated a particular cross
, he obtained
similar results
; this told him that
the

principles of probability could be used to explain the results of genetic crosses.
The likelihood that a particular event will occur is called probability
. As an example of probability, think of the scenario of
flipping a coin
. When you do this
there are

two possible outcomes: the coin will land heads up, or tails up.
If you flip a coin
three times in a row
, what is the probability that it will land heads up every time? Because each coin flip is an
independent event, the probability
of each coin's landing heads up is
1/2. Therefore the probability of flipping three heads in a row is:
The chances, or probabilities, of either outcome are equal. Therefore, the probability that a single coin flip will come up heads is one chance in two, that is 1/2 or 50%.
The important thing to note is that
past outcomes do NOT affect future ones.
Punnet Squares and Probabilities
The
variation
of
gene combinations
that
might result
from a person's
genetic cross
can be
determined
by a particular diagram known as a
Punnett square.
Organisms
that have
two identical alleles for a particular trait (such as TT or tt)
are said to be
homozygous.
This means they got the
same trait
from both
mother and father,
which is why
homozygous
organisms are
true-breeding for a particular trait.
Organisms
that have
two different alleles for the same trait
are
heterozygous.
This means they got
two different traits
from
mother and father
, which is why
heterozygous
organisms are
hybrid for a particular trait.
All
tall plants
have the same
phenotype
, or
PHYSICAL characteristics
.
However, just because they are all
tall
does
not
mean they necessarily have the
same genotype or GENETIC makeup.
Notice in this
F2 cross
how a
tall plant
can have a
homozygous TT genotype as well as a heterozygous Tt genotype?
Also, it is evident in this
F2 cross
that
3/4
of the predicted offspring show the
dominant "tall" characteristic (TT/ Tt)
, while
1/4
demonstrates the
recessive "short" characteristic (tt)
F2 Cross Demo
After showing that
alleles segregate and recombine during the formation of gametes (the law of segregation)
, Mendel wondered if they did so independently. In other words,
did the segregation of one pair of alleles affect the segregation of another pair of alleles
?
For example, does the gene that determines whether a seed is
round or wrinkled
in shape have anything to do with the
gene for seed color
? Does a round seed also have to be yellow?
To answer these questions, Mendel performed an experiment to follow
two different genes
as they passed from
one generation to the next
. This is known as a
two-factor, or dihybrid, cross.
First, Mendel crossed
true-breeding
plants that produced only
round (R) yellow (Y) peas (genotype RRYY)
with
true-breeding
plants that produced
wrinkled (r) green (y) peas (genotype rryy)
The Dihybrid Cross
All the
F1 offspring produced round, yellow peas.
This shows that the
alleles
for
yellow and round peas
are
dominant
over the
alleles
for
green and wrinkled peas.
However, this cross
did not indicate whether genes assort, or segregate, independently.
But it did provide the
hybrid plants
needed for the next cross.
F1 cross demonstration
Mendel knew that the
F1 plants
had
genotypes of RrYy
, which means that they were
all heterozygous
for both the
seed shape and seed color genes.
Remember,
each plant in the F1 generation
was formed by the
fusion of a gamete carrying the dominant RY alleles with another gamete carrying the recessive ry alleles.
But did this mean that the
two dominant alleles would always stay together
? Or would they
"segregate independently"
so that any combination of alleles was possible?
When Mendel crossed plants that were
heterozygous dominant for round yellow peas
, he found that there were
allele combinations not found in either parent
; which means the
alleles for seed shape segregated independently of those for seed color
.
F2 cross demonstration
This showed that the
genes for seed shape and seed color in plants do not influence each other's inheritance.
The
principle of independent assortment

states that genes for different traits can segregate independently during the formation of gametes
.
Independent assortment
helps account for the
many genetic variations observed
in plants, animals, and other organisms.
Independent Assortment
Fun Fact:

In the world, blonde hair is exceptionally rare outside Europe. However, Melanesians of some islands are one of the few non-European peoples and the only dark-skinned group of humans known to have blonde hair. This has been traced to an allele of TYRP1 unique to these people
Beyond Dominant and Recessive Alleles
Despite the importance of Mendel's work, there are
important exceptions
to most of his principles. For example,
some alleles are neither dominant nor recessive.
In most organisms,
genetics is more complicated
because
the majority of genes have more than two alleles.
In addition,
many important traits are controlled by more than one gene.
Incomplete Dominance
A cross between two Mirabilis plants shows one of these complications. The
F1 generation produced
by a cross between
homozygous red-flowered (RR) plants and homozygous white-flowered (WW) plants consists of pink colored flowers (RW).
Which allele is dominant in this case? Neither.
Cases like this in which one allele is not completely dominant over the other are called incomplete dominance.
In
incomplete dominance, the heterozygous phenotype is somewhere in between the two homozygous phenotypes.
Codominance
A similar situation is
codominance, in which both alleles contribute to the phenotype.
For example, in certain varieties of
chicken
, the
allele for black feathers is codominant with the allele for white feathers.
This means that
heterozygous chickens have speckled black and white feathers.
Unlike the blending of red and white flowers in heterozygous Mirabilis plants, the white and black colors appear separately on the chicken.
Roan cows, made by crossing a red cow with a white cow, are another example of codominance.
Multiple Alleles
Many genes have more than two alleles
, and are therefore said to have
multiple alleles
. This does not mean that an individual can have more than two alleles, it only means that
more than two possible alleles can exist in a population.
The human genes for blood type are an example of this.
In humans, there are
four blood types (phenotypes):
A, B, AB, and O

Blood type
is controlled by
three alleles: A, B, O

O
is
recessive
, which means
two O alleles must be present
for the person to have
type O
blood

A and B are codominant
. If a person receives an
A allele and a B allele, their blood type is type AB
When doing
blood type crosses
, you will need to know whether a
type A or B person is heterozygous or homozygous.

Type O's are automatically OO and type AB is automatically AB.
Polygenic Traits
Many
traits
are produced by the
interactions of several genes
.
Traits controlled by two or more genes are known as polygenic traits
, which means "having many genes."
Polygenic traits often show a wide range of phenotypes.
For example,
the wide range of skin color in humans
comes about
partly because more than four different genes probably control this trait.

When each
F1 plant produces gametes
,
the two alleles segregate
from each other so that
each gamete carries only a single copy of each gene
. Therefore,
each F1 plant produces two types of gametes: those with the allele for tallness, and those with the allele for shortness.
Full transcript