Download law of segregation

Through his work with peas, Mendel came up with
three principles:
•The Law of Dominance
•The Law of Segregation
•The Law of Independent Assortment
The Law of Dominance
The principle of dominance states that some alleles
are dominant and others are recessive
An organism with a dominant allele for a particular
form of a trait will always have that form.
Dominant: form of a gene (allele) that is expressed
even if present with the contrasting recessive allele.
They overpower the expression of the other form of
the allele.
Always use an uppercase letter to represent the
dominant allele.
An organism with a recessive allele for a particular
form of a trait will have that form only when the
dominant allele for the trait is not present
Recessive – Description of a form of a gene (allele)
that is only expressed in the homozygous state.
Always use a lower case letter to represent the
recessive allele.
In Mendel’s experiments, the allele for tall plants was
dominant and the allele for short plants was recessive.
In his first experiments, Mendel looked at only one
trait at a time.
This type of cross is called a monohybrid cross.
We’re looking at only ONE trait.
A simple way to represent a cross is to set up a
Punnett square.
It is a chart showing the possible combinations of
genes in the offspring of a cross.
P generation
T
T
t
T t
Tt
t
T t
Tt
F1 generation
T
t
T
T T
Tt
t
T t
t t
Phenotype
Physical characteristics of an organism.
What the organism physically looks like
How the genes are expressed.
Ex.: Pea plants with Tt are always Tall – look at them
– they are tall. You can SEE the trait.
Genotype
What the genes look like – the genetic makeup of an
organism.
Ex.: a heterozygous tall pea plant would have the
genes (alleles) T and t (Tt)
A homozygous tall pea plant would have the alleles T
and T (TT)
One of the offspring was a short pea plant!!!!
The short pea trait reappeared in the F2 generation.
How did that happen???
Once again, the alleles separated and recombined to
produce a new combination
The cross resulted in one pair of recessive alleles, tt.
Although all of the F1 plants appeared to be tall, the
alleles separated and recombined during the cross.
This is an example of the LAW OF SEGREGATION.
Segregation – the separation; in genetics, the
separation of alleles during gamete formation.
When each F1 plant flowers, the two alleles are
segregated 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