Transcript Mendel`s Principle of Independent Assortment

Dihybrid Crosses looking at 2 traits
Mendel’s dihybrid crosses:
1. Mendel also performed crosses involving two pairs of traits,
e.g., seed shape (smooth vs. wrinkled) and color (yellow vs.
green).
2. If alleles sort independently, four possible phenotypes (2n)
appear in the F2 generation in a 9:3:3:1 ratio.
“Mendel’s Principle of Independent Assortment”:
Alleles for different traits assort independently of one another.
Modern formulation of independent assortment:
Genes on different chromosomes behave independently in gamete
production.
The Independent Alignment of
Different Pairs of Homologous
Chromosomes At Meiosis Accounts for
the Principle of Independent
Assortment
The alignment of one pair of homologs is
independent
of any other.Assortment: The
Principle
of Independent
assortment of one pair of genes into gametes
is independent of the assortment of another
pair of genes.
Possible Gametes:
To determine the number of different gametes a parent can have use the FOIL
method.
RREE
RE
RE
RE
RE
All gametes are RE
Possible Gametes:
To determine the number of different gametes a parent can have use the FOIL
method.
RREe
Re
RE
RE
Re
You have 2 possible gametes:
Re and RE
Possible Gametes:
To determine the number of different gametes a parent can have use the FOIL
method.
rrEe
re
rE
rE
re
You have 2 possible gametes:
rE and re
How many gametes?
To determine the number of different gametes a parent can have use
the FOIL method.
Number and kind of gametes
TTYY
=
one
TtYY
=
two
TtYy
=
four
TY
TY and tY
Ty, tY, ty, TY
Possible Gametes (sperm/egg):
If I cross:
What are the different possible gametes for these parents??
RRTT x RrTt
RT
RT, Rt, rT, rt
There are 4 total different gametes
If I cross:
What are the possible gametes?
RrTt x RrTt
I have 16
possible
offspring
9:3:3:1
RT
Rt
rT
rt
RT
RRTT
RRTt
RrTT
RrTt
Rt
RRTt
RRtt
RrTt
Rrtt
rT
RrTT
RrTt
rrTT
rrTt
rt
RrTt
Rrtt
rrTt
rrtt
If I cross:
What are the possible gametes?
RRTT x RrTt
RT
I have 4
possible
offspring
RT
Rt
rT
rt
RRTT
RRTt
RrTT
RrTt
If I cross:
What are the possible gametes and offspring?
rrTt x RRTt
RT
Rt
rT
RrTT
RrTt
rt
RrTt
Rrtt
I have 3 possible
offspring
Why Did Mendel Conclude That The
Inheritance of one Trait is Independent
of Another?
Phenotype Ratio:
9 yellow/smooth
3 yellow/wrinkled
3 green/smooth
1 green/wrinkled
Because it’s the
only way to explain
the pattern of
inheritance.
What Works
for Peas Also
Works for
Humans
Consider a cross
between parents
heterozygous for
both deafness and
albinism.
This is the same
9:3:3:1 ratio seen
for Mendel’s cross
involving pea
color and shape.
Take out:
Exercise: Dihybrid Crosses
Complete all questions and check
your answers with me.
Exceptions to the rule…
Some alleles are related through -
Incomplete Dominance
Dominance relationships may differ,
but the Law of Segregation remains
the same.
Multiple Alleles
Many genes are present in 3 or
more versions (alleles) – this is
known as multiple alleles.
The human ABO blood group is
determined by three alleles (IA, IB,
and i) of a single gene.
The AB phenotype (genotype
IA IB) is an example of
codominance
Codominance
The human ABO blood group
illustrates another genetic
phenomenon – codominance.
Codominance occurs when the
phenotype associated with each
allele is expressed in the
heterozygote. Neither allele is
dominant or recessive
Incomplete or Codominant?
+
Pleiotropy – When One Allele Influences Many Traits
Examples:
1. Sickle Cell Anemia
2. Albinism
Pleiotropy in Action
Anemia,
infections,
weakness,
impaired
growth, liver
and spleen
failure,
death.
Traits (phenotypes)
associated with the
sickle cell allele.
Polygenic Inheritance – when a Single Trait is
influenced by many genes
Examples:
1. Height
2. Hair texture
3. Skin color
4. eye color
Corn Lab – Dihybrid Cross
On a sheet of paper, count 200 kernels of corns and
perform a chi-square analysis:
Your options are:
Purple, smooth
Purple, wrinkled
Yellow, smooth
Yellow, wrinkled
What ratio did Mendel see on his dihybrid corn? That is your expected ratio….