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Exploring Mendelian Genetics
11-3
Independent Assortment
When Mendel was
looking at his
principal of
segregation he ended
up with another
question
Do genes for traits
segregate
independently from
one another?
Independent Assortment
In order to find this
out Mendel needed to
look at two genes at
the same time.
This process is called
a two-factor cross
Independent Assortment
In Mendel’s first twofactor cross he crossed
a plant that was true
breeding for round,
yellow seeds and
crossed it with a plant
that was true breeding
for wrinkled, green
seeds.
Two factor cross between F1
hybrids
Because the true
breeding cross does
not show us the
answer we are looking
for we will create an
F2 generation from
our F1 hybrids just
like in Mendel’s
original experiments
Two factor cross between F1
hybrids
When Mendel did this
experiment he came
created 556 seeds from
the two original hybrid
F1 seeds
His results were very
similar to what we see
in our phenotypic
probability ratio of
9:3:3:1
A summary of Mendel’s
Principles
The inheritance of biological characteristics is
determined by individual units known as genes,
which are passed from parents to offspring.
In cases in which two or more forms of the gene for a
single trait exist, some forms of the gene may be
dominant and others may be recessive.
A summary of Mendel’s
Principles
In most sexually reproducing organisms, each adult
has two copies of a gene -- one from each parent.
These genes are segregated from each other when
gametes are formed.
The alleles for different genes usually segregate
independently of one another.
Beyond Dominant and Recessive
Some alleles are
neither dominant nor
recessive, and many
traits are controlled by
multiple alleles or
multiple genes
This can easily be seen
in many complex
organisms such as
animals
Incomplete Dominance
Cases in which one
allele is not dominant
over the other is called
incomplete dominance
An example is the four
o’clock flower
Neither red or white is
recessive, so when
crossed, it produces
pink flowers
Codominance
Codominance is
similar to incomplete
dominance.
Both alleles will
contribute to the
phenotype of the
offspring
This creates a mixture
effect in the offspring
Multiple Alleles
Many genes have more
than two alleles and
therefore are said to
have multiple alleles
This does not mean
that they can be given
more than two, just
that there are more
than two types known
to exist
Multiple Alleles
Multiple alleles is commonly seen in the fur color of
rabbits
We can see here our pattern of dominance as well as the
phenotypes and genotypes of the organism
Polygenic Traits
Many traits are
produced by the
interaction of several
genes
These are said to be
polygenic, meaning
“many genes”
Polygenic Traits
Examples of polygenic
traits can be seen
when looking at the
skin color of humans
Many genes come
together to create the
possible phenotypes
that are seen