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Biology 2012
 In
some cases, an intermediate phenotype is
shown
 Neither allele is dominant
 In snapdragons, flower color can be red,
pink, or white. The heterozygous condition
results in pink flowers (or an intermediate
trait)
 A white snapdragon crossed with a red
snapdragon produces all pink offspring
 Two pinks crossed together

produce 1/4 white, 2/4 pink, and 1/4 red
 Try
These Crosses in your Notes!
•Sickle cell disease is incompletely dominant in humans. AA x aa = Aa
(sickle cell trait), where some blood cells will have abnormal shapes
In Andalusian fowls, black individuals (B) and
white individuals (b) are homozygous.
 A homozygous black bird is crossed with a
homozygous white bird. The offspring are all
bluish-gray. Show the cross as well as the
genotypes and phenotypes of the parents and
offspring.
 What
results if a black individual is crossed
with a bluish-gray individual? (SHOW YOUR
WORK)
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Both alleles can be expressed
For example, red cows crossed with
white will generate roan cows. Roan
refers to cows that have red coats
with white blotches.
This phenotype might seem to
support the blending theory. (The
blending theory predicts pink F1
progeny.)
The phenotypic outcomes for cow
color and incomplete dominance in
general can be explained
biochemically.
One allele of the gene codes for an
enzyme that functions in the
production of the red color. The
other allele codes for the gene to
make white color. If both alleles are
present, both are expressed,
resulting in a cow that has some red
and some white.
In shorthorn cattle, when a red bull (RR) is
crossed with a white cow (WW), all the
offspring are roan—a spotted, red and white
or milky red color.
 What offspring are expected from mating a
roan bull and a roan cow?
 What
phenotypes would you expect from a
cross between a red bull and a white cow?
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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, 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
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Crosses involving blood
type often use an I to
denote the alleles - see
chart.
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.
Crosses are performed
the same as any other.

Blood type determines what
antibodies are located within
the blood.
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Type A =B antibodies.
If type B blood is put into their
bodies, their immune system
reacts as if it were a foreign
invader, the antibodies clump the
blood - can cause death.
Type AB blood has no
antibodies, any blood can be
donated to them="universal
acceptors"
Type O blood has no surface
markers on it, antibodies in
the blood do not react to type
O blood, ="universal donors"