Transcript 2/15 Advanced Mendelian Genetics

Codominance and Incomplete
Dominance
Incomplete Dominance
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Simple dominant-recessive doesn’t happen
often in real life. Genetics is more complicated.
You’re going to learn several more advanced
patterns of inheritance, how to recognize them,
and how to make Punnett Squares for them:
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Incomplete Dominance
Codominance
Sex-Linked Traits
Multiple Alleles
Polygenic Traits
Incomplete Dominance
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Incomplete dominance: Neither allele is
dominant. A heterozygote has a blend of the
two traits.
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For instance, incomplete dominance in coat color
may be B for black, W for white, and individuals who
are BW would be grey.
If T is for an abnormally long tail, and S is for an
abnormally short one, TS individuals could have a
normal medium tail length.
If B codes for a blue feather, and Y codes for a
yellow feather, then BY could code for a greenish
feather.
Incomplete Dominance
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Punnett Squares for Incomplete Dominance
work the exact same way as Punnett Squares
for normal dominance, just that heterozygotes
have a blended phenotype.
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With your partner: If the R allele is for red flowers,
and the W allele is for white flowers, make a Punnett
Square crossing a red flower with a pink one. Give
phenotype probabilities.
Codominance
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Codominance: If you have the allele for a trait,
you will have that trait. If you have two different
alleles, you have both traits at the same time.
NOT blended, literally both.
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For instance, if the C allele codes for making normal
catalase, and the c allele codes for making abnormal
catalase, a person who is Cc would make some
normal catalase and some abnormal catalase.
If the S allele codes for making sickle-shaped red
blood cells, and the R allele codes for making normal
round cells, a person who is SR makes some sickle
cells and some normal ones.
The most famous example is probably blood type…
Incomplete vs Codominance
FF
Incomplete
Dominance
Codominance
Normal
Dominance
FG
GG
Blood Type
What do you know about blood types?
 Does anyone know why you can only get
transfusions of certain kinds of blood?
 Why, on House, Grey’s Anatomy, etc., are
ER docs always asking for “O Neg”?
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Blood Type
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Red blood cells have many proteins embedded
into their cell membranes, just like any other cell
(including invading bacteria). These proteins
are called antigens.
And your blood plasma (fluid) is full of defensive
proteins, antibodies, that are shaped to fit just
one certain shape of antigen.
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You don’t have antibodies that fit to your antigens.
Why not?
Blood Type
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Human blood cells can have Atype proteins in their cell
membranes, or B-type proteins.
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People who have A-type antigens
will not have anti-A antibodies or
else they’d be attacking
themselves. They could have antiB antibodies, though.
Likewise, people with B-type
antigens in their cells will not make
anti-B antibodies.
Blood Type
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You get A-antigens and Bantigens from one allele
each. If you have a
genotype that is AA, you will
have blood cells with Aantigens.
If you have a genotype that
is BB, you will have blood
cells with B-antigens.
Blood Type
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But A and B are
codominant. If you
have both alleles, you
have type AB blood your red blood cells
have BOTH kinds of
protein in them. Each
cell has some A
proteins and some B
ones. And you don’t
make either anti-A or
anti-B antibodies - why?
Blood Type
The gene for blood type actually has
THREE alleles. A and B are codominant.
The O allele is recessive. The O allele is
essentially the “don’t make blood cell
proteins” allele.
 People who are OO make blood cells that
don’t have any proteins in them. And their
blood has BOTH kinds of antibodies.
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Blood Type
Blood Type
Because O is recessive, if you are AO,
you have type A blood.
 If you are BO, you have type B blood…
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Blood Type
Genotype
Phenotype
AO
Type A
AA
Type A
BO
Type B
BB
Type B
AB
Type AB
OO
Type O
Blood Type
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There’s another gene, different from the ABO
gene, that also makes proteins for the cell
membranes of red blood cells.
The Rh antigens are simpler, there’s a normal
dominant and recessive allele.
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The allele Rh+ is dominant. It codes for having Rh
antigens on red blood cells.
The allele Rh- is recessive. It codes for not having
any Rh antigens.
So, what are the three genotypes and phenotypes
for this gene?
Rhesus Disease
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People who are Rh- don’t automatically make
antibodies against Rh antigens, but they can. If they
are ever exposed to Rh+ blood, they may make antiRh+ attacking antibodies.
This can lead to some trouble for women pregnant in
certain situations:
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If Mom is Rh- and she has an Rh+ baby, it’s not going to be
very problematic the first time. But after contact with that
baby, her body will start to make anti-Rh+ antibodies…
If she gets pregnant with a second Rh+ baby, she now has
antibodies that will attack its blood. This can be fatal to the
developing fetus. She will need injections to prevent that
from happening.
Codominance Practice
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With your partner, make a Punnett Square
to predict offspring of parents who are AB
and OO (don’t worry about +/- for now).
Codominance Practice
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Now, make a Punnett Square to predict
offspring of parents who are +- and --.
Codominance Practice
To figure out a complete blood type, we
just do a dihybrid cross.
 Dihybrid cross, AB+- x OO--. Give
phenotype and genotype probabilities.
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Blood Type
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EARN A STICKER! Answer all 4
questions correctly…
Blood Type
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1. If you have type A blood, can you safely
receive type B blood?
2. If you have type AB blood, can you safely
receive type O blood?
3. The antigen and the antibody. Which is the
“attacker” in the blood stream and which is the
marker on the cell?
4. If you have type AB blood, is it possible for
you to have a type O parent?
Blood Type
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1. No! Type A blood has antibodies against B.
2. Yes, type AB blood has no antibodies, and
type O blood has no antigens to bond to.
3. Antibodies are the attackers, antigens are
the markers on the cell.
4. No, it isn’t. Type O means a genotype of
OO, but Type AB means a genotype of AB. If
you have a Type O parent, one of your alleles
must be O.