Codominance and Incomplete Dominance

Homework

Objectives Distinguish between codominance and incomplete dominance

Be able to make Punnett Squares for cases of codominance and incomplete dominance

Inheritance Patterns 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: Incomplete Dominance Codominance Sex-Linked Traits Multiple Alleles Polygenic Traits

Incomplete Dominance Incomplete dominance: Neither allele is dominant. A heterozygote has a BLEND of the two traits. 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 Punnett Squares for Incomplete Dominance work the exact same way as Punnett Squares for normal dominance, just that heterozygotes have a blended phenotype. 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 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. 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 FG GG

IncompleteDominance

Codominance

NormalDominance

Blood Type What do you know about blood types? Do you know yours?

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”?

Blood Type 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. You don’t have antibodies that fit to your antigens. Why not?

Blood Type Human blood cells can have A-type proteins in their cell membranes, or B-type proteins. People who have A-type antigens will not have anti-A antibodies or else they’d be attacking themselves. They could have anti-B antibodies, though.

Likewise, people with B-type antigens in their cells will not make anti-B antibodies.

Blood Type You get A-antigens and B-antigens from one allele each. If you have a genotype that is AA, you will have blood cells with A-antigens.

If you have a genotype that is BB, you will have blood cells with B-antigens.

Blood Type 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.

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…

Blood TypeGenotype Phenotype

AO Type A

AA Type A

BO Type B

BB Type B

AB Type AB

OO Type O

Blood Type 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. 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.

Not-So-Fun Fact: Rhesus Disease

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 anti-Rh+ attacking antibodies.

This can lead to some trouble for women pregnant in certain situations: 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 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 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.

Blood Type EARN A STICKER! Answer all 4 questions correctly…

Blood Type

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 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.