Rules for Genetic Crosses: Codominance and Numerous Alleles

Red Blood Cells

As much as I hate doing this, I’m going to stick with the traditional example of codominance for this article. But I have only decided to do so because it’s such a great way to learn this concept. The ABO blood group is widely known and used, so if you don’t get it here, then there exist 9872934598723405973984572034570923465 other sites or individuals who can explain it to you. So, moving right along…

Here in America, our laws state that we don’t discriminate. You can’t have an advantage over another person simply because of your race, gender, religion, or anything. (Our citizens disagree on some of these, but that’s an article for a different webzine.) HOWEVER, in America, you’re pretty much not allowed to do anything legally until you’re 18 years old. You can’t buy a house, start a business, or even call that 1-800 number on the tv commercials.

Codominance works in much the same way. Codominant genes don’t descriminate. If they are the dominant form of the gene, then they are all on the same playing field, and no one gene is any more powerful than another… unless you’re the under-18 form of the gene. Then, you’re recessive to ALL of the codominant forms.

As for the point of multiple alleles, think of them as all the different races of adults in America. They all have the same abilities (legally), but they come in so many different flavors! Codominant genes are the same way. One gene is no more powerful than the other, unless one is the under-18 form, and there can be many different types!

Let’s get right into the example of the ABO blood group. In this system, there are three main alleles that decide what type of blood cell clothing that is produced. Remember that genes code for proteins, so the genes of the ABO blood group code for proteins. The thing to know, though, is that these proteins are presented on the outside of the cell, much like clothing for us. These three types are “make an A sock” (A), “make a B sock” (B), and “go barefoot” (o).

{If you’ll notice, both the “A sock” (A) and “B sock” (B) alleles are capitalized, and the “barefoot” (o) allele is not. This is codominance. In this case, we would say that both (A) and (B) are dominant to (o). }

Do you remember how to figure out the possible combinations? There are three possible alleles, but we can only choose two because there are two copies of each gene in the final cell. If you don’t know how to figure this out, then check out my post on permutations. To save time, I’ll just tell you that there are six combinations: (AA), (AB), (BB), (Ao), (Bo), and (oo).

So, lets do some practice. Suppose a type (AB) woman had a son with a type B man – the kid is a type A. What is dad’s genotype?

Step 1: Gather given info.

So, here’s what we know:

• Mom = (AB)
• Dad = (B?)
• Kid = (A?)

Step 2: Figure out genotypes

The kid already has one (A) allele, and we need to know what that other one is in order to figure out dad’s genotype. Since we know that the kid is type A, he only has 2 possibilities: (AA) and (Ao). We could use this information in two ways – we could either run two punnett squares, or we could use common sense.

That other allele, either (o) or (A), had to come from Dad… who we already know is a type B. Which of those two alleles could he have given the boy in order to still be type B? Since the (A) allele is codominant to the (B), possession of this allele would mean that dad is a type AB. Since we already know that he’s a type B, and that the (o) allele is recessive to the (B), then dad’s other allele must be the (o).

Step 3: Check answer against question

Whoops, we never properly answered the question, which asked us for dad’s genotype… not his other allele. So the formal answer to this question is: Dad’s genotype is (Bo).

… The OTHER half of ABO blood typing…

Yes, there’s more. I can’t leave you here without covering the Rh group component of the ABO blood system. You see, just as (A) and (B) and (o) code for the red blood cell’s socks, the Rh gene codes for the shoes. You may have head people talking about their blood types as AB positive or O negative. This positive (R) and negative (r) refer to the Rh gene… and any crosses that you perform concerning blood types will cover the ABO gene and the Rh gene, meaning blood type crosses are actually dihybrid crosses.

Let’s solve a problem so that we can all go home.

Jenny Anderson, Janie Anderson, and Josie Anderson (all unrelated) each go to the same veterinarian at the same time on the same day so that their Yorkies can have pups. Each has only one pup. The nurses lose track of which pup belongs to whom, since the babies happened to each look alike by some awful coincidence. You are the head vet, and it’s your job to figure out which pup belongs to which dog. You remember reading about the ABO/Rh blood groups when you were in introductory bio; although you don’t remember whether or not its the same in dogs, you decide to give it a try anyway. Your blood test results are below. Which pup belongs to which dog? (Assume that each parent is a heterozygote.)

• Jenny’s Female = O+
• Jenny’s Male = B+
• Janie’s Female = B-
• Janie’s Male = A-
• Josie’s Female = A+
• Josie’s Male = O-
• Pup 1 = O+
• Pup 2 = AB-
• Pup 3 = B-

Since step one has already been done for you, I guess it’s time to move on to step 2.

Step 2: Figure out genotypes

In this case, we can’t really do anything until we figure out exactly what types of children each couple can have. For this, we’ll employ the use of three punnett squares.

Now, let’s examine the results and the parent types. (Since the problem said to, we assumed that everyone that COULD be a heterozygote was a heterozygote.) Let’s start with the first pup, type O+. Since both Jenny and Josie have dogs that COULD have had O+ pups, we can’t really decide who’s dog it is yet. So let’s move on to pup 2, AB-. Janie was the only one of the three dogs that COULD have had AB- pups, so pup 2 must be hers. In the case of pup 3, only Jenny’s dogs could have had a pup with B- blood. So pup 3 must be Jenny’s. This only leaves pup 1 and Josie, so they must go together.

Good job, Doc.

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Best of Luck,

Grey