Genetics

Lesson 5.3

Monohybrid Cross

5.3.4 Blood Typing Traits

There are some traits that show a combination of traits expressions. One of the most common and well understood is that of human blood type. Human blood type comes in four main varieties A, B, AB, O. For each variation there is also a positive(+) and negative (-). In figure 5.16 you can see how common each blood type is in the United States.

Figure 5.16^[1]

The positive and negative symbol signifies if a person has (is positive for) or does not have (is negative for) a blood protein called the Rh factor. Being positive or negative is neither good or bad. For much of our lives it simple does not matter if you are Rh+ or Rh-. The only time this information becomes important is during childbirth and blood donation/transfusions. Blood from a - person can not be mixed with the blood of a + person. For our purposes we will not be using the + or - symbols, because it does not affect how blood type is determined.

So how is our blood type determined? Because the alleles in blood can be both dominant/recessive and incompletely dominant things can get a little confusing. A and B are both dominant to O, but they are incompletely dominant to each other and form AB. You can see the possible genotypes and phenotypes of human blood in figure 5.17.

Figure 5.17

There are two genotypes for A blood type, as well as two for B blood type. A person could be homozygous (AA or BB), in which two dominant alleles give the dominant trait or they could be heterozygous (Ao or Bo), because the the O is recessive, it is overpowered by the A and B. Similarly the only way to have a recessive trait is to have two recessive letters, in this case oo.

Practice problem:

What is the predicted blood types of the children of born to Alicia and James? Alicia has AB blood and James has O blood,

1) label the alleles

*The genotypes and phenotypes for blood type will always be the same, so we can skip this step and refer to table 5.17.

2) Label the cross

        Alicia X James

        AB X oo

3) Do the Punnett square

4) List all possible results

        50% Ao, A blood type

        50% Bo, B blood type

It is predicted that if Alicia and James have children each child will have a 50% chance of having A blood type and a 50% chance of having B blood type.