MadSci Network: Genetics

Re: If the father has B+ blood type and the mother has O- can the child have O-

Date: Tue Feb 16 17:04:36 1999
Posted By: Carolyn Pettibone, grad student, Genetics, Harvard Medical School
Area of science: Genetics
ID: 915921864.Ge

Question: If the father has B+ blood type and the mother has O- can the child have O-?

Absolutely -- it happens all the time! Blood type inheritance is a little tricky, so I will try to explain it so you will understand how this can happen. Let’s deal with the some basic definitions first, then we can tackle the ABO blood type system, and then finish with the Rh (+ or -) system.

Useful definitions:
Gene: A specific location in your DNA that encodes hereditary information.

Allele: An alternate form of a gene. Each person has two alleles of each gene, one inherited from their mother, and one inherited from their father.

Dominant: A “strong” allele. One that can suppress a weaker recessive allele.

Recessive: A “weak” allele. One that is suppressed by dominant alleles.

Codominant: Alleles that are equal in strength. They are expressed together.

These terms are easier to understand if you have some examples to think about. I will use plants because they are simplest and easy to visualize.

The pea plant has a gene that determines how tall it grows. It has two copies of this gene, one allele inherited from its mother and one allele from its father (yup, even plants have parents!). If it inherited two tall alleles, it will grow up to be tall. If it inherited two short alleles, it will be small. But what if it has one tall allele and one short allele? In this case, the tall allele is dominant and the short allele is recessive. If the plant has one of each, it will grow up to be tall. It is as if the tall allele was “stronger” than the short allele, so it won. Does that make sense? Try reading the definitions and this paragraph again if it is not quite clear yet.

In the ABO blood system, the O allele is recessive (like the short allele), and the A and B alleles are dominant (like the tall allele). If someone has type B blood, they either have two B alleles, or one B allele and one O allele. It is just like the pea plant. There are two ways to be tall -- either you have two tall alleles or one tall allele and one short allele.

It is the same for type A blood. You can have type A blood with either two A alleles or one A allele and one O allele. The only way to have type O blood is to have two O alleles, because O is recessive to every thing else. AB blood type occurs when a person has one A allele and on B allele. A and B are both dominant to O, but they are codominant with each other. Think of it this way: it is as if A and B are in a power struggle, and they are of equal strength. When someone has inherited the A allele from one parent, and the B allele from the other, there isn’t a clear winner. The person has AB blood.

So now you can probably figure out how someone with O blood could have a father with type B blood and a mother with type O blood. That person inherited an O allele from his/her father (who has one B allele and one O allele), and an O allele from his/her mother (who has two O alleles). Those parents could also have a child with blood type B, if the child inherited a B allele from the father and an O allele from the mother.

Here’s another way of listing the blood type possibilities. You can write down what alleles someone has, and then say what blood type they have.

AA or AO = blood type A BB or BO = blood type B AB = blood type AB OO = blood type O

Now the Rh blood type will be trivial to understand. Either you are Rh positive (+) or Rh negative (-). You inherit an Rh allele from each parent. The positive allele is dominant to the negative allele. Someone who is Rh positive has either two positive alleles or one positive allele and one negative allele. That person can pass on either of those two alleles to their children. Someone who is Rh negative has two negative alleles.

In the example you gave, the dad is Rh positive, but he must have one positive allele and one negative allele. That means he could pass on either a positive allele or a negative allele to his child. So the child got two Rh negative alleles, one from mom and one from dad.

I hope this was helpful! You can get more information from any basic genetics textbook and most basic biology textbooks too. The advantage of looking in a textbook is that you’ll get lots of helpful figures and pictures.


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