Co-Dominance

OAC Biology MULTIPLE ALLELE SYSTEMS: CODOMINANCE

MR.GALAJDA

A diploid organism has only 2 alleles for most loci. Where only 2 alleles exist in a population, only 3 possible genotypes can occur. For example:

At some loci, there may be more than 3 alleles present in a population. REMEMBER that each individual has ONLY 2 alleles at any one time. Obviously, if 3 or more alleles are present in a population the number of genotypes will be greater than that found in two allele systems. When 3 or more alleles are present in a population, the loci are said to form multiple allele systems.

MULTIPLE ALLELES: A trait is determined by a combination of more than one allele type.

An example of a multiple allele system found in humans is the ABO blood typing system. This system involves 3 different alleles. This system also shows CODOMINANCE.

In the ABO System there are 3 well-known alleles:

IA IB i (O)

These three alleles can produce 4 possible blood phenotypes:

A B AB O

Allele IA produces antigen A on the surface of red blood cell membranes.

ANTIGEN: (Ag) special proteins present on the surface of cell membranes. An antigen is a foreign substance that comes in contact with the body. Since the body cannot identify foreign substances as self, it activates the immune system in order to destroy the foreign antigen. ANTIBODIES (Ab), found in the blood plasma, is the body’s response to antigen. Every antibody produced is specific to every antigen (like a lock and key).

Allele IB produces antigen B on the surface of red blood cell membranes.

Allele i (O) produces no antigen on the surface of the red blood cell membranes.

The ABO system shows dominance (IAIA, IAi) (IBIB, IBi), codominance (IAIB) and the recessive function of a third allele (IAi, IBi. ii).

This information is obviously important during blood transfusions. In this instance, knowledge of the genetics of blood is essential since the infusion of the wrong blood type could possibly lead to death.

SAMPLE PROBLEMS:

#1. Write the genotypes of the following individuals:

(a) homozygous type B ______________

(b) heterozygous type AB _______________

#2. A man with type A blood is attempting to determine whether or not he can possibly be the father of a certain child. The child has type O blood and it is known that its mother has type B blood. Show how you would determine whether the man could be the father of this child. (Even if it can be shown, it does not necessarily follow that the man is the father)

RHESUS FACTOR ( Rh factor ):

Rh factors are other antigens that may be present on the membranes of red blood cells. Most people have these antigens and are said to be Rh positive. People who lack these antigens are said to be Rh negative.

Phenotype Genotype

Rh positive DD, Dd

Rh negative dd

SAMPLE PROBLEM:

#1. A man with the blood phenotype O Rh positive marries a woman with the blood phenotype AB Rh negative. What possible blood phenotypes may their children develop?

E-mail Mr. Galajda at: david.galajda@sac.on.ca