Patterns of inheritance 6
Joanna George-Johnson, Contributor

Members of Belair High's basketball team show off their trophy, recently, following a ceremony to celebrate their victory in the all-island championships. At right is coach Mike Stephenson and beside him is Miss Jamaica Universe 2007, Zahra Redwood. Also with the team are assistant coach Phillip Johnson, left, and principal Trevor Brown, second right. - Ricardo Makyn/Staff Photographer

In previous lessons, we talked about when traits do not follow the dominant recessive rule. Moreover, we looked closely at incomplete dominance. This week we will, however, look at co-dominance - another case when genes do not always follow the dominant recessive rule.

• Co-dominance is similar to incomplete dominance in that there is no dominant allele.
  
  
• However, the phenotypic expression is quite different.
  
  
• If two alleles have a co-dominance relationship, in the heterozygote state, both alleles will be completely expressed.

ABO blood type - An example of co-dominance

For example, in human ABO blood types, two of the three alleles (the A and B alleles are co-dominant).

This gene controls the deposition of antigen markers on cells.

A person with blood type A has one kind of antigen marker, while a person with blood type B has a slightly different kind of antigen marker. A person with blood type O has no antigen marker.

A person with blood type AB has both A antigen and B antigen markers.

There is no 'in-between' antigen, as would be expected if the alleles showed incomplete dominance. Both of the alleles are completely expressed, and the person has both blood types at the same time.

With this in mind, let's do the cross for the following:

Two parents: one heterozygous for blood type A, and the other heterozygous for blood type B.

*I indicate the gene for blood type. The super script represents the allele on the gene.

Random Fertilisation
Possible Genotype of offspring (2n)

Possible Phenotype: from the punnett square it can be seen that there is a 1 : 1 : 1 : 1 chance that these parents may produce offspring that have any of the four blood types.

Questions

1. Differentiate between incomplete and co-dominance?

2. A woman with blood type A sued a man with blood type B (IBIB) for child support. The child had blood type O. A simple blood test was done to determine paternity. Using a genetic diagram, such as a punnett square, determines if the man sued could possibly be the child's father. If he is not the father determine the possible genotype of the true father. Also determine the genotype of the mother.

Joanna George-Johnson teaches at Ardenne High School.
Email: Masterbio@gmail.com.