Blood Grouping and Inheritance

An example of how we show multiple alleles and codominace

Genotypically speaking blood is quite similar to the other genes we have discussed.

It uses a capital letter to represent a dominant trait

It also uses a lower case letter to represent a

recessive trait

I i

I i

From a dominant and recessive allele we can assume that we have 3 possible combinations

•II – Homozygous dominant

•Ii – Heterozygous

•ii – Homozygous recessive

But with blood there is more than just two possible alleles to consider

When we talk about the ABO blood groups there are actually 3 alleles that are responsible for the different groups

The “O” trait has the first trait and is the lone recessive allele and is written as “i”

The other two traits, “A” and “B” are both dominant so they both use the capital letter “I”

PROBLEM - But with “I” written as is there is no way to decipher if it is an A allele or a B allele

SOLUTION – For each separate trait the allele will have an accompanying superscript letter.

IA IB

This is not done to confuse you

The capital “I” tells you the trait is a dominant allele and the superscript just helps you figure out which dominant allele it is.

So instead of just three possible genotypes, II – Homozygous dominantIi – Heterozygousii – Homozygous recessive

We actually have 6 possible genotypes

IAIA IBIB

IAi ii IBi

IAIB

And four possible Phenotypes

Hopefully that helps you out