Inheritance of Blood Groups

Multiple Alleles and Blood Groups

Different forms of the same gene.

Multiple Alleles
When more than two alleles control a trait these are known as multiple alleles.

Blood Groups
The alleles controlling blood groups are a well known example of multiple alleles.
There are three alleles for blood groups within the human population
A, B and o. (A and B are equally dominant and o is recessive).
Every person will have just two of these three alleles. So the possible combinations are as follows
Alleles  Blood Group
AA or


BB or


Blood groups are inherited according to simple Mendelian patterns. They are another example of a Monohybrid Cross (involving one trait only).


A man whose blood group is A marries a woman with blood group O. Show the possible genotypes and phenotypes of their children.
(Remember: The man can have the genotype AA or possibly Ao. The woman can only be oo
Therefore we must do out two crosses to show the possible offspring.)

Cross 1
Parents:  Group A  Group O
AA oo
Gametes: A  o
F1 Genotype: All Ao
Phenotype: All Group A
Cross 2
Parents: Group A  Group O
Ao  oo
Gametes: A  o  o
F1 Genotype:  Ao  oo
Phenotype:  Group A Group O
1   1

Following the same layout try the following cross:

Parents:                              Group AB                         Group O


F1 Genotype: 



Parents:                                     Group A                  X                                  Group B (Hint: you will have to do more than two crosses)

Show how two parents who both are Group A could have a child that is Group O