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Blood Groups

Blood Groups

Blood Groups

Every person has a blood type within the ABO blood groups. Blood groups are formed by multiple alleles (A, B and O), which display codominance - where more than one allele is seen in the phenotype.

Previously, we have focused on traits that have one gene controlling the trait, with two possible alleles.

For example in pea plants, a plant could have the tall phenotype or the short phenotype. One allele coded for a tall plant, and one allele coded for a short plant.

However, instead of just having two alleles for one trait, some traits have three or more different types of alleles. What do you think that is called?

One common example of multiple alleles is blood groups in humans. Every human belongs to a specific blood group, made from A and/or B and/or O alleles. Every person will have two chromosomes each with an allele on it. The alleles may be the same, or they may be different.


This image shows three chromosomes with a different allele in the same position on the chromosome.

The red allele codes for a protein A on red blood cells, the blue allele codes for protein B on red blood cells, and the white allele doesn't code for any proteins.


Remember that genes are just sections of DNA. A chromosome pair have the same sequences of genes (e.g. hair length next to eye colour next to toenail length). If the chromosomes had the exact same copy of the genes, they'd have the same coding DNA sequences.

Alleles are different forms of genes. Different alleles will have a different bases in their DNA, which code for a different protein (brown eye protein or blue eye protein).


For each trait, in this instance it is blood group, every person will have a gene on a chromosome, with a particular allele on it. 

It's 'twin', the other chromosome in the chromosome pair, will have another gene, either with a different allele or the same.

Why is it not correct to represent the genotype like this IAI^{A} for a person in blood group A?

Remember- a gene is a section of DNA, an allele is a variant of that gene (so a variant of that DNA sequence). So if you represented the gene for blood group as IA BI^A\space^B instead of IA IBI^A\space I^B , it would be saying that both the A blood variant and B blood variant is on the same gene, the same stretch of DNA.

The correct way of showing a genotype for one person's blood group would be IA IBI^A\space I^B . If someone has this genotype, we would say their blood group is AB. Which one of A or B is recessive?

Blood groups follow the inheritance method of codominance, which means that both alleles are equally expressed in the phenotype.

Pick all the options you think are correct about codominant alleles.

You can select multiple answers


This image shows all the possible genotypes of blood groups in humans.

Recall that allele A codes for a protein A on red blood cells, allele B codes for protein B on red blood cells, and allele O doesn't code for any proteins at all.


If allele O doesn't code for any proteins on the red blood cell, what proteins do you think an AO blood group would have?

A) No proteins B) A proteins only C) O proteins only


What proteins do you think the blood group AB has on it's red blood cells?

A) A proteins only B) B proteins only C) Both A and B proteins

So here's a recap of the different blood groups and the alleles that can make them.


Blood group A

AA and AO


Blood group B

BB and BO


Blood group AB



Blood group O



Here we have a blood group AB and a blood group A with an O allele.

The diagram shows how the blood groups are inherited.


The bottom right hand box doesn't have any alleles on its genes. Which option is the correct one?

A) O; B B) B; O C) B; B


Likely, what percentage of offspring could be blood group A?


Likely, what percentage of offspring could be blood group B?

Some examples of blood crosses may show the symbol ii instead of IOI^{O} to show a blood group O... Just something to bear in mind so you don't get confused if you come across an ii.