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MULTIPLE ALLELES &CODOMINANCE

• The classic example of both multiple alleles and codominance is the human ABO blood group system.

• Red blood cells have glycoproteins on their surface which serve as identification tags. There are many types of tags which are made by genes however, the tags that we’ve determined are most important in blood transfusion are the ABO Blood Group tags.

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• There are three different alleles associated with the ABO Blood group system. – IA make type A glycoproteins (antigens)– IB make type B glycoproteins (antigens)– i makes no glycoproteins (antigens)

• Because there are more than two alleles, we use this as an example of multiple alleles.

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• Like always, human cells have two copies of each gene and the combination of genes make up the various blood groups.

• There are 6 combinations of these three alleles which create 4 blood groups

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• Because the genotype IAIB is composed of two different alleles which both produce antigens, we say those alleles are codominant.

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• Your body also manufactures antibodies that prevent another type of blood from working in your body. When an blood cell with a different type of antigen comes into contact with an antibody, the antibody agglutinates (clots) the blood.

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• During most blood transfusions, only the red blood cells are provided to the patient. The blood plasma, and its associated antibodies are not given. So, when there is a mistake in providing blood transfusions, it’s the patient’s antibodies that react with the donor’s red blood cells to cause the clotting.

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• The ABO system is as an example of multiple alleles because;a) it is caused by IA, IB, and i.b) it produces 6 different genotypes.c) it produces 4 different phenotypes.d) both IA and IB produce antigens when they are

present.e) it causes clotting when the wrong type of blood

is transfused to a patient.

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• The ABO system is as an example of codominant alleles because;a) It is caused by IA, IB, and i.b) It produces 6 different genotypes.c) It produces 4 different phenotypes.d) Both IA and IB produce antigens when they are

present.e) It causes clotting when the wrong type of blood

is transfused to a patient.

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• How many different genotypes are present in the ABO blood group system?a) 2b) 3c) 4d) 5e) 6

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• Which of the following is true regarding type A blood?a) It has type B antigens.b) It has type anti-A antibodies.c) It can be donated to someone with type O

blood.d) It is caused by a recessive allele.e) It will clot if given AB blood.

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