Prepared for B. Sc. III Botany students, based on the syllabus prescribed by Dr. Babasaheb Ambedkar Marathwada University, Aurangabad.

1. B. Sc. III: Semester-VI Paper-XIX:
Genetics and Biotechnology
Blood group Inheritance
Dr. Anjali Naik
Head, Dept. of Botany,
SBES College of Science

2. Inheritance of blood group.
u It’s a variation of mendelian monohybrid inheritance.
u The ABO blood group system in man provides an excellent illustration of a
multiple allelic system.
u Bernstein in 1924 found that the blood group antigens are controlled by an
autosomal gene designated I (iso-haemaglutinin) which has 3 alleles IA, IB and
IO.
u So, instead of one gene with two alleles controlling the inheritance of one
character, (as seen in mendelian monohybrid inheritance), here three alleles
control the inheritance of blood group.
u This also presents a case of variation of codominance.
u IA and IB are codominant, and both express themselves in F1 as a third
phenotype- Blood group AB

3. Explanation of inheritance of blood
groups.
u Human population has 4 blood groups—A, B, AB and O.
u These are identified by a specific glycoprotein substance,
called antigen on the surface of RBCs.
u The blood group is determined by a gene “I” which has
three forms or alleles viz., IA, IB, IO.
u The genes IA and lB are dominant to gene IO but are co-
dominant to each other and both are expressed when
present together.

4. HOW BLOOD GROUPS ARE TESTED?
u Antigens- Proteins present on the surfaces of
RBCs.
u Antibodies- Proteins present in the plasma.
u Antibodies and antigens show clumping
(Agglutination) when mixed indicating blood
group.
u Bl.gr. A persons have antigens A on RBCs and
anti B antibodies in plasma.
u Bl.gr. B persons have antigen B on RBCs and
anti A antibodies in plasma.
u Bl.gr. AB have both A and B antigens on their
RBCs but no antibodies in plasma.
u Blo.gr. O have no antigens but both anti A
and anti B antibodies in plasma.
u Bl.gr O: Universal donors and Bl gr AB are
universal accepters.

6. Working of alleles
u Blood group phenotype. Possible genotypes
1. Blood gr. A (Homozygous) IAIA
2. Blood gr. A (Heterozygous) IAIO
3. Blood gr. B (Homozygous) IBIB
4. Blood gr. B (Heterozygous) IBIO
5. Blood gr. AB IAIB
6. Blood gr. O IOIO
IA = IB (Codominant)

8. Parents: Mother- Heterozygous for blood A: IAIO
Father- Heterozygous for blood B: IBIO
IAIO X IBIO
Gametes of parents: IA and IO; IB and IO
F2:
Genotypic ratio: IAIB: IBIO : IAIO : IOIO in 1:1:1:1 proportion
Phenotypic ratio: AB:A:B:O also in 1:1:1:1 proportion.
Male Parent Female parent
IA
IO
IB IAIB
(Blood gr. AB)
IBIO
(Blood group B)
IO
IA
IO
(Blood gr. A)
IO
IO
(Blood gr. O)

9. Assignment problems on blood group inheritance
1. List all the possible genotypes for each of the 4 blood types:
2. A man who has type B blood (genotype: BB) is married to a woman with type
O blood. What blood type will their children have?
3. A woman with type A blood (genotype: AO) is married to a type B person
(genotype: BO). What blood types will their children have?
4. A woman with type A blood is claiming that a man with type AB blood is the
father of her child, who is also type AB. Could this man be the father? Show the
possible crosses; remember the woman can have AO or AA genotypes.