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GCSE level biology exam revision notes on basic genetics Part 6

More complex genetics: 6.4

Explaining the genetics of blood group inheritance

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INDEX of biology notes on more complex genetics - inherited sex/non-sex linked examples

(6.4) The inheritance of blood groups (not sex-linked, an example of codominance)

Generally speaking

(i) most characteristics are determined by several genes,

(ii) most genes have two possible alleles - dominant, recessive or codominant with each other.

However, as in the case of blood groups, you sometimes get more than two alleles for a single gene.

In the case of human blood, two from three possible alleles control which blood group you inherit.

The three alleles controlling blood type are denoted by codominant A and B and recessive O.

Codominant means that when an individual has both dominant alleles, they have the blood type AB, NOT A or B, because one cannot dominate the other. Allele O is recessive.

Therefore you can potentially have one of four blood types: AB (from AB), A (from AA/AO), B (from BB/BO) and O from (double recessive OO). (allele pairings).

 

Possible genotypes and phenotypes of blood groups (simplified e.g. there are subgroups of A alleles)

Phenotype

blood group

 From possible genotypes
A AA or AO
B BB or BO
AB AB
O OO

In some genetic diagrams the genotypes are indicated with a different notation for dominant alleles

e.g.  IA, IB and recessive allele  IO, but I've used the 'usual' simpler notation i.e.

AO  as IAIO,  BO as  IBIO,  AA as IAIA, BB as  IBIB, AB as  IAIB  and OO as  IOIO

 

Worked examples of the genetic inheritance of blood groups

 

Example 1: Parent of blood group AO crossed with a parent blood of group BO.

Genetic Punnett square table for crossing a parent of blood group A with a parent of blood group B
Parent genotypes cross: AO  x  BO
Gamete alleles: A, O , B and O
Genotypes of parents - alleles A O
B AB BO
O AO OO

Since O is a recessive allele and A and B are dominant alleles,

this parental cross will give the ratio of the phenotype blood groups as:

AB : A : A : O or 1 : 1 : 1 : 1, in other words a 1 in 4 chance (25%) of any of the offspring having ANY of the four possible blood groups. A matching diagram is shown below.

 

Example 2: Parent of blood group AO crossed with a parent blood of group AO.

Genetic Punnett square table for crossing a parent of blood group A with a parent of blood group B
Parent genotypes cross: AO  x  BO
Gamete alleles: A, O , A and O
Genotypes of parents - alleles A O
A AA AO
O AO OO

In the offspring: 75% chance of blood group A (O recessive) and 25% chance of blood group O (double recessive).

 

Example 3: Parent of blood group AA crossed with a parent blood of group OO.

Genetic Punnett square table for crossing a parent of blood group A with a parent of blood group B
Parent genotypes cross: AO  x  BO
Gamete alleles: A, O , B and O
Genotypes of parents - alleles O O
A AO AO
A AO AO

In the offspring: 100% chance of blood group A (O recessive).

 

There are other possible permutations.

Key points Source of information is based on textbooks & syllabus-specifications for students taking the AQA GCSE, Edexcel GCSE and OCR GCSE level biology examinations (~US grades 9-10).

Key points on the genetics of blood groups

Blood Group Inheritance: Understanding Human Genetics

Blood group inheritance demonstrates how genes are passed from parents to offspring and how different blood types are determined.

1. The ABO Blood Group System

The ABO blood group system is controlled by a single gene with three alleles:

  • Iᴬ (produces antigen A)

  • Iᴮ (produces antigen B)

  • Iᴼ (produces no antigen)

The Iᴬ and Iᴮ alleles are codominant, meaning both can be expressed together (blood type AB). The Iᴼ allele is recessive, meaning it only shows when inherited from both parents.

2. Blood Group Inheritance

Each person inherits two alleles—one from each parent. The possible genotypes and phenotypes are:

Genotype

Phenotype (Blood Group)

IᴬIᴬ or IUIᴼ

A

IUIᴮ or IUIᴼ

B

IUIᴮ

AB

IUIᴼ

O

  • Blood group A: Can inherit IUIᴬ or IUIᴼ.

  • Blood group B: Can inherit IUIᴮ or IUIᴼ.

  • Blood group AB: Must inherit Iᴬ from one parent and Iᴮ from the other.

  • Blood group O: Must inherit Iᴼ from both parents.

3. Importance in Human Genetics

Studying blood group inheritance helps scientists:

  • Understand genetic inheritance patterns.

  • Improve blood transfusion compatibility.

  • Investigate population genetics and ancestry.

Students should be familiar with Punnett squares, codominance, and inheritance patterns.

Summary of learning objectives and key words or phrases

Be able to explain the genetics of blood group inheritance using Punnett square tables and genetic diagrams.

Know what codominant gene alleles are and how dominant, codominant and recessive gene alleles are involved in the inheritance of human blood type.

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