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More complex genetics: 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

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(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 IAIOBO as  IBIOAA 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).

Do more examples? lots of other permutations.


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