Modes of Inheritance

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•  Dominant alleles mask the effect of recessive alleles but co-dominant alleles have joint effects

The gene composition (i.e. allele combination) for a specific trait is referred to as the genotype 

  • The genotype of a particular gene will typically be either homozygous or heterozygous

The observable characteristics of a specific trait (i.e. the physical expression) is referred to as the phenotype

  • The phenotype is determined by both the genotype and environmental influences 

Complete Dominance

Most traits follow a classical dominant / recessive pattern of inheritance, whereby one allele is expressed over the other

  • The dominant allele will mask the recessive allele when in a heterozygous state
  • Homozygous dominant and heterozygous forms will be phenotypically indistinguishable
  • The recessive allele will only be expressed in the phenotype when in a homozygous state

When representing alleles, the convention is to capitalise the dominant allele and use a lower case letter for the recessive allele

  • An example of this mode of inheritance is mouse coat colour – black coats (BB or Bb) are dominant to brown coats (bb)

Complete Dominance (Mouse Coat Colour)

complete dominance


Co-dominance occurs when pairs of alleles are both expressed equally in the phenotype of a heterozygous individual

  • Heterozygotes therefore have an altered phenotype as the alleles are having a joint effect

When representing alleles, the convention is to use superscripts for the different co-dominant alleles (recessive still lower case)

  • An example of co-dominance is feathering in chickens – black (CB) and white (CW) feathers create a speckled coat (CBCW)

Co-dominance (Chicken Feathering)



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•  Inheritance of ABO blood groups

Human red blood cells can be categorised into different blood groups based on the structure of a surface glycoprotein (antigen)

  • The ABO blood groups are controlled by a single gene with multiple alleles (A, B, O)

The A, B and O alleles all produce a basic antigen on the surface of red blood cells

  • The A and B alleles are co-dominant and each modify the structure of the antigen to produce different variants
  • The O allele is recessive and does not modify the basic antigenic structure

When representing blood group alleles, the letter
I is used to represent the different antigenic forms (isoantigens)

  • A allele = IA  ;  B allele = IB  ;  O allele = i (recessive)

The genotypes for the different blood groups can be summarised as follows:

blood type table

As humans produce antibodies against foreign antigens, blood transfusions are not compatible between certain blood groups

  • AB blood groups can receive blood from any other type (as they already possess both antigenic variants on their cells)
  • A blood groups cannot receive B blood or AB blood (as the isoantigen produced by the B allele is foreign)
  • B blood groups cannot receive A blood or AB blood (as the isoantigen produced by the A allele is foreign)
  • O blood groups can only receive transfusions from other O blood donor (both antigenic variants are foreign)

Summary of the ABO Blood Groups

ABO blood groups

The Consequence of an Incompatible Blood Transfusion

blood transfusion