Brent Cornell

    
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)

Co-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 (C^B) and white (C^W) feathers create a speckled coat (C^BC^W)

Co-dominance (Chicken Feathering)

 

    
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 = I^A  ;  B allele = I^B  ;  O allele = i (recessive)

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

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

The Consequence of an Incompatible Blood Transfusion