Multiple Alleles Revision Notes for AQA A-Level Biology

Multiple Alleles

What are multiple alleles?

Most genes exist in just two versions (alleles), but some genes can have multiple alleles - meaning they have more than two different versions. This creates more complex inheritance patterns than simple dominant-recessive relationships.

chatImportant

An important point to remember is that although a gene may have multiple alleles in a population, each individual can only carry two alleles for any gene. This is because we have two homologous chromosomes, which means only two gene loci are available for each gene.

The ABO blood group system

The human ABO blood group system provides an excellent example of multiple alleles in action. The gene controlling ABO blood groups (the immunoglobulin gene) has three different alleles:

$I^A$ allele: produces antigen A on red blood cell surfaces
$I^B$ allele: produces antigen B on red blood cell surfaces
$I^O$ allele: produces no antigens on red blood cell surfaces

These antigens are proteins found on the cell membrane of red blood cells and determine blood group compatibility for transfusions.

Blood group genotypes and phenotypes

Blood Group	Possible Genotypes
A	$I^A I^A$ or $I^A I^O$
B	$I^B I^B$ or $I^B I^O$
AB	$I^A I^B$
O	$I^O I^O$

The $I^A$ and $I^B$ alleles are codominant, meaning both are expressed when present together. The $I^O$ allele is recessive to both $I^A$ and $I^B$ .

Inheritance patterns in ABO system

lightbulbExample

Worked Example: ABO Blood Group Crosses

Cross 1: Blood group AB ( $I^A I^B$ ) × Blood group O ( $I^O I^O$ )

All offspring receive $I^O$ from O parent
Half receive $I^A$ from AB parent → Blood group A
Half receive $I^B$ from AB parent → Blood group B
Result: Only blood groups A and B offspring

Cross 2: Blood group A ( $I^A I^O$ ) × Blood group B ( $I^B I^O$ )

Possible offspring genotypes:
- $I^A I^B$ → Blood group AB (25%)
- $I^A I^O$ → Blood group A (25%)
- $I^B I^O$ → Blood group B (25%)
- $I^O I^O$ → Blood group O (25%)
Result: All four blood groups possible

This occurs because the blood group A parent might be heterozygous ( $I^A I^O$ ) and the blood group B parent might also be heterozygous ( $I^B I^O$ ).

Codominance in cattle coat colour

Another clear example of multiple alleles involves coat colour in shorthorn cattle. The gene C has two main alleles:

$C^R$ allele: produces red pigment when homozygous ( $C^R C^R$ = red coat)
$C^W$ allele: produces no pigment when homozygous ( $C^W C^W$ = white coat)

When both alleles are present ( $C^R C^W$ ), neither dominates the other. Instead, both alleles are expressed equally, producing a roan coat. This coat appears as a mixture of both red and white hairs distributed across the animal.

infoNote

The roan phenotype demonstrates codominance perfectly. Each individual hair is either completely red or completely white - there are no pink hairs. The roan appearance comes from having both red and white hairs mixed together across the coat.

When two roan-coated animals (both $C^R C^W$ ) are crossed, the offspring appear in a 1:2:1 ratio - 25% red coat, 50% roan coat, and 25% white coat.

Environmental influence on gene expression

Even when genotype is known, the final phenotype can be influenced by environmental factors. A good example is found in Siamese cats, where fur colour is affected by temperature.

lightbulbExample

Example: Temperature Effects in Siamese Cats

The production of dark pigment in Siamese cats is controlled by an enzyme called tyrosinase. This enzyme is temperature-sensitive, working better at lower temperatures. This explains why Siamese cats have:

Darker fur on cooler body parts (ears, face, feet, tail)
Lighter fur on warmer body parts (main body)
Kittens born with completely light coats that develop their characteristic markings as they grow

This shows that phenotype results from both genetic factors (the genotype) and environmental influences.

Key principles of multiple alleles

Understanding multiple alleles requires remembering several important principles that govern how these genetic systems work:

chatImportant

Key Principles:

Population vs individual: A gene may have many alleles in a population, but each individual only carries two
Dominance relationships: Multiple alleles can show various dominance patterns - complete dominance, codominance, or more complex relationships
Inheritance complexity: More alleles mean more possible genotype combinations and inheritance patterns
Real-world applications: Multiple alleles are common in nature and have practical importance in areas like blood transfusions and animal breeding

Links to immune system function through ABO blood groups and antigen recognition.

bookmarkSummary

Key Points to Remember:

Multiple alleles means a gene has more than two versions in a population, but individuals still only carry two alleles
The ABO blood group system demonstrates multiple alleles with $I^A$ , $I^B$ , and $I^O$ alleles creating four different blood groups
Codominance occurs when both alleles are expressed equally, as seen in roan coat colour in cattle
Environmental factors can influence how genes are expressed, affecting the final phenotype
Multiple alleles create more complex inheritance patterns than simple dominant-recessive relationships

Multiple Alleles (AQA A-Level Biology): Revision Notes