Exceptions to Mendel's Principles > Multiple alleles

Multiple alleles are the three or more alternative forms of the same gene that occupy the same locus on homologous chromosomes and control the same character in a population, though only two alleles occur together in an individual.

• Alleles are different forms of the same gene present at the same locus on homologous chromosomes.
• When three or more alleles of a gene that control the same trait occupy the same locus in a population, they are called multiple alleles.
• Any one diploid individual still carries only two alleles for that gene at a time, one on each homologous chromosome.
• In simple Mendelian inheritance, most genes have two alternative forms – a dominant (wild) allele and a recessive (mutant) allele.
• A gene can undergo repeated mutations, producing several alternative forms (alleles) over time.
• These alleles are all mutant forms of the same original gene (wild type) and together form a multiple-allelic series.

Key idea:

• The wild-type allele is usually dominant to most or all mutant alleles.
• The alleles in a series may show dominant–recessive, codominance, or incomplete dominance relationships among themselves.

• Multiple alleles are a population-level phenomenon: the gene has many allelic forms in the gene pool of the population.
• At the individual level, a diploid organism can possess only two alleles of that gene at any time.

• In Drosophila, wing size is controlled by multiple alleles of a single gene.
• Normal wing is the wild-type phenotype and is dominant.
• Vestigial wing (very reduced or absent wings) is recessive and expressed in the homozygous condition for that allele (vg in homozygous form).
• Intermediate phenotypes (like nicked, notched, and strap) form part of an allelic series between wild type and fully vestigial.

Normal (wild type) → intermediate variants → vestigial (recessive) – all controlled by multiple alleles of one gene.

The ABO blood group in humans is the classical example of multiple alleles.

(a) Alleles and Antigens:

The gene responsible for ABO blood group has three alleles: Iᴬ, Iᴮ, i.

These alleles determine the presence or absence of antigens (agglutinogens) on the surface of red blood cells.

• Iᴬ → synthesises A antigen.
• Iᴮ → synthesises B antigen.
• i → does not produce A or B antigen.

(b) Dominance Relationship:

Iᴬ and Iᴮ are codominant:

• In genotype IᴬIᴮ, both A and B antigens appear on RBCs → blood group AB.

Iᴬ and Iᴮ are dominant over i:

• Iᴬi behaves as group A (Iᴬ expressed, i masked).
• Iᴮi behaves as group B.

Dominance order:

• Iᴬ = Iᴮ > i.

(c) Genotype–Phenotype Table:

(d) Cross Giving All Four Blood Groups:

Cross between parents heterozygous for A and B:

• Parent 1 (A): Iᴬi
• Parent 2 (B): Iᴮi

This cross can produce A, B, AB, and O blood groups in the offspring (all four phenotypes possible).

Parents’ Blood Groups and Children’s Possible Blood Groups:

1. If both parents are O (ii × ii).
   The children can have only blood group O (ii).

2. If one parent is AB and the other is O (IᴬIᴮ × ii).
   The children can have only A or B blood groups
   and cannot have AB or O.

3. If one parent is A and the other is B,
   and both are heterozygous (Iᴬi × Iᴮi).
   The children can show all four blood groups – A, B, AB, and O.

4. If both parents are AB (IᴬIᴮ × IᴬIᴮ).
   The children can have A, B, or AB.
   But never O, because no parent has i in homozygous form.

Child and Parent Blood Group Logic:

Multiple alleles also help to check whether a particular blood group is possible in the child from given parents.

1. If both parents are O (ii).
   A child with A, B, or AB blood group cannot be born in this family.

2. If the mother is O (ii) and the child is AB (IᴬIᴮ).
   This combination cannot arise from this mother because she has only 'i' alleles.

3. If the child is O (ii).
   Then each parent must contribute an i allele.
   So at least both parents must carry i (they may be O, A, or B but must have i).

These patterns come directly from the genotype–phenotype relationship of the multiple alleles Iᴬ, Iᴮ, and i in the population.

• Multiple alleles are defined as three or more alternative forms of the same gene that control a specific character in a given population.
• Despite the presence of multiple alleles in a population, a single individual will only carry two of these alleles at any given time.
• All alleles in a multiple allele series occupy the exact same position, or locus, on homologous chromosomes, which means crossing over does not occur between them.
• When any two mutant alleles from the series are crossed, the resulting phenotype will always be a mutant type rather than the original wild type.
• The human ABO blood grouping system serves as a classic and practical example of multiple allelic inheritance.