Multiple Alleles | Zoology Optional Notes for UPSC PDF Download

Multiple Alleles

An allele, a shorter term for allelomorph (another form), represents an alternate form of a gene. While many genes have two alternate forms, some have more than two. The presence of more than two alleles at the same locus results in a multiple allelic series. Multiple alleles refer to a series of gene forms located at the same locus on homologous chromosomes, impacting the same trait.

According to Mendel, genes typically have two alternate forms, with one being dominant and the other recessive. The dominant form, known as the wild type, serves as the baseline from which the recessive mutant evolves through mutation. Similarly, a wild type can undergo various mutations, producing multiple mutant forms. Mutants can further undergo additional mutations, leading to the emergence of new mutants. Consequently, a gene can exist in more than two allelomorphs, with the wild type allele usually exerting dominance over its recessive counterpart, represented as "+".

Multiple alleles are characterized as different forms of the same gene, with slight differences in the sequence of bases in genes located at the same chromosome locus. These alternative states at the same locus result in each individual having only two alleles for a trait, despite several alleles being available. Examples of multiple alleles include human blood group, self-incompatibility in tobacco, coat color in rabbits, and self-incompatibility genes in brassica.

The number of possible genotypes in a series of multiple alleles is given by ½ n (n + 1), where n represents the number of alleles.
For instance:

• Di-allelic genes can generate 3 genotypes.
• Genes with 3 alleles can generate 6 genotypes.
• Genes with 4 alleles can generate 10 genotypes.
• Genes with 8 alleles can generate 36 genotypes.

Important features of multiple alleles

• Multiple alleles consistently occupy the same locus on a chromosome, with only one allele present at a given time.
• Multiple alleles invariably govern the same characteristic in an individual.
• The wild type allele consistently exerts dominance over other alleles.
• Crossing over does not occur in the context of multiple alleles.
• In a series of multiple alleles, the wild type is consistently dominant.
• When crossing two mutant types, the recovery of the wild form is not possible.
• The mating of two mutant alleles will consistently result in a mutant phenotype. Examples of multiple alleles include:
  a) Fur color in rabbits
  b) ABO blood group in humans
  c) Wing type in Drosophila
  d) Eye color in Drosophila.

In rabbits, the coat color is controlled by three alternative forms of genes, with "C" representing the wild type and its alleles.

Skin colour in rabbit

In rabbits, four kinds of skin colour are known.

Agouti

• Also known as the wild type, agouti exhibits full coloration and is dominant over all other colors.
• When crossed with any of the other three colors in rabbits, it produces agouti color in F1 and follows a 3:1 ratio in F2.
• Represented by the symbol "C."

Chinchilla

• Lighter than agouti, chinchilla is dominant over Himalayan and albino.
• When crossed with either Himalayan or albino, it produces chinchilla in F1 and follows a 3:1 ratio in F2.
• Represented by the symbol "cch."

Himalayan

• Characterized by a white main body and colored tips on the ears, feet, tail, and snout.
• Dominant over albino and produces a 3:1 ratio in F2 when crossed with albino.
• Represented by the symbol "ch."

Albino

• Features pure white fur color and is recessive to all other types.
• Represented by the symbol "c."
• The order of dominance for fur color in rabbits is represented as Agouti > Chinchilla > Himalayan > Albino (C > cch > ch > c).

ABO Blood group in man

Antibody

• A type of protein commonly known as immunoglobulin.
• Typically found in the serum or plasma.
• The presence of an antibody can be identified through its specific reaction with an antigen.

Antigen

• Refers to a substance or agent that, when introduced into the system of a vertebrate animal (e.g., cow, goat, man), induces the production of a specific antibody.
• Antigens are located in red blood corpuscles (RBC).
• In human RBCs, two types of antigens, A and B, are present.
• Blood groups in humans are categorized into four types (A, B, AB, and O) based on the presence or absence of antigens A and B.
• Blood group A individuals have antigen A on the surface of their RBCs.
• Blood group B individuals have antigen B.
• Blood group AB individuals have both antigens A and B.
• Blood group O individuals lack antigens on the surface of their RBCs.

Recent studies shows that antigen is galactosamine and B is galactose Antibodies A, B, AB and None and are naturally present in the serum of individuals having A,B,AB, and O blood group respectively. The agglutination or coagulation of RBCs leads to clotting of blood due to interaction between antigen antibody. The blood group B cannot be transferred to an individual having blood group A because the recipient has antibody against antigen B which is present on the RBCs of blood group B. Similarly the reverse transfusion is not possible. The blood group AB does not have antibody A and B. Hence individuals with AB blood group can accept all types of blood, viz., A, B, AB and O. Such individuals are known as universal acceptors or recipients. The O blood group does not have any antigen and has antibody against antigen A and B, It cannot accept blood group other than O. Individuals with blood group O are known as universal donors, because transfusion of blood group O is possible with all the four blood types. The consideration of Rh (rhesus) type is important in blood transfusion. Each blood group has generally two types of Rh group, viz positive and negative. The same type of Rh is compatible for blood transfusion Opposite type lead to reaction resulting in death of the recipient. These are few examples of multiple alleles Now it is believed that multiple alleles are present almost for all genes. 

Multiple alleles in plants 

The classical example of multiple alleles in plants is ‘self incompatability alleles’ which prevents self fertilization. 

Multiple alleles in Maize 

Multiple allelic series affecting seed color is seen in Maize.

Multiple allele in cotton

For information:

• About 30% of the genes in humans are di-allelic, that is they exist in two forms.
• About 70% are mono-allelic, they only exist in one form and they show no variation. 
• A very few are poly-allelic having more than two forms.

Pleiotropism

In general, a single gene typically influences a specific characteristic. However, there are genes known as pleiotropic genes that can impact more than one trait, and this phenomenon is termed pleiotropy. An example of a pleiotropic gene in humans is the recessive gene s, which leads to sickle cell anemia in individuals with ss homozygotes. When a gene causes changes in two or more unrelated characteristics, it is considered pleiotropic. For instance, in cotton, the Punjab hairy lintless gene lic not only produces seeds without lint but also results in incomplete lancinations of the leaf, reduced boll size, and fertility issues. It's important to note that if a gene produces a specific effect, such as red pigment in various organs of a plant, it is not considered pleiotropic because it has only one general effect—the production of pigment. Genes related to traits like wings, vestigial features, or fecundity may be termed accordingly, and some recessive genes in humans causing significant and often detrimental effects are known as syndromes.

Penetrance

The majority of genes result in consistent phenotypes across all individuals with the appropriate genotype. For instance, in peas, the w gene, which influences seed shape, produces uniformly wrinkled seeds when in the homozygous state (ww), while seeds with either WW or Ww genotype exhibit consistently round shapes. This ability of a gene to generate uniform phenotypes in individuals with the correct genotype is referred to as complete expressivity. In contrast, some genes exhibit incomplete expressivity, leading to variable phenotypes among individuals with the relevant genotype.

Expressivity

In general, genes typically manifest their effects in all individuals with the correct genotype, a phenomenon referred to as complete penetrance. However, there are genes that fail to produce the expected phenotype in all individuals carrying them in the appropriate genotype, leading to a condition known as incomplete penetrance. The percentage of individuals in which a gene is capable of expressing itself when present in the correct genotype is a measure of its penetrance. For instance, the gene responsible for chlorophyll deficiency in lima beans exhibits a penetrance of 10%. It's worth noting that most genes displaying incomplete penetrance also exhibit incomplete expressivity. Essentially, incomplete penetrance reflects incomplete expressivity, where some individuals express the gene to such a minimal extent that the trait is not detectable. 

Isoalleles

These alleles, which are similar but on testing it proves to be a different one. Blood group A person have three slightly different types such as IA1, IA2, IA3 which are similar but found to be different after testing. 

Pseudoalleles 

The genes that are so closely linked can be separatable only by rare crossing over. Such genes are called pseudoalleles.