Question

How do multiple alleles occur?

Answer 1

Variants of gene that exist in two or more forms occupying a specific locus on chromosome are called alleles. Generally, there are only two alleles for a gene in a diploid Organism. If a gene exists in more than two allelic forms, the condition is called multiple allelism. They all are found on the same gene locus on the same chromosome or its homologue. For designating various alleles, there are two conventions.
(1) Using capital letters for wild type dominant allele with or without superscript.
(2) Using small letters based on the most recessive allele with different superscripts over it indicating various traits of alleles. A sign of (+) is used as a superscript for the most dominant wild-type allele.

Complete answer:
New alleles are created by spontaneous mutation in both haploid and diploid organisms. The spontaneous mutation can arise in a variety of ways, but the effect is a different sequence of nucleic acid bases in the DNA.
The genetic code is read as a triplets of nucleic acid bases (or series of codons) that correspond to individual amino acids. Due to mutation, the sequence of amino acid changes, either in a simple or drastic way. Simple changes that only affect a few amino acids can produce multiple alleles in a population, all of which function in almost the same way just to a different degree.
Other mutations cause large changes in the protein created, and it will not function at all. Other mutations, by giving novel forms of protein, allow organisms to develop new pathways, structure and functions.
Most of the time, scientists classify alleles by the phenotypes created by them. However, a phenotype can be created by a large number of mutations. Humans have thousands of genes, over three billion base pairs. Each gene consists of many base pairs. Any mutated base pair can cause a new allele.
In a population, different phenotypes are produced by different combinations of multiple alleles. Proteins encoded by various alleles give rise to these phenotypes. Although each gene of multiple alleles encodes for the same type of Protein, the different alleles can cause high variability in the functioning of these Proteins.

Note:
Characteristics of multiple alleles:
(1) They are more than two forms of the same gene.
(2) All of them are mutants of the same wild allele.
(3) They express different alternative traits of the same character.
(4) Out of the various multiple alleles, one is dominant over all others while another is recessive to all the alleles. The remaining alleles show codominance, incomplete-dominance and dominance recessive.
(5) All the alleles show mendelian inheritance.
(6) The various multiple alleles of a gene occur on the same gene locus.
(7) A gamete or a chromosome carry only one allele.
(8) An individual possess only two alleles of a gene.