Alleles, the abbreviated form of the term "allelomorphs" (meaning one form or the other) indicates alternative form of the same gene. For instance, in the above example "d" and "D" are two allelomorphs of the gene for plant height. In pure tall or pure dwarf plants, same allele is duplicated (DD or dd), while in hybrid tall both the alleles will be present (Dd). An individual, having only one allele or in other words two identical alleles, is known as homozygous (DD or dd). Similarly, an individual, having two different alleles will be called heterozygous or hybrid (Dd).
You should know that each individual receives genes from its ancestors and the set of alleles that are inherited decides the individual's genotype. The alleles come in pairs and each allele comes from one parent. If there exists two different alleles of a single gene then the organism is heterozygous.
If the alleles combine their characteristics in the phenotype then incomplete dominance occurs and if both alleles participate, manifesting both traits in the same time , then co-dominance occurs.
It is important to mention that if an individual receives dominant and recessive alleles then the dominant alleles manifest over the recessive alleles.
The main cause of genetic disorders is the inheritance of two recessive alleles over a single gene characteristic.
An allele, as you might know, is related to the concept of a gene. Hence, let us start by defining what a gene is. The gene is the molecular unit of heredity. It is the part of our DNA that dictates certain things about how we become - e.g. color of skin, or blood type.
Alleles, on the other hand, are alternative forms of a single gene. An allele, therefore, allows for a different result in terms of observable phenotypic traits. Let me give an example. As I've mentioned, the genes control our blood type. The blood type is actually determined by the presence (or absence) of certain antigens in the blood. There are three alleles for the gene that dictates this and we label/refer to them as `I^A` ,`I^B`, and `I^O` . These result to the blood types. Note that the rules of dominance then apply to determine the blood type. Alleles A and B are co-dominant over O. Hence, we have blood types A (from a combination of alleles A and A, or A and O), B (from B and B or B and O), O (from O and O), and AB (from A and B)
Allele is a form of a gene. For example, the gene for eye color may have more than five alleles. The five alleles may be black eye, brown eye, blue eye, green eye, and hazel eye color. In human, both of our parent each contributes one allele for a gene. The are dominant alleles and recessive alleles. Only the dominant allele is expressed unless both parents contribute recessive alleles to the child. For instant, black eye is a dominant trait, which will be expressed in the existence of a recessive allele for eye color. As a result, this person will have black eye even though he or she has a recessive allele for another eye color.
Alleles are simply different versions of the same gene.