What are the differences between genes and alleles?

Gene

Genes are short DNA sequences that encode for or determine a specific trait in an individual. 

They occur as individual units and are present in all known organisms. 

Genes play a key role in determining an individual’s traits such as skin, hair, or eye color. A single gene determines a specific trait. 

A gene can have several different alleles. 

Allele

An allele is a variant of a specific gene. 

They always occur in pairs and can only be identified in multi-genome organisms. 

Alleles are responsible for the variations in which a given trait may be expressed such as blue, black or brown eye color or curly or straight hair.  Two or more alleles bring about variations to the trait. 

An individual contains a pair of alleles for a particular gene.