Gene vs. Allele: What’s the Difference?

Close-ups of 9 eyes of various colors
Eye color is determined by multiple genes.

Anthony Lee / Getty Images

A gene is a portion of DNA that determines a trait. A trait is a characteristic, or a feature, passed from one generation to another, like height or eye color.

Genes come in multiple forms, or versions. Each of these forms is called an allele. For example, the gene responsible for the hair color trait has many alleles: an allele for brown hair, an allele for blonde hair, an allele for red hair, and so on.

 GeneAllele
DefinitionA gene is a portion of DNA that determines a certain trait.An allele is a specific form of a gene.
FunctionGenes are responsible for the expression of traits.Alleles are responsible for the variations in which a given trait can be expressed.
PairingGenes do not occur in pairs.Alleles occur in pairs.
ExamplesEye color, hair color, hairline shapeBlue eyes, blonde hair, V-shaped hairline

Function

Genes govern the traits of an organism. They do so by acting as instructions to make proteins. Proteins are the diverse molecules that play many critical roles in our bodies, such as producing hormones and creating antibodies. 

Humans have two copies (or alleles) of each gene, one inherited from each parent. Alleles play a significant role in shaping each human’s individual features. Alleles are versions of the same gene with slight variations in their sequence of DNA bases. These small differences among alleles of the same gene contribute to each person’s unique characteristics.

Heredity

Heredity is how traits are passed to offspring. Genes determine your traits, like how tall you are, what color your eyes are, and what color your hair is. But a single trait is usually determined by a number of genes, rather than just one. For example, height alone is determined by over 400 genes.

Humans and other multicellular organisms have two alleles at the same site of a chromosome. Chromosomes are very long strands of DNA wrapped around special proteins called histones. Humans have 46 chromosomes; each parent passes on 23 of those chromosomes. Accordingly, the expression of any given trait would depend on two sources of information. These two sources are a paternal allele and a maternal allele.  

Genotypes and Phenotypes

A genotype is all the genes passed onto an individual by their parents. But not all of the genes you carry end up being translated into visible traits. The set of physical characteristics an individual has is called a phenotype. An individual’s phenotype is made up exclusively of expressed genes.

For example, take an individual who has one allele for blonde hair and one allele for brown hair. Based on this information, we know that their genotype includes blonde hair and brown hair. If we observe that the individual has blonde hair – in other words, blonde hair is the expressed trait – then we know that their phenotype includes includes blonde hair, but not brown hair.

Dominant and Recessive Traits

Genotypes can be either homozygous or heterozygous. When the two inherited alleles for a given gene are identical, this specific gene is called homozygous.

Alternatively, when the two genes are different, the gene is said to be heterozygous.

Dominant traits require the presence of only one allele in order for the given trait be expressed. Recessive traits can only be expressed if the genotype is homozygous. For example, a V-shaped hairline is a dominant trait, while a straight hairline is recessive. In order to have a straight hairline, both hairline alleles need to be straight hairlines. However, in order to have a V-shaped hairline, only one of the two hairline alleles need be V-shaped.