What Is the Relationship Between a Phenotype and an Allele?

The physical appearance of a trait, such as hair color, eye color, or height, is called a phenotype. The phenotype is determined by the genotype, or set of genes inherited for that trait. Two copies of every gene are inherited by an offspring—one from each parent. Each of these copies is called an allele. The genotype for a trait is the combination of the two alleles inherited for that trait.

The relationship between phenotype and allele depends on the presence of dominant or recessive traits. If a particular trait is dominant, only one copy (or allele) of the gene is required for that trait to appear in the body. If a trait is recessive, then two alleles are required for the trait to appear. Note that this is a simplified explanation; many traits are determined by multiple genes that work together.

Let's look at the example of eye color. The allele for brown eyes is dominant; we will represent this with a capital "B." The allele for blue eye is recessive; we will represent this with a lowercase "b." Since having brown eyes ("B") is a dominant trait, only one copy of this allele is necessary for brown eyes to appear. Therefore, a genotype of either "BB" or "Bb" will result in a phenotype of brown eyes.

Since having blue eyes "b" is a recessive trait, it can easily be overshadowed by a dominant allele. In order for the phenotype of blue eyes to appear, a genotype of "bb" is required.

Let's look at the example of eye color. The allele for brown eyes is dominant--we will represent this with a capital "B". The allele for blue eye is recessive--we will represent this with a lower case "b". Since brown eyes ("B") is dominant, only one copy of this allele is necessary for brown eyes to appear. Therefore, a genotype of either "BB" or "Bb" will result in a phenotype of brown eyes.

Since blue eyes "b" is recessive, it can easily be overshadowed by a dominant allele. In order for the phenotype of blue eyes to appear, a genotype of "bb" is required.

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For some traits, the inherited alleles can show co-dominance or incomplete dominance. This means that the phenotype displays some of each allele. For example, a black-and-white-spotted cow is exhibiting a phenotype of co-dominance. The allele for white fur and the allele for black fur are expressed separately. A pink snapdragon flower exhibits a phenotype of incomplete dominance, in which the alleles for red and white petals are equally blended together.