Recessive Trait: Definition & Example

Recessive Trait: Definition & Example
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  • 0:00 What Is a Recessive Trait?
  • 1:07 An Example
  • 2:47 Some Recessive Traits
  • 4:25 Lesson Summary
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Lesson Transcript
Instructor: Angela Lynn Swafford

Lynn has a BS and MS in biology and has taught many college biology courses.

Can you bend your thumb backwards like a hitch-hiker? Do you have small, attached earlobes? Are your eyes blue? If you answered yes to any one of these, then you have a recessive trait. Learn what recessive traits are in this lesson.

Definition

The unit of heredity of all organisms is called a gene, and it can have many different versions called alleles. Normally, individuals have two alleles for one gene. A dominant allele is usually designated by an uppercase letter and masks the effects of another allele, the recessive allele. Recessive alleles are normally represented by lowercase letters and is - as you can no doubt guess - the non-dominant allele.

If an individual has the same two alleles for a gene, this is called homozygous. If an individual has two different alleles for a gene, this is called heterozygous. The combination of alleles an individual inherits is called the genotype. What that individual actually looks like is called the phenotype. A recessive trait is the phenotype that is seen only when a homozygous recessive genotype for the trait of interest is present. This means that an individual must have two recessive alleles for the gene that determines this trait of interest.

Example

Let's look at an example. Humans have at least two types of earlobes: detached and attached. The dominant allele for determining earlobe type, which we will write as 'E,' specifies free or detached earlobes. The recessive allele, which we will call 'e,' is code for attached earlobes. Remember, individuals must have two alleles for a gene, so someone could be homozygous dominant (EE), homozygous recessive (ee), or heterozygous (Ee). In this example, the recessive trait is having attached earlobes and is only seen in someone with the genotype ee (homozygous recessive).

Humans have two earlobe phenotypes: detached and attached. Having attached earlobes is the recessive trait and is shown in blue in this figure. (Having detached earlobes is considered the dominant trait.)
Earlobe Genotypes and Phenotypes

Another way to define a recessive trait is one that is never expressed by the heterozygous genotype. For example, the heterozygous genotype for earlobe type (Ee) produces detached earlobes because the dominant allele (E) hides the effects of the recessive allele (e). The homozygous dominant genotype (EE) also produces detached earlobes, so detached earlobes are considered the dominant trait. While there are two genotypes that express a dominant trait (homozygous dominant and heterozygous), there is only one genotype that expresses a recessive trait (homozygous recessive); however, this does not necessarily mean that recessive traits are less common than dominant traits.

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