Dominant Trait: Definition & Example

Lesson Transcript
Instructor: Angela Lynn Swafford

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

A dominant trait is seen when two different alleles for a gene are present but one allele overpowers or masks the existence of the other. Study the definition of dominant trait, dominant traits and heredity, and examples of dominant traits. Updated: 09/21/2021

Definition of Dominant Trait

Before we can define exactly what a dominant trait is, we must first define and discuss many genetics terms. A gene, or unit of heredity, can have many different versions called alleles.

There are often two kinds of alleles:

  1. Dominant allele: masks the effects of another allele; designated by an uppercase letter
  2. Recessive allele: effects are hidden by a dominant allele; designated by a lowercase letter

Normally, individuals have two alleles for one gene: one from their mom and one from their dad. 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, and the physical appearance associated with a genotype is called the phenotype.

Let's use an example in humans to illustrate all these new terms we've learned. Humans have at least two earlobe phenotypes: detached and attached. The dominant allele (E) for determining earlobe type specifies detached earlobes. The recessive allele (e) codes for attached earlobes.

Because individuals must have two alleles for a gene, there are three possible genotypes:

  1. EE is homozygous dominant (two dominant alleles)
  2. Ee is heterozygous (one dominant and one recessive allele)
  3. ee is homozygous recessive (two recessive alleles)

Remember that there are only two possible phenotypes and that dominant alleles hide recessive allele effects. This means that both EE and Ee are seen as detached earlobes, while only ee is seen as attached earlobes.

A recessive trait is the phenotype that is not seen in the heterozygous genotype, and attached earlobes are a recessive trait. This brings us back to our main topic, which we can now define. A dominant trait is the phenotype that is seen when the heterozygous genotype is present. So, detached earlobes are a dominant trait.

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  • 0:00 Definition of Dominant Trait
  • 2:27 Dominant Traits and Heredity
  • 3:26 Examples of Dominant Traits
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Dominant Traits and Heredity

Dominant and recessive traits are heritable, meaning they can be passed on to offspring. Since there are two genotypes that show the dominant trait (homozygous dominant and heterozygous), it is sometimes possible for two individuals with dominant traits to have children with recessive traits.

When two parents are heterozygous for earlobe shape (Ee), there are three possible combinations of alleles they can give to their children (EE, Ee, and ee). So, if a child has attached earlobes (recessive trait) and both parents have detached earlobes (dominant trait), you know that those parents are heterozygous for the earlobe trait. This is the only way for children to get the recessive trait from parents with the dominant trait. On the other hand, it is never possible for children to have dominant traits if both parents have recessive traits.

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