Lynn has a BS and MS in biology and has taught many college biology courses.
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:
- Dominant allele: masks the effects of another allele; designated by an uppercase letter
- 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:
- EE is homozygous dominant (two dominant alleles)
- Ee is heterozygous (one dominant and one recessive allele)
- 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.
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.
Examples of Dominant Traits
While there are two genotypes that express a dominant trait and only one that expresses a recessive trait, this does not necessarily mean that dominant traits are more common than recessive traits. For example, you probably have five fingers on each of your hands. Most people do, but having more than five fingers is actually the dominant trait.
The following lists some common and uncommon traits in humans that appear to follow the rules for being dominant traits:
- Detached or free earlobes
- Capable of tongue rolling
- Polydactyly: having more than five fingers
- Type A and B Blood
- Straight thumb (unable to bend thumb backwards)
- Brown eyes
- Achondroplasia: a form of dwarfism
- Huntington disease: a progressive brain disorder
- Hypercholesterolemia: having very high levels of cholesterol in the blood
Here is a similar list of traits for animals:
- Cattle: black coat color, absence of horns
- Chickens: white skin, black feathers
- Horses: smooth hair, black coat
- Pigs: erect ears, black hair
- Sheep: white wool, brown eyes
- Cats: short hair, black hair
Finally, here are traits in pea plants:
- Purple flowers
- Yellow seeds
- Smooth seeds
- Green pea pod
- Tall stem
Dominant traits are those that are expressed or seen when a heterozygous genotype is present. Heterozygous just means having two different alleles for a gene. One is usually dominant while one is recessive. Remember, recessive alleles are masked by dominant alleles. So, if a heterozygous genotype is present, then only the dominant trait will be seen. Dominant traits are also expressed in homozygous dominant individuals. One more thing to remember is that just because a trait is dominant, it does not have to be the common trait.
As you come to the end of the video, you should try to:
- Define what a dominant trait is
- List the two different kinds of alleles
- Discuss dominant traits and how heredity impacts them
- Cite examples of dominant traits
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