Simple Dominance: Definition & Concept

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  • 0:00 What Is Simple Dominance?
  • 1:15 An Example
  • 2:05 Simple Dominance In Humans
  • 3:35 Dominant Does Not Mean Common
  • 4:25 Lesson Summary
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Lesson Transcript
Instructor: Joanne Abramson

Joanne has taught middle school and high school science for more than ten years and has a master's degree in education.

Discover the meaning of simple dominance and learn how different variations of a gene influence the traits we see. Find out whether simple dominance is expressed in humans.

What Is Simple Dominance?

Let's start with an experiment. Ask your friends and family the following questions:

Do you have hair on your middle finger?
When you clasp your hands, is your left thumb on top?
Can you roll the sides of your tongue upward?

You'll probably find that most of the people you survey answer 'Yes!' to these questions. That's because each of these traits is dominant.

Humans have diploid cells, meaning they contain two sets of chromosomes. We get one chromosome from our mother and one from our father. Simple dominance occurs when an inherited trait is coded for by a single gene and that gene has two versions, or alleles: the dominant version and the recessive version.

The dominant allele of the gene hides the presence of the recessive allele. For example, if you receive the dominant 'tongue-rolling' allele from your father and the recessive 'non-tongue-rolling' allele from your mother, you're still able to roll your tongue. This is because the tongue-rolling allele will hide the presence of the non-tongue-rolling allele. The only way that you would not be able to roll your tongue is if you inherited recessive alleles from both of your parents.

An Example

Phew. That was a lot of information. Let's use some diagrams to make the process a little bit clearer.

When a man and woman produce gametes, or sperm and egg cells, each gamete receives only one allele. In genetics, we refer to this as segregation.

Here, both the mother and father show the dominant trait of tongue rolling. The dominant allele, represented by a capital 'T', hides the presence of the recessive allele, represented by the lowercase 't'.

It's likely that their children will show the dominant trait, too. In fact, there's only a 1-in-4 chance that any of their children will exhibit the recessive, non-tongue-rolling trait. For this to happen, a recessive female gamete must be fertilized by a recessive male gamete.

Simple Dominance in Humans

Humans are complex organisms, so very few aspects of our makeup can be considered 'simple.' As such, especially in an area as messy as genetics, real life doesn't always operate as you would initially predict. In fact, the majority of traits in humans are not expressed through simple dominance. Most geneticists now argue that even the examples discussed in the introduction don't show simple dominance. Rather, they're influenced by a number of factors, including other genes and the environment.

For example, some people can teach themselves to roll their tongues as they get older. The fact that this trait can be learned suggests that there is more at play than the dominance of a single allele.

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