This lesson examines complete dominance, which is one of the ways the genes you get from your parents can show up.
It also covers the vocabulary needed to understand complete dominance as well as examples.
What Is Complete Dominance?
Why do you look the way you do? Why do you have curly hair, but your sister has straight hair? Questions like this have confused and intrigued scientists for hundreds of years. One such scientist, Gregor Mendel, asked these questions and found some answers.Mendel performed his experiments during the 1800s. While he was working with pea plants, he found that when he crossbred different kinds of pea plants, the offspring would only exhibit the traits, or characteristics of one parent.
With these experiments, he discovered complete dominance, which happens when one trait masks another trait.
Genes and Alleles
In order to understand complete dominance, we’ll need to learn a few other vocabulary words first.Genes are a section of DNA, or deoxyribonucleic acid, that code for something like eye color or flower color. Let’s say that you have the gene for blue eyes. That gene tells your body to make blue eyes, or codes for blue eyes. Or let’s say you have the gene for curly hair.
Your body makes curly hair, or codes for curly hair. Oftentimes there are several genes coding for the same thing but for simplicity sake, let’s just focus on one.Alleles are different forms of a gene.
For example, eyes come in many colors. You may have the allele for brown eyes, or the allele for blue eyes, or you may have some combination.
Dominant and Recessive Alleles
Just like Mendel did, let’s look at pea plants to see dominant and recessive alleles at work. A particular pea plant that Mendel worked with had two varieties: plants with violet flowers and plants with white flowers. He crossed these two plants and found that all of the offspring had violet flowers.
Even though the offspring had inherited the allele for white flowers, they only had violet flowers. The term dominant was coined to describe the violet flowers because they completely covered up, or dominated, the white flowers. The white flowers were referred to as recessive because they were completely covered up by the violet flowers.
Complete Dominance and Other Types of Dominance
The flowers on Mendel’s pea plant are an example of complete dominance, or when the dominant allele completely covers up the recessive allele. In addition to complete dominance, scientists have found incomplete dominance, where there is a blending, and codominance, where both alleles show up. An example of incomplete dominance is when a red carnation is crossed with a white carnation and the offspring are pink or a blending of the parents.An example of codominance is a roan cow where one parent is red and one parent is white.
The roan cow has white and red because both alleles are expressed.When you Google complete dominance, you may get a search that includes pictures of tongue rolling or a hairline with a widow’s peak. These two traits were once thought to be examples of complete dominance; however, this is no longer the case. Today, scientists believe that tongue-rolling can be learned (in some cases) and widow’s peaks do not follow the complete dominance rules of recessive and dominant.
Examples of Complete Dominance
So, other than Mendel’s violet and white flowers, what are some examples of complete dominance? Pea plants give us more examples, such as the shape of the pea: round or wrinkled.
The round pea is dominant, and the wrinkled pea is recessive. Another example is the color of the pea pod: green or yellow. The green pea pod is dominant over the yellow, recessive pea pod.