This lesson provides an overview of allelic variation, complete with real-world examples, definitions, and explanations. When you have finished reading the lesson and taking the accompanying quiz, you should be able to tell the difference between a gene and a trait, or explain why some alleles are dominant or recessive.

Genes and Traits

In the study of genetics, variety is everywhere! Let’s consider a classroom full of students. Are all the students exactly the same? Of course they aren’t. Students may be similar, but all have differences unique to themselves. This is because of the genes they inherited from their parents. Genes are segments of DNA that code for a particular trait.

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Classroom variation
Classroom variation

Traits are physical characteristics that we see in each other. They include characteristics such as eye color, hair color, height, weight, skin tone, etc. Now let’s focus on those traits for a minute. There are many different eye colors, hair colors, and other physical attributes. That’s because there is more than one version of a particular trait, such as blue eyes and brown eyes. Alleles are those different versions of a trait. So while everyone has the gene for eye color, people may possess different alleles (or versions) of that gene.

Alleles and Physical Appearance

Alleles come in all shapes and sizes. One of the more common ways to introduce people to genetics is with the concepts of dominant and recessive alleles. Dominant alleles show up in the physical appearance of organisms. Recessive alleles are hidden or masked by the dominant allele. They only show up where there isn’t a dominant allele present.

All people have two alleles for every gene (one from each parent). For example, take a look at human ear lobes. The allele for unattached ear lobes is dominant to the allele for attached ear lobes. Therefore, if someone has unattached ear lobes, they possess at least one dominant allele. If they have attached ear lobes, they possess two recessive alleles.

Unattached and Attached Ear Lobes
Unattached and Attached Ear Lobes

It’s important to remember that the concepts of dominant and recessive alleles are useful for understanding basic genetics. While humans have approximately 23,000 genes, most of them do not fall into the dominant or recessive categories. As discussed earlier, alleles are versions of a gene, and there are many ways these can be expressed.

A third kind of allele is the co-dominant allele. Co-dominance is when both versions of the allele present in the organism. Since genetics are more personable when we look at ourselves, let’s use human blood type as our example. Human blood group can generally be categorized using three letters: A, B, and O. These letters represent your alleles (versions) of the blood type trait. In this case, both the A and B alleles are dominant; O is recessive. Remember that everyone has two alleles, so, if a person were to receive an ‘A’ allele from mom and a ‘B’ allele from dad, that person would be blood type ‘AB.’ If that same person had instead received an ‘A’ from mom and an ‘O’ from dad, that person would be blood type ‘A’, since A is dominant to O.

For our next example, I’m going to change things up and use roses rather than people. Roses come in two common colors: red and white. Suppose you breed a red rose producing plant with a white rose producing plant, what color flowers will the offspring plant have? Red? White? Actually, you’d have neither. That’s because roses don’t fall into the dominant, recessive, or co-dominant categories. They use something called incomplete dominance. Incomplete dominant is when neither version of the allele presents in the organism; instead, the two versions blend together.

Incomplete Dominance in Roses
Incomplete dominance in roses

See how the middle rose in the above image is neither red nor white; it’s a blend between the two. That tells us the allele is incompletely dominant.

Lesson Summary

Variation exists among all forms of life. Variation occurs because our genes aren’t all the same. They have different versions called alleles. Those alleles can be dominant, which will present in the organism, recessive, which are hidden or masked, co-dominant, where both versions show up, or incompletely dominant, where the versions blend. An understanding of these concepts will provide you with a strong foundational understanding of basic genetics.