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Polymorphism: Definition, Types & Examples

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  • 0:05 What Is Polymorphism?
  • 0:34 Types of Polymorphism
  • 1:10 Examples of Polymorphism
  • 2:53 Lesson Summary
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Lesson Transcript
Instructor
Dominic Corsini
Expert Contributor
Jillian Conte

Dr.Conte has taught biology and forensic science courses. She holds a PhD in Cell & Molecular Biology, a MS in Forensic Science, and a BS in Biology.

In this lesson, we'll explore the concept of polymorphism. We'll learn the definition of polymorphism, examine the different types of polymorphism in nature, and see some real-world examples.

What is Polymorphism?

Polymorphism occurs when several different physical forms or types of individuals exist among the members of a species. Take the peppered moth, for example, a nocturnal insect often used by biology teachers in their lessons about evolutionary biology and natural selection. Peppered moths are an excellent example of polymorphism in nature. For instance, while two peppered moths belong to the exact same species, they may look completely different from one another.

Types of Polymorphism

There are several different types of polymorphism, and the two we'll discuss in this lesson can be further subdivided. However, we'll set aside the more complex issue of subdivision and instead focus on two basic types of polymorphism: sexual dimorphism and allelic polymorphism.

Sexual dimorphism occurs when physical variations are found between male and female members of the same species. Allelic polymorphism occurs when there are multiple alleles expressed within the population; alleles are different versions of a trait or physical characteristic.

Examples of Polymorphism

Sexual dimorphism is a very common form of polymorphism. It occurs in many sexually reproducing animals, such as humans, birds, deer, and insects. And while not all members of these groups may show physical variations between males and females, most do. For example, consider these pheasants:

Sexual Dimorphism in Pheasants

Notice the clean phenotypic variation between the male and female pheasant, with the male being both bigger and much more colorful. This is an example of sexual dimorphism. As we learned earlier, polymorphism expressed as sexual dimorphism is very common in nature. In fact, depending on your familiarity with the animal kingdom, it may be challenging to think of animals that don't exhibit sexual dimorphism, although they do exist.

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Additional Activities

Applications of Polymorphism in Forensic Science

Much of the genetic material does not vary from person to person. Less than 0.1% does vary between individuals, so we call these areas 'polymorphic'. Let us look at a real world application of detecting polymorphism in forensic science. DNA testing of evidence from crimes is considered the "gold standard" of testing. The scientists that test the evidence for DNA are called 'forensic geneticists'. They look at highly polymorphic regions of the DNA. These locations are called 'loci' for plural, or 'locus' for singular. The loci used by forensic geneticists are considered highly polymorphic because they have many different alleles expressed within the population. By looking at multiple loci, about 20, forensic geneticists can begin to tell two people apart.

Metaphor for Polymorphism

A good metaphor for a polymorphic locus is going for ice cream. If you go to a fast-food place, there is one flavor of ice cream available - vanilla. Think of the fast-food place as a locus in our DNA, and it has one allele, or flavor - vanilla. It does not have polymorphic ice cream. Now, let's go to the local ice cream stand. Here we find flavors like rocky road, chocolate peanut butter, cookies and cream, bake batter, strawberry, and vanilla. At the ice cream stand (another locus), we have many more ice cream flavors available, or in our metaphor, alleles. The ice cream stand is highly polymorphic.

Activity for Polymorphism in Forensics

Short tandem repeats or STRs are polymorphic regions in the genome targeted by forensic geneticists. The way alleles are assigned is based on the number of times a series of DNA bases repeats.

Let us assume an evidence sample has the following alleles at the named loci:

  • Locus 1: 9, 12
  • Locus 2: 32, 36
  • Locus 3: 10, 15

There are two suspects:

  • Suspect A
    • Locus 1: 12, 12
    • Locus 2: 32, 36
    • Locus 3: 14, 15
  • Suspect B
    • Locus 1: 9, 12
    • Locus 2: 32, 36
    • Locus 3: 10, 15

Match up which of the two suspects left DNA on the evidence sample by looking for the same alleles at the same loci.


Answer

Suspect B contributed to the evidence sample.

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