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What Is a Phenotype? - Definition & Example

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  • 0:01 Definition of a Phenotype
  • 0:27 Examples of Phenotypes
  • 1:40 How Genotypes Cause Phenotypes
  • 2:20 One Trait, Many Genes
  • 2:57 Nature vs. Nurture
  • 3:45 Lesson Summary
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Lesson Transcript
Instructor: Beth Skwarecki
Do you have blue eyes or brown eyes? Or something else, like hazel? That's your phenotype for eye color. Discover examples of phenotypes, and how they are (partly) determined by your genes.

Definition of a Phenotype

An individual's phenotype consists of the traits we can observe. These can include features of appearance, behavior, metabolism, or anything else we can detect.

On the other hand, an individual's genotype is what we call the genes that help to create that phenotype. There may be one gene at work, or more than one. And genes don't always tell the whole story; sometimes your environment also affects what phenotype you end up with.

Examples of Phenotypes

Traits related to appearance are sometimes the easiest to observe. When Gregor Mendel was doing his famous experiments with pea plants, he observed the plants' appearance: the peas might be green or yellow, smooth or wrinkly. The plants could also be regular height or dwarfed.

Humans have appearance phenotypes, too; for example, your height and your eye color are both phenotypes controlled, at least partly, by your genes.

Behavior can be a phenotype, too. Border collies were bred to herd sheep, so even if they have never seen a sheep in their life, they will display herding behaviors - like running around your house collecting all your pillows.

Most genes don't do anything as flashy as changing our eye color; instead, they make enzymes deep inside our cells. These enzymes do different little jobs that are important to keeping us alive, like running chemical reactions to help us digest our food or burn energy. These chemical reactions are called our metabolism.

One phenotype related to metabolism is lactose intolerance. If you have a gene that makes the enzyme lactase, you can easily digest the sugar (lactose) in milk. But if you are lactose intolerant, you don't have that enzyme, so you can't digest lactose and will feel sick when you drink milk.

How Genotypes Cause Phenotypes

If your DNA is a cookbook, each gene is a recipe. And each recipe is for something your body needs - maybe a pigment to make your eyes brown, or an enzyme to digest lactose.

The different alleles for a gene are slightly different versions of recipes. Sometimes an allele is a broken recipe - imagine if your chocolate cake recipe comes out great every time, but the version in my cookbook says to bake at 500 degrees for 24 hours. I'd burn mine, and I'd never serve chocolate cake at my house.

In the same way, you might have a perfectly functional gene for brown pigment in your eyes, but if mine is broken, I won't have brown eyes. Eyes with no pigment appear blue, so I would have the phenotype of blue eyes, and you would have the phenotype of brown eyes.

One Trait, Many Genes

We can observe a phenotype whether or not we understand the genetics behind it. In real life, there is a gene called HERC2 that works like I described, and most people with blue eyes have the broken allele of this gene. But that's not the end of the story, because that doesn't explain all of the different eye colors in the world. You may have friends with green or hazel eyes, or even blue with brown spots. HERC2 is one piece of the puzzle, but there are many more.

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