Phenotypic Ratio: Definition & Overview

Instructor: Dominic Corsini
This lesson addresses the concept of phenotypic ratio. It includes definitions, examples, and illustrations. A brief quiz accompanies the lesson and highlights its major points.

Feathered Friends

A friend of mine recently purchased some chickens. I'm not sure what type of chicken he bought, but I know his intention is to collect and sell their eggs. Hopefully this earns him a few dollars. I suppose his purchase means we live in a fairly rural community, which is perfectly acceptable. Sometimes rural communities are the best places to study biology. Yet, despite our location and my love of biology, I have no desire to own chickens or collect eggs. When it comes to farm animals I'm more about reading and less about doing. But I can see their usefulness in scientific studies. For example, take a look at the chickens in this picture.

Phenotypic Variation in Chickens
Phenotypic Variation in Chickens

Notice how all three chickens have different phenotypes. A phenotype represents one of the physical characteristics of an organism. In this example, each chicken displays a different phenotype for feather color.

Physical Appearances

Phenotype is a common term in genetic studies. It's often associated with Punnett squares. Punnett squares are diagrams used to assist people in determining the phenotypes of a genetic cross. For example, suppose you allow two black chickens to breed. A Punnett square could help predict the likelihood of producing offspring with a specific phenotype. Here is an example to guide you.

Chicken offspring phenotypes
Punnett square

For the purposes of this lesson, you needn't worry about why the offspring appear as they do. Instead, focus on the end result. In the example there are three black offspring and one white offspring produced. So in genetic terms you'd say these two black chickens have a 75% chance of producing black offspring, or a 25% chance of producing white offspring. That is the function of a Punnett square.

Predictions made by the example Punnett square (feather color) were given in percentages. However, they could also be presented as a ratio. A ratio is the comparison between two numbers. The ratio of black to white chicken offspring in our example cross is 3:1.

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