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Intermediate Inheritance: Definition & Example

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  • 0:00 What Is Intermediate…
  • 0:42 Genetic Inheritance
  • 2:05 Predicting Inheritance
  • 2:34 Examples
  • 4:34 Lesson Summary
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Lesson Transcript
Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

In this lesson, we'll explain what intermediate inheritance, or incomplete dominance, is. We will also review the basics of genetic inheritance and Punnett squares before looking at examples of incomplete dominance in plants, animals and humans.

What Is Intermediate Inheritance?

When painting, it's often necessary to mix paint colors. You've probably learned the basics: mixing white with a color will lighten it, while mixing black with the same color will darken it. For example, making pink paint for flowers involves combining equal parts red and white paint.

Although the process is a little bit different, we inherit some traits from our parents in a similar way. The trait from dad mixes with the trait from mom and the child gets an intermediate appearance. This type of inheritance, or passing down of traits from parent to offspring, is called intermediate inheritance, also known as incomplete dominance. To fully understand how this process happens we need to look at how traits are inherited first.

Genetic Inheritance

All living things have DNA, or genetic blueprints, inside their cells. DNA is composed of genes, which are the individual instructions for a trait. For example, there is a gene that specifies eye color. However, there are different versions of this gene called alleles. The eye color is the trait and the alleles are the specific colors possible, like green, blue and brown. Since you get half your DNA from your dad and half from your mom, you get one allele for each trait from either of your parents. So using our eye color example, you might get an allele for green eyes from your mom and an allele for brown eyes from your dad.

Usually, like with eye color, one allele masks another, and you only see one trait or the other. For example, one parent may have brown eyes and the other may have blue eyes, but the allele for brown eyes masks the allele for blue eyes, and you'll likely only see brown eyes in the offspring. But with intermediate inheritance the traits blend together, creating an intermediate appearance.

The way the organism looks, or its appearance, is called a phenotype. The actual alleles the organism has is called its genotype, for genes. The alleles the organism has for each gene can be represented by letters, one from each parent. In intermediate inheritance two different letters are used for each allele.

Predicting Inheritance

When two organisms are mated, we can predict the offspring with a Punnett square, or a diagram that gives the probability of getting offspring with a particular phenotype. We put the letters representing the genotype of each parent on the outside of the box. Then we fill in the letters down and across. The genotype in each of the four boxes is the expected genotype for the offspring. Next, let's look at some examples of intermediate inheritance.

Examples

Four-o-clock flowers are a classic example of intermediate inheritance. These pretty flowers get their name because their petals open right around 4pm in the summer and remain closed during the day. The flowers come in white, red and pink. Pink is the intermediate phenotype, and a pink four-o-clock flower results from mating a red four-o-clock with a white four-o-clock.

We can represent the genotypes here with two capital Ws for a white flower and two capital Rs for a red flower. When they are mated, all of the offspring have the genotype RW. This genotype indicates the flowers will be pink, since red mixes with white in intermediate inheritance.

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