Dihybrid Crosses — Definition & Examples

Crossing Two Traits at the Same Time

A dihybrid cross allows us to look at the pattern of inheritance of two different traits at the same time.

For example, say we are crossing two pea plants. The two traits we are looking at are the seed color and shape. The first seed is green and wrinkly, and the second is yellow and round.

Notice that both parent seeds are homozygous for the traits that we are studying. This means that all of their offspring will be heterozygous for those traits (they each have one dominant allele and one recessive allele). This makes it easy to tell which variation is the dominant trait: It's whichever one shows up as the phenotype of the offspring. The offspring are carriers of the recessive trait, but the recessive phenotype will not show up in their generation.

The dihybrid cross shows us all the possible offspring that can come from two given parents. The cross is based off of Mendel's Law of Independent Assortment which states that the inheritance of one trait does not affect the inheritance of another.

Crosses with multiple traits can also help us understand what type of inheritance pattern governs each trait. This is because every cross gives us a specific phenotypic ratio, as in how many of the possible offspring will have a specific trait.

What Is a Dihybrid Cross?

You’ve used Punnett squares to track how a trait is inherited in a monohybrid cross. But what if you want to track how two traits are inherited? A dihybrid cross tracks two genes at the same time.

A great way to remember this is by breaking down the word dihybrid:

The most famous dihybrid experiment is Mendel’s pea plant experiment. He tested two traits: seed shape and seed color.

First, Mendel crossed a pea plant that produced round, yellow peas with one that produced wrinkly green peas. The baby pea plants from these parents all had round, yellow peas. Mendel concluded that yellow and round are dominant traits over green and wrinkly.

Next, he crossed two of the baby pea plants that made round, yellow peas. Each pea plant in this couple had the genotype RrYy, making the plants identically heterozygous.

Most of the offspring of the RrYy pea parents had $\color{#29A603}{\text{round, yellow peas}}$. Equal numbers of baby pea plants had $\color{#29A603}{\text{round, green peas}}$ and $\color{#29A603}{\text{wrinkly yellow peas}}$. Only a few of the offspring had $\color{#29A603}{\text{wrinkly green peas}}$. In fact, the exact ratio of $\color{#29A603}{\text{round, yellow}}$ : $\color{#29A603}{\text{round, green}}$ : $\color{#29A603}{\text{wrinkly yellow}}$ : $\color{#29A603}{\text{wrinkly green}}$ that Mendel found was 9:3:3:1. Because of this ratio, he concluded that seed shape and seed color must sort independently.

What Makes Two Traits Independent?

Two traits are independent if they are inherited separately. In other words, the alleles for the traits don't physically affect one another. Your eye color doesn't affect your height.

How Does a Dihybrid Cross Work?

Let’s do our own dihybrid experiment. Imagine we have two cats, one with brown fur and green eyes, and one with spotted blond fur and blue eyes. Let’s pretend we know two things:

(1) The genotypes for our kitties.

Brown fur and green eyes is FFEE, while spotted blond fur and blue eyes is ffee.

(2) Fur color and eye color sort independently.

Our cats have kittens. This first generation of offspring is called the F1 generation. Their kittens all have the genotype FfEe. They are heterozygous for both fur color and eye color, so they all have brown fur and green eyes.

What happens if one of those kittens meets another identically heterozygous cat, and they have kittens? We can set up a Punnett square based on the gametes each cat parent can make.

What Does the Punnett Square Tell Us?

It’s important to remember exactly what information we can get from a Punnett square. It won’t tell us how many kittens our cute cat couple has or guarantee what those kittens will look like. Punnett squares give the probabilities of different offspring genotypes and phenotypes based on parent genotypes for whatever trait or traits you are looking at.

What If Traits Aren't Independent?

If fur color and eye color did not sort independently or both parents were not identically heterozygous, then the ratio will not be 9:3:3:1. When one trait affects another, genetics gets more complicated.

(Video) Dihybrid Genetic Cross

by RicochetScience

A dihybrid genetic cross is a cross involving two traits at the same time. It can be pretty confusing at first, so take a look at the video above before you try it yourself.