Test Crosses - Online Tutor, Practice Problems & Exam Prep

Test crosses are essential for determining the genotype of an organism with a dominant phenotype, such as a yellow pea plant. By crossing this unknown genotype with a homozygous recessive parent, scientists can analyze the offspring's phenotypes. If all offspring display the dominant trait, the mystery parent is homozygous dominant. Conversely, if there is a mix of phenotypes, the mystery parent is heterozygous. This method is crucial for understanding genetic inheritance and the underlying principles of alleles and phenotypes.

Test Crosses

In this video, we're going to begin our lesson on test crosses. And so what you may have noticed from our previous lesson videos is that phenotypes do not always reveal the genotype. For example, we could have a yellow pea, but we don't necessarily know just from looking at that yellow pea if that yellow pea has a homozygous dominant genotype with 2 dominant alleles, 2 capital Y's, or if that yellow pea has a heterozygous genotype with only 1 dominant allele and one recessive allele. Once again, just because we know the phenotype is yellow does not mean we know what the genotype is. It could either be homozygous dominant or heterozygous. And so the question is, how do scientists determine what the genotype is in a scenario like this one?

Well, in some cases, scientists can use what are known as test crosses to test or identify the genotype of an unknown mystery parent with an unknown genotype. The way that test crosses work is that the mystery parent's genotype can be identified by cross fertilizing, cross fertilization with a known homozygous recessive genotype. If we take a look at our image down below, notice on the left we're showing you a mystery parent who you can see we have a detective here trying to figure out what is the genotype of this mystery parent. We have a yellow pea plant, or yellow peas, but we want to determine what is their genotype. Is it homozygous dominant or is it heterozygous? And, again, we don't know which one of these it would be, so this is why this is the mystery parent. And, we can take this mystery parent and cross this mystery parent with a known parent, a parent whose genotype we know is homozygous recessive, 2 lowercase ys, which would create these green peas here. And so you can see this light bulb here saying, I know because this parent has the known genotype.

When you cross the mystery parent with the known parent who is homozygous recessive, that will reveal the genotype of the mystery parent and we'll be able to talk about exactly how that will work and exactly how test crosses work in our next video. So I'll see you guys in that video.

Steps to Performing a Test Cross

In this video, we're going to talk about the steps to performing a test cross. Let's say that we have a yellow pea plant, but we're not sure about the genotype of this yellow pea plant. Perhaps this yellow pea plant has a homozygous dominant genotype or maybe it has a heterozygous genotype. How do we figure that out? Well, we need to perform a test cross. There are three steps when it comes to performing a test cross, and notice that step three also has parts 3a and 3b.

The very first step in performing a test cross is to cross the mystery parent with the unknown genotype with a homozygous recessive parent. The second step of performing a test cross is to analyze the offspring or, in other words, analyze the offspring phenotypes. The third and final step of the test cross is to make a conclusion about the mystery parent's genotype based on the results. If all the offspring have the dominant phenotype, then the mystery parent's genotype must be homozygous dominant (two capital Y's). However, if the offspring show mixed phenotypes, then the mystery parent's genotype is likely heterozygous, meaning it has one dominant allele and one lowercase or recessive allele.

Let's take a look at our example below to see how this test cross precisely works to determine the genotype of this mystery parent, this yellow pea. Notice that on both the left-hand side and the right-hand side, we're showing you this mystery parent. We know it is yellow, but we don't know its genotype. We know it has at least one dominant allele, but is it homozygous dominant or heterozygous? The first step is again to cross the mystery parent with a homozygous recessive parent. Analyzing the results of the offspring, if all the babies come out 100% with the dominant phenotype of yellow peas, then tracing this backwards to see the possible results of the offspring, we know they must have at least one dominant allele, one capital Y, and this indicates that the gamete must have had a capital Y here as well, meaning that the mystery parent is homozygous dominant.

If the offspring show a mix of phenotypes where 50% of the offspring display the dominant trait of yellow peas, but the other 50% show the recessive trait for green peas, then half of these offspring must be homozygous recessive, possessing two lowercase y's. They must have gotten this lowercase y from one of the father's gametes. This means that the mystery parent must have a heterozygous genotype. So, by analyzing the results, whether we see mixed offspring or 100% of the offspring showing the dominant trait, we can determine whether the mystery parent was homozygous dominant or if there was a mixture, that the mystery parent is heterozygous. That's how you determine the genotype of the mystery parent by performing a test cross.

This concludes our introduction to how to perform a test cross, and we'll be able to get some practice applying these concepts as we move forward in our course. I'll see you all in our next video.

To identify the genotype of a yellow-seeded pea plant as either homozygous dominant (YY) or heterozygous 

(Yy), you could do a test cross between your mystery plant and a plant with a genotype of _______. 

• A. YY
• B. yY
• C. Yy
• D. yy

A single gene test cross is conducted to determine the genotype of a pea plant that shows the dominant phenotype for height (T = tall, t = short). If all offspring of the cross show the dominant phenotype, then the genotype of the unknown parent is _______.

a) TT.

b) tT. 

c) Tt. 

d) tt.

• A. TT.
• B. tT.
• C. Tt.
• D. tt.

You want to determine the pea color genotype of a pea plant with yellow peas. You conduct a test cross with your mystery pea plant. The test cross results in 50% of the offspring possessing yellow peas and 50% of the offspring possessing green peas. What is the genotype for pea color of the mystery parent?

a) YY.

b) Yy.

c) yy.

d) None of the above.

• A. YY.
• B. Yy.
• C. yy.
• D. None of the above.

Here’s what students ask on this topic:

What is a test cross and why is it used in genetics?

A test cross is a genetic technique used to determine the genotype of an organism exhibiting a dominant phenotype. This is achieved by crossing the organism with a homozygous recessive individual. The purpose of a test cross is to reveal whether the organism with the dominant phenotype is homozygous dominant (having two dominant alleles) or heterozygous (having one dominant and one recessive allele). By analyzing the phenotypes of the offspring, scientists can infer the genotype of the mystery parent. If all offspring display the dominant trait, the parent is homozygous dominant. If there is a mix of dominant and recessive phenotypes, the parent is heterozygous.

How do you perform a test cross step by step?

Performing a test cross involves three main steps:

1. Cross the mystery parent (with an unknown genotype) with a homozygous recessive parent.
2. Analyze the phenotypes of the offspring produced from this cross.
3. Draw conclusions about the mystery parent's genotype based on the offspring's phenotypes. If all offspring show the dominant phenotype, the mystery parent is homozygous dominant. If there is a mix of phenotypes, the mystery parent is heterozygous.

This method helps determine whether the mystery parent carries two dominant alleles or one dominant and one recessive allele.

What are the possible outcomes of a test cross and what do they indicate?

There are two possible outcomes of a test cross:

1. If all offspring display the dominant phenotype, it indicates that the mystery parent is homozygous dominant (having two dominant alleles).
2. If the offspring show a mix of dominant and recessive phenotypes, it indicates that the mystery parent is heterozygous (having one dominant and one recessive allele).

These outcomes help scientists determine the genotype of an organism with a dominant phenotype by analyzing the traits expressed in the offspring.

Why is a homozygous recessive parent used in a test cross?

A homozygous recessive parent is used in a test cross because it provides a clear and predictable genetic background. Since the homozygous recessive parent carries two recessive alleles, any dominant phenotype observed in the offspring must come from the mystery parent. This allows scientists to determine whether the mystery parent is homozygous dominant or heterozygous. If all offspring show the dominant trait, the mystery parent is homozygous dominant. If there is a mix of phenotypes, the mystery parent is heterozygous. The use of a homozygous recessive parent ensures that the results are straightforward and interpretable.

How can test crosses help in understanding genetic inheritance?

Test crosses are crucial for understanding genetic inheritance as they allow scientists to determine the genotype of an organism with a dominant phenotype. By crossing the organism with a homozygous recessive parent and analyzing the offspring's phenotypes, scientists can infer whether the organism is homozygous dominant or heterozygous. This helps in understanding how alleles are passed from parents to offspring and how traits are inherited. Test crosses also provide insights into the principles of Mendelian genetics, such as dominance, segregation, and independent assortment, which are fundamental to the study of genetics.