Module 6 Lesson 2 - 4

Test Cross

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In a test cross, an individual with the recessive phenotype is crossed with an individual with unknown genotype displaying the dominant phenotype. Phenotypically dominant individuals can be either homozygous dominant or heterozygous. No way is available to determine which genotype they have by looking at them. To help with this problem, geneticists conduct test crosses in which the unknown is crossed with a homozygous recessive individual (such as aa or bb). The results of the cross indicate if the unknown was heterozygous or homozygous.

Two outcomes are possible:

1. If all resultant offspring express the dominant trait, then the parent of unknown genotype is homozygous dominant.
   
2. If half the offspring express the recessive trait, then the parent of unknown genotype is heterozygous.
   

Read more about a test cross and how to perform one on page 591 of your textbook.

How to Perform a Test Cross

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You were given a yellow seeded pea plant. To determine the genotype of yellow seeded plants, you performed a test cross. After the test cross, you obtained 50% yellow and 50% green seeded pea plants. What is the genotype of the P1 yellow seeded pea plants?

1. Let Y represent the dominant allele for yellow seed. Let y represent the recessive allele for green seed.

   The phenotypically yellow seeded P1 plant can have YY or Yy genotype.
   
   

2. There are two possibilities (YY or Yy), and we will cross both with a homozygous recessive green pea plant (yy).


If unknown is YY:

P1 Cross = YY x yy


	Y	Y
y	Yy	Yy
y	Yy	Yy

F1 Generation Phenotype = 100% yellow seeded plants

If unknown is Yy:

P1 Cross = Yy x yy


	Y	y
y	Yy	yy
y	Yy	yy

F1 Generation Phenotype = 50% yellow seeded plants : 50% green seeded plants



3. Analyze the result: Because the F1 generation yielded 50% yellow and 50% green seeded plants, the genotype of the unknown yellow seeded plant is heterozygous dominant (Yy).

Note: A large sample size is required to determine the unknown parental genotype to ensure that the F1 generation phenotype ratio did not occur due to chance.

As you watch the following video on monohybrid crosses, look for answers to the questions below.

 

1. Explain Mendel's first law in your own words.
   
   
2. What does a Punnett square tell you in a cross?
   
   
3. Given two heterozygous parents for pea type (such as Rr), what phenotypic and genotypic ratios are expected in their offspring?
   
   
4. What does a test cross help to determine?