Module 6 Lesson 4 - 3

Test Cross (Two Traits)

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As you have studied, a test cross is performed when an individual has an unknown genotype displaying the dominant phenotype. Phenotypically dominant individuals can be either homozygous dominant or heterozygous for the trait. The genotype they have cannot be determined by looking at them. To help with this problem, geneticists conduct test crosses where the unknown is crossed with a homozygous recessive for both phenotypes (such as aabb or yyrr). The results of the cross indicate if the unknown was either heterozygous or homozygous.

How to Perform a Dihybrid Test Cross

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You are given a yellow, round seed pea plant. To determine the genotype of yellow, round seed plant, you performed a test cross from which you obtained 50% yellow, round and 50% yellow, wrinkled seed pea plants. What is the genotype of the P1 yellow, round seed pea plants?

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

   Let R represent the dominant allele for round seed. Let r represent the recessive allele for the wrinkled seed.
   



2. The four possibilities are YYRR or YyRR or YYRr or YyRr. Cross them with a homozygous recessive wrinkled, green pea plant (yyrr).




If the unknown is YYRR: P1 Cross = YYRR x yyrr   YR  yr  YyRr F1 Generation Phenotype = 100% yellow, round seed plants

If the unknown is YyRR: P1 Cross = YyRR x yyrr   YR  yR  yr  YyRr  yyRr F1 Generation Phenotype = 50% yellow, round seed plants 50% green, round seed plants

If the unknown is YYRr: P1 Cross = YYRr x yyrr   YR   Yr  yr  YyRr   Yyrr F1 Generation Phenotype = 50% yellow, round seed plants 50% yellow, wrinkled seed plants

If the unknown is YyRr: P1 Cross = YyRr x yyrr   YR   Yr yR  yr  yr  YyRr  Yyrr  yyRr  yyrr F1 Generation Phenotype = 25% yellow, round seed plants 25% yellow, wrinkled seed plants 25% green, round seed plants 25% green, wrinkled seed plants




3. Analyze the result: Because the F1 generation yielded in 50% yellow, round and 50% yellow, wrinkled seed pea plants, the genotype of the unknown yellow seeded plant is YYRr.

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Consider the following video: 

 Answer the following questions for your own understanding.

1. What was the letter assignment used here for widow's peak / straight hair line, and for normal thumb/hitch hiker's thumb?
   
   
2. What hypothesis (if/then statement) did the students use to test whether attachment of ear lobes was a trait controlled by a dominant or a recessive allele?
   
   
3. Did they find many people with all three traits recessive?
   
   
4. In your own words, state or explain Mendel's second law, the Law of Independent Assortment.
   


5. Does the movement of the alleles for plant height affect the movement of the alleles for flower colour?