Test Crosses and Determining Genotype in Mendelian Genetics

Mendelian Genetics

Test Crosses

A test cross is a genetic technique used to determine the genotype of an individual expressing a dominant trait. Since dominant phenotypes can result from either homozygous dominant or heterozygous genotypes, a test cross helps reveal the actual genetic makeup.

• Phenotype: The observable trait (e.g., yellow peas).

• Genotype: The genetic constitution (e.g., YY or Yy).

• Test Cross: Crossing the individual with a homozygous recessive (yy) organism.

Purpose: To determine if the individual with the dominant phenotype is homozygous dominant or heterozygous.

Steps to Performing a Test Cross

1. Cross the mystery parent (unknown genotype) with a homozygous recessive parent (yy).

2. Observe the phenotypes of the offspring (progeny).

3. Analyze the results:

   • If all offspring show the dominant phenotype, the unknown parent is likely homozygous dominant (YY).

   • If offspring show a 1:1 ratio of dominant to recessive phenotypes, the unknown parent is heterozygous (Yy).

Example: Test Cross to Determine Genotype of a Yellow Pea

Parent Genotypes	Possible Offspring	Phenotype Ratio
YY (mystery) × yy (known)	All Yy	100% Yellow Peas
Yy (mystery) × yy (known)	Yy and yy	50% Yellow Peas, 50% Green Peas

Practice Questions

1. 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

   Correct answer: a) yy

2. 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

   Correct answer: a) TT

3. 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

   Correct answer: b) Yy

Key Terms and Concepts

• Homozygous dominant: Both alleles are dominant (e.g., YY).

• Heterozygous: One dominant and one recessive allele (e.g., Yy).

• Homozygous recessive: Both alleles are recessive (e.g., yy).

• Phenotypic ratio: The ratio of different observable traits among the offspring.

Summary Table: Test Cross Outcomes

Unknown Parent Genotype	Test Cross with	Offspring Genotypes	Offspring Phenotypes
YY	yy	All Yy	All dominant (yellow)
Yy	yy	Yy, yy	50% dominant (yellow), 50% recessive (green)

Relevant Equation

For a monohybrid test cross:

$\text{Phenotypic Ratio} = 1:1$ (if the unknown is heterozygous) $\text{Phenotypic Ratio} = 1:0$ (if the unknown is homozygous dominant)

Additional info: Test crosses are fundamental in classical genetics and are used to confirm Mendel's laws of inheritance. They are also applied in plant and animal breeding to ensure desired traits are passed on to offspring.