What is a test cross? Definition and purpose in genetics.

📚 What is a Test Cross?

A test cross is a fundamental procedure in genetics used to determine the genotype of an individual displaying a dominant phenotype. Since a dominant phenotype can arise from either a homozygous dominant (AA) or heterozygous (Aa) genotype, a test cross helps distinguish between these possibilities.

• 🌱 Definition: A test cross involves crossing an individual with an unknown genotype (but expressing the dominant phenotype) with a homozygous recessive individual (aa).
• 🔬 Purpose: To identify whether the individual with the dominant phenotype is homozygous dominant or heterozygous.
• 🧬 How it Works: By observing the phenotypes of the offspring, we can infer the genotype of the parent in question.
• 📈 Interpreting Results:
  • If all offspring display the dominant phenotype, the parent is likely homozygous dominant (AA).
  • If approximately half of the offspring display the dominant phenotype and half display the recessive phenotype, the parent is heterozygous (Aa).
• 🧮 Example: Consider a plant with purple flowers (dominant trait). If crossed with a white-flowered plant (pp) and all offspring are purple, the purple plant's genotype is likely PP. If half are purple and half are white, the purple plant's genotype is Pp.

Quick Study Guide

• 🧪 Test cross: Cross of an unknown genotype with a homozygous recessive individual.
• 🎯 Purpose: Determine if the dominant phenotype is homozygous dominant (AA) or heterozygous (Aa).
• 📉 If all offspring show dominant trait: Likely homozygous dominant (AA).
• ⚖️ If ~50% show dominant, ~50% show recessive: Heterozygous (Aa).
• 📝 Genotype ratios in offspring reveal the unknown parent's genotype.

Practice Quiz

1. What is the primary purpose of a test cross?
   1. A) To determine the phenotype of an individual
   2. B) To determine the genotype of an individual with a dominant phenotype
   3. C) To create new genetic variations
   4. D) To identify recessive traits
2. In a test cross, an individual with an unknown genotype is crossed with which type of individual?
   1. A) Homozygous dominant
   2. B) Heterozygous
   3. C) Homozygous recessive
   4. D) Any genotype
3. If a test cross results in all offspring displaying the dominant phenotype, what is the likely genotype of the parent with the unknown genotype?
   1. A) Homozygous recessive
   2. B) Heterozygous
   3. C) Homozygous dominant
   4. D) Cannot be determined
4. If a test cross results in approximately half of the offspring displaying the dominant phenotype and half displaying the recessive phenotype, what is the genotype of the parent with the unknown genotype?
   1. A) Homozygous dominant
   2. B) Homozygous recessive
   3. C) Heterozygous
   4. D) Triploid
5. A plant with purple flowers (dominant) is test crossed with a white-flowered plant. All offspring have purple flowers. What is the likely genotype of the purple-flowered parent?
   1. A) Pp
   2. B) pp
   3. C) PP
   4. D) Cannot be determined
6. In a certain species, tall (T) is dominant to short (t). A tall plant is test crossed and produces 50% tall and 50% short offspring. What is the genotype of the tall parent?
   1. A) TT
   2. B) Tt
   3. C) tt
   4. D) Cannot be determined
7. Why is it important to use a homozygous recessive individual in a test cross?
   1. A) To ensure the dominant trait is expressed
   2. B) To mask the effects of the dominant allele in the unknown genotype
   3. C) To reveal the genotype of the unknown parent through the offspring's phenotypes
   4. D) To promote genetic mutations