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16.07 F1, F2, and Test Crosses

Posted14/11/2024
Updated29/12/2024
ByBMF
Views15

Introduction to Monohybrid Crosses

  • A monohybrid cross involves the study of inheritance patterns for a single gene with two alleles. This type of cross helps predict the genotypic and phenotypic ratios of offspring based on the genetic makeup of the parents.

Example Scenario:

  • Trait Studied: Coat color in rabbits.
  • Alleles:
    • B (dominant allele for brown coat)
    • b (recessive allele for white coat)

Key Terms and Definitions

  1. Monohybrid Cross
    • Definition: A genetic cross between two organisms focusing on a single trait with two alleles.
    • Example: Crossing a homozygous brown rabbit (BB) with a homozygous white rabbit (bb).
  2. Allele
    • Definition: Alternative forms of a gene that determine distinct traits.
    • Types:
      • Dominant Allele (B): Expressed in the phenotype when present.
      • Recessive Allele (b): Expressed in the phenotype only when homozygous.
  3. Genotype
    • Definition: The genetic makeup of an organism concerning a particular trait.
    • Examples:
      • BB: Homozygous dominant
      • Bb: Heterozygous
      • bb: Homozygous recessive
  4. Phenotype
    • Definition: The observable physical or biochemical characteristics of an organism, determined by both genotype and environment.
    • Examples:
      • Brown Coat: Resulting from BB or Bb genotypes
      • White Coat: Resulting from bb genotype
  5. F1 Generation
    • Definition: The first generation of offspring resulting from a cross between two parental (P) generation organisms.
    • Example: Crossing BB (brown) with bb (white) results in all Bb (brown) offspring.
  6. F2 Generation
    • Definition: The second generation of offspring, produced by crossing individuals from the F1 generation.
    • Example: Crossing two Bb (brown) rabbits yields a mix of BB, Bb, and bb offspring.
  7. Homozygous
    • Definition: Having two identical alleles for a particular gene (e.g., BB or bb).
  8. Heterozygous
    • Definition: Having two different alleles for a particular gene (e.g., Bb).
  9. Test Cross
    • Definition: A cross between an individual with a dominant phenotype but unknown genotype and a homozygous recessive individual to determine the unknown genotype.
    • Purpose: To reveal whether the dominant phenotype individual is homozygous or heterozygous.

Genetic Diagrams and Examples

  • Visualizing genetic crosses helps in understanding the inheritance patterns. Below are detailed examples of monohybrid crosses, including Punnett squares to predict outcomes.

1. F1 Generation: Crossing Homozygous Parents

Parental Genotypes:

  • Parent 1 (P1): BB (homozygous dominant, brown coat)
  • Parent 2 (P2): bb (homozygous recessive, white coat)

Cross:

B B
—————-

b | Bb Bb
b | Bb Bb

F1 Generation Outcome:

  • Genotype: All offspring are Bb (heterozygous)
  • Phenotype: All offspring display the dominant brown coat

Summary:

  • Genotypic Ratio: 100% Bb
  • Phenotypic Ratio: 100% brown

2. F2 Generation: Crossing F1 Individuals

Parental Genotypes:

  • Parent 1 (F1): Bb (heterozygous brown)
  • Parent 2 (F1): Bb (heterozygous brown)

Cross:

B b
—————-

B | BB Bb
b | Bb bb

F2 Generation Outcome:

  • Genotypic Ratio:
    • 1 BB : 2 Bb : 1 bb
  • Phenotypic Ratio:
    • 3 brown : 1 white

Summary:

  • Genotypic Distribution:
    • BB: Homozygous dominant (brown)
    • Bb: Heterozygous (brown)
    • bb: Homozygous recessive (white)
  • Phenotypic Distribution:
    • Majority display the dominant trait (brown), with a minority displaying the recessive trait (white)

Test Cross: Determining Unknown Genotypes

  • A test cross is a powerful tool to determine the genotype of an individual exhibiting the dominant phenotype.

Purpose of a Test Cross

  • Objective: To ascertain whether an individual with a dominant phenotype is homozygous dominant (BB) or heterozygous (Bb).
  • Method: Cross the individual with a homozygous recessive (bb) individual and analyze the offspring.

Test Cross Examples

Example 1: Test Cross with a Heterozygous Brown Rabbit (Bb)

Parental Genotypes:

  • Unknown Parent: Bb (brown)
  • Known Parent: bb (white)

Cross:

b b
—————-

B | Bb Bb
b | bb bb

Offspring Outcome:

  • 50% Bb (brown)
  • 50% bb (white)

Interpretation:

  • Presence of white offspring indicates that the unknown parent is heterozygous (Bb).

Example 2: Test Cross with a Homozygous Brown Rabbit (BB)

Parental Genotypes:

  • Unknown Parent: BB (brown)
  • Known Parent: bb (white)

Cross:

b b
—————-

B | Bb Bb
B | Bb Bb

Offspring Outcome:

  • 100% Bb (brown)

Interpretation:

  • All offspring display the dominant phenotype, suggesting the unknown parent is homozygous dominant (BB). However, to confirm, larger sample sizes are recommended as rare mutations or sampling errors could occur.

Practice Questions

F1 Generation Prediction:

  • Task: Construct a Punnett square to show all offspring from a cross between a homozygous brown rabbit (BB) and a white rabbit (bb).
  • Expected Outcome: All offspring will be Bb (heterozygous brown).

F2 Generation Prediction:

  • Task: Use a Punnett square to predict the phenotypic ratios of offspring from two heterozygous brown rabbits (Bb x Bb).
  • Expected Outcome: 75% brown, 25% white (3:1 ratio).

Test Cross Interpretation:
a. Scenario: Crossing a brown rabbit with a white rabbit results in both brown and white offspring.

  • Question: What does this indicate about the genotype of the brown rabbit?Answer: The brown rabbit is heterozygous (Bb).
b. Scenario: Crossing a brown rabbit with a white rabbit results in all brown offspring.

  • Question: What does this suggest about the genotype of the brown rabbit?
  • Answer: The brown rabbit is likely homozygous dominant (BB), but further testing with a larger sample size is needed for confirmation.

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