3-3

Dihybrid Crosses

  • Introduction to Dihybrid Crosses

    • Previous discussions focused on monohybrid crosses
    • Monohybrid crosses involved two plants differing in only one characteristic
    • Mendel used homozygous parents (both alleles are the same) for each trait in monohybrid crosses
    • Example:
    • Homozygous round seeds (dominant allele)
    • Homozygous wrinkled seeds (recessive allele)

  • Monohybrid Cross Results

    • F1 generation offspring were all round and heterozygous
    • F2 generation exhibited a 3:1 ratio of round to wrinkled seeds

Characteristics in Dihybrid Crosses

  • Introduction of Dihybrid Crosses

    • Focus shifts to crossing two traits:
    • Seed shape (round vs. wrinkled)
    • Seed color (yellow vs. green)
    • Main question: Do alleles encoding different traits assort independently?

  • Example Parental Generation

    • Selected traits:
    • Round and yellow (dominant)
    • Wrinkled and green (recessive)
    • Genotypes of Parents:
    • Round yellow: RRYY
    • Wrinkled green: rryy

  • Gametes Produced from Parents

    • Homozygous parents produce one type of gamete:
    • Round yellow gamete: RY
    • Wrinkled green gamete: ry

F1 Generation

  • F1 offspring genotype is heterozygous for both traits: RrYy
  • Gametes formed in F1 generation:
    • Possible combinations of alleles:
    • RY
    • Ry
    • rY
    • ry
    • Total of four types of gametes possible from F1 generation

Mendel's Principle of Independent Assortment

  • Definition:
    • Alleles on different pairs of chromosomes assort independently during meiosis.
  • Explanation via Meiotic Cycle
    • During anaphase I of meiosis: homologous chromosomes separate independently
    • Alleles can assort in various combinations: Example
    • If A and B are on different chromosomes:
    • Possible combinations include:
      • A with capital B
      • A with lowercase b
      • Lowercase a with capital B
      • Lowercase a with lowercase b
    • Illustrates diverse gametic combinations due to independent assortment

Meiosis Overview

  • Diagram representation of meiosis
    • G1 phase: Diploid cell with homologous chromosomes
    • S phase: DNA replication (results in two copies of each chromosome)
    • Anaphase I: Separation of homologous chromosomes leading to independent assortment

Generation of Gametes

  • Examples of possible gametes from F1 during anaphase II:
    • RY, Ry, rY, ry
    • Total of 8 gamete combinations considered from homologs
    • Results in 4 distinct gametes due to independent assortment of alleles

Punnett Square for Dihybrid Cross

  • Larger Punnett square than monohybrid cross
    • F1 generation is heterozygous for both traits (RrYy)
    • Produces 4 kinds of haploid gametes:
    • RY, Ry, rY, ry

F2 Generation and Phenotypic Ratios

  • Filling out a Punnett square for F2 generation to observe potential genotypes
    • Resulting phenotypes are impacted by dominance
    • Phenotypic ratio for offspring from the F2 generation: 9:3:3:1
    • 9 round yellow
    • 3 round green
    • 3 wrinkled yellow
    • 1 wrinkled green

Challenges with Multiple Traits

  • Complexity with more than two traits in a Punnett square
    • Larger squares become unwieldy
    • Alternative strategy involves applying probability rules

Utilizing Probability Rules

  • Breakdown multiple traits into monohybrid crosses
    • Analyze each trait individually
    • Apply previously discussed probability rules

Calculation of Ratios

  • Example of calculating probabilities individually for traits
    • Round and wrinkled (shape) and yellow and green (color)
  • Calculation summarization:
    • Probability of round is 3/4 (dominant trait)
    • Probability of wrinkled is 1/4 (recessive)
    • Probability of yellow is again 3/4
    • Probability of green is 1/4

Combining Individual Probabilities

  • Combine probabilities of traits:
    • Example of seed being round and yellow:
    • Round: 3/4, Yellow: 3/4
    • Combined probability: (3/4) * (3/4) = 9/16
    • Other combinations include:
    • Round and green: (3/4) * (1/4) = 3/16

Additive and Multiplicative Rules

  • Multiplicative Rule: Used for “and” situations
  • Additive Rule: Used for “or” situations
    • Example: Probability of being round or wrinkled
    • Combined probability: 3/4 + 1/4 = 1

Complex Scenarios

  • Considerational example with height, seed color, and texture
    • Dominant alleles indication and recessive phenotypes explained
    • Specific combination questioned: Probability of tall, green, and wrinkled
    • Calculation strategy mirrors earlier examples

Conclusion of Lecture

  • Recap of methods for determining probabilities for dihybrid crosses using probability rules instead of Punnett squares
  • Acknowledgement of complexities in genetic combinations and importance of individual trait analysis.