Genetics: Dihybrid Crosses and Mendel's Principles

  • Punnett Squares

    • Used to determine the possible genetic combinations from two parents.
    • For a dihybrid cross (2 traits), a 4x4 Punnett square yields 16 cells, many representing the same trait.

  • Dihybrid Cross Example

    • Traits considered: Yellow vs. Green, Wrinkled vs. Smooth.
    • Four gametes can be formed from each parent, leading to combinations:
    • For example, from a pure dominant (YYWW) and pure recessive (yyww) parent, you would fill out the square based on the gametes.
    • Results in a phenotypic ratio of:
    • 9 Yellow Smooth : 3 Yellow Wrinkled : 3 Green Smooth : 1 Green Wrinkled.
    • This ratio can be found in related diagrams and textbooks.

  • Law of Independent Assortment

    • Genes assort independently, meaning the inheritance of one trait does not influence another.
    • Note that in reality, genes on the same chromosome may not assort independently but this is valid in pure Mendelian genetics.
    • This independent assortment occurs during Meiosis (Metaphase I and Anaphase I).

  • Pedigree Charts

    • Used to trace genetic traits through generations.
    • Squares represent males, circles represent females.
    • Connecting lines indicate mating events, and lines descending from mating pairs indicate offspring.
    • Will examine in future lessons for disease tracing in human genetics.

  • Mendel's Contributions

    • Gregor Mendel tested around 28,000 plants to establish genetic principles.
    • Developed the laws of Segregation and Independent Assortment, summarizing dominant and recessive traits.
    • His work was rediscovered in 1900 by scientists Hugo DeVries and Karl Correns.

  • Summary of Key Genetic Principles

    • Each trait is governed by genes, which exist in alleles (variants).
    • Traits for different characteristics are inherited independently.
    • Each individual has two alleles for each gene (one from each parent), which can be the same (homozygous) or different (heterozygous).
    • Genes typically remain unaltered through generations, while alleles can be dominant or recessive.
    • The structure of DNA carries genes, discovered by Watson and Crick in 1953, utilizing Rosalind Franklin's earlier work.