Genetics

Practice Problems for Punnett Squares

  • If struggling, repetition is key to mastering Punnett squares.
  • Practice various crosses:
    • Recessive x recessive
    • Recessive x heterozygous
    • Heterozygous x heterozygous
    • Homozygous dominant x homozygous dominant
  • Start with one trait, then move to two or three traits.
  • Some may grasp it intuitively, others may struggle.
  • Due to the condensed summer class schedule, practice is essential.
  • Repetition leads to muscle memory in solving these problems.

Textbook Approach

  • The textbook presents various concepts in the chapter's end.
  • Many concepts are introduced at once.

Course Progression

  • After Mendelian genetics, the course shifts to molecular genetics.
  • If struggling with Mendelian genetics, it's primarily on the first or second exam.
  • The majority of the course covers completely different material.
  • Work through the material, understand conceptually, and perform simple math.

Mendel's Postulates

  • Unit factors exist in pairs (e.g., A vs. a).
  • Dominance vs. recessive:
    • Dominant allele: capital letter.
    • Recessive allele: lowercase letter.
    • Dominant allele dictates phenotype.
    • Recessive phenotype requires homozygous recessive genotype.
  • Factors segregate independently during gamete formation.

Punnett Squares

  • Reginald C. Punnett devised the Punnett square approach for visualization.
  • Cross between two heterozygotes:
    • Parental alleles placed on the grid's axes.
    • Possible alleles from each parent (e.g., big A, little a).
  • Bring down and across the alleles to create possible genotypes.
  • Four possible genotypes:
    • Homozygous recessive (aa)
    • Homozygous dominant (AA)
    • Heterozygous (Aa)
  • Genotype ratio: 1:2:1 (homozygous dominant: heterozygous: homozygous recessive).
  • Phenotype ratio: 3:1 (dominant trait: recessive trait).
  • Dominant allele leads to the dominant trait.
  • Recessive phenotype appears only in homozygous recessive individuals.

Mathematical Prediction

  • Mendel crossed F1 generations (heterozygotes) and observed a 3:1 ratio.
  • Punnett squares can be used for any combination of crosses.
  • Example: crossing a heterozygote with a homozygous recessive.
  • Homozygous recessive parent donates only recessive alleles.
  • Heterozygous parent donates dominant or recessive alleles.
  • Genotype ratio: 1:1 (heterozygous: homozygous recessive).
  • Phenotype ratio matches genotype ratio (1:1).

Monohybrid Cross Example

  • Darwin's monohybrid cross: tall plants x dwarf plants.
  • Tall is dominant (D), dwarf is recessive(d).
  • True breeding plants are homozygous (DD or dd).
  • F1 generation: all heterozygotes (Dd) with dominant phenotype (tall).
  • Crossing F1 (Dd x Dd) leads to:
    • Genotype ratio: 1:2:1 (DD:Dd:dd).
    • Phenotype ratio: 3:1 (tall: dwarf).

Understanding the Ratios

  • Crossing two heterozygotes (F1 generation) yields predictable results.
  • Possible combinations: DD, Dd, and dd.
  • Homozygous dominant (DD) and heterozygous (Dd) plants are tall.
  • Homozygous recessive (dd) plants are dwarf.
  • Genotypic ratio: 1:2:1.
  • Phenotypic ratio: 3:1.
  • This holds true for any trait with a dominant-recessive relationship when crossing F1 generations.

Test Cross

  • Tall plants can be either homozygous dominant (DD) or heterozygous (Dd).
  • Dwarf plants are always homozygous recessive (dd).
  • Cross a plant with a dominant phenotype with a homozygous recessive plant.
  • If the unknown plant is homozygous dominant (DD):
    • All offspring will be heterozygous (Dd) and exhibit the dominant phenotype.
  • If the unknown plant is heterozygous (Dd):
    • Half of the offspring will be heterozygous (Dd) and exhibit the dominant phenotype.
    • Half of the offspring will be homozygous recessive (dd) and exhibit the recessive phenotype.
  • By observing the offspring, you can determine the genotype of the original plant.

Dihybrid Crosses

  • Looking at two traits simultaneously (e.g., seed color and seed shape).
  • Seed color:
    • Yellow (Y) is dominant to green (y).
  • Seed shape:
    • Round (R) is dominant to wrinkled (r).
  • Cross true breeding plants:
    • Yellow and round (YYRR) x green and wrinkled (yyrr).
    • F1 generation: all heterozygous (YyRr) - round and yellow.
  • Law of Independent Assortment.
  • Switching combinations (yellow/wrinkled x green/round) doesn't matter.
  • F1 generation will always be 100% yellow and round (heterozygous for both).

Understanding the Combinations

  • F2 generation ratios (YyRr x YyRr):
    • (\frac{3}{4}) yellow and \frac{3}{4} round.
  • Predictable ratios:
    • \frac{9}{16} carry both dominant traits.
    • \frac{3}{16} dominant phentotype and recessive phentotype yellow and wrinkled.
    • \frac{3}{16} recessive phentotype and dominant phentotype green and round.
    • \frac{1}{16} are going to have both recessive phenotypes.
    • 9:3:3:1 ratio in F2 generation
    • 9 - both dominant phenotypes
    • 3 - dominant phenotype with recessive phenotype
    • 3 - recessive phenotype with dominant phenotype
    • 1 - both recessive phenotypes
    • Use a larger Punnett square (16 boxes) for visualization.

Law of Independent Assortment

  • Confirmed by the 9:3:3:1 ratio.
  • Unit factors assort independently during gamete formation.
  • Chromosomes separate independently during meiosis I.
  • What happens to color has no impact on the shape.
  • Maternal chromosomes are segregated independently.