genetics 4 (punnett squares)

Objectives

  • Explain how probability can be used to predict inherited traits.
  • Describe how alleles segregate when more than one gene is involved.
  • Identify Mendel’s contributions to our understanding of genetics.

Probability and Heredity

  • Principles of probability predict outcomes of genetic crosses.
    • Punnett Squares: A graphical method to help predict genotype and phenotype combinations in offspring.
    • Statistical odds example: What are the odds of getting an aa individual in the next generation?

Probability Questions

  1. Probability of a gamete receiving g?
  2. Probability of offspring being green?
  3. Probability of offspring being yellow?
  4. Probability of offspring having a heterozygous genotype?

Punnett Squares

  • Monohybrid Cross: Examines one trait (2 x 2 grid).
  • Dihybrid Cross: Examines two traits (4 x 4 grid).

Steps to Solving Punnett Squares

  1. Determine genotypes of parents.
  2. Set up the Punnett square:
    • Monohybrid: 2 x 2
    • Dihybrid: 4 x 4
  3. Fill in squares with letters: Represent possible gamete combinations from fertilization.
  4. Write out the possible genotypic ratio of offspring.
  5. Use genotype ratio to determine phenotype ratio.

Monohybrid Cross Setup

  • Example: Cross of two heterozygous organisms (Aa):
    • Possible genotypes: AA, Aa, aa

Steps for Monohybrid Cross

  1. Combine parental gamete genotypes.
  2. Example: Ospreys with beak sizes (B, b).
  3. Gametic combinations yield results:
    • Combined outcomes yield genotypes:
      • BB, Bb, bb

Genotype and Phenotype Ratios

  • Genotype Ratio:
    • Example: TT = ¼ (25\%), Tt = ½ (50\%), tt = ¼ (25\%)
  • Phenotype Ratio:
    • Example: Brown eyes = ¾ (75\%), Blue eyes = ¼ (25\%)

Examples of Monohybrid Cross

  1. Flowers Cross:
    • P = purple (dominant), p = white (recessive)
    • Genotype Ratio: PP = 25\%, Pp = 50\%, pp = 25\%
    • Phenotype Ratio: Purple: 75\%, White: 25\%
  2. Pea Plant Example:
    • Yellow peas dominant over green. Cross a heterozygous yellow with a homozygous green:
    • Determine genotype and phenotype ratios.

Test Cross

  • Cross between individual of unknown genotype and a homozygous recessive.
  • Used to determine the unknown genotype.

Test Cross Example

  • Spongebob's yellow color could be homozygous dominant or heterozygous. Perform a test cross:
    • If all offspring are yellow, deduce Spongebob's genotype is YY (homozygous dominant).

Dihybrid Cross

  • Predict outcomes for two traits simultaneously.
  • Example: Cross a brown-haired, brown-eyed male (BB//bb) with a blonde-haired, blue-eyed female:
    • Calculate phenotype combinations:
      • Brown hair & brown eyes
      • Brown hair & blue eyes
      • Blonde hair & brown eyes
      • Blonde hair & blue eyes

Setting Up Dihybrid Crosses

  1. Determine parent genotypes.
  2. Determine gametes using FOIL method.
  3. Set up a 4 x 4 Punnett square.
  4. Determine outcomes and ratios.

Additional Insights on Dihybrid Crosses

  • Example Traits:
    • Y = Yellow, y = green, R = Round, r = wrinkled
    • Analyze phenotype and genotype ratios based on common traits in plants or animals.

Practice Problems

  1. Predict offspring from a cross of homozygous brown mussel (BB) with blue mussel (bb): Calculate for blue offspring.
  2. Analyze color traits of organisms (like Spongebob).

Resources

  • Bozeman Science: A Beginner’s Guide to Punnett Squares Video (YouTube)
  • Khan Academy: Probabilities in Genetics Notes