Lesson 6 - Monohybrid Crosses

Monohybrid Cross

Objective

  • Determine the genotypes and phenotypes of expected offspring in a monohybrid cross.

  • Successful if I can:

    • Identify the parent genotypes

    • Use a Punnett square

    • Determine genotypes and phenotypes in ratios, fractions, and/or percent values.

Warm-Up

Reasons for Using Pea Plants

  • Reproduce quickly

  • Cost-effective

  • Sexual reproduction allows for pollination

  • Traits are expressed in two ways

Mendel’s Findings

  • Each parent contributes one allele during fertilization.

  • The dominant allele is expressed when the recessive allele is present.

  • Alleles segregate independently during gamete formation.

Monohybrid Cross Details

  • Involves one trait (one gene).

  • Usually two alleles (e.g., blue and brown eyes).

  • A Punnett Square is used to predict offspring genotypes and phenotypes.

Biology Behind Mendel’s Experiment

  • P Generation: Purebred round seeds crossed with purebred wrinkled seeds.

  • F1 Generation: All round seeds.

  • F2 Generation: Crossing two round seeds from F1:

    • ¾ round seeds and ¼ wrinkled seeds.

    • Genotype of P generation seeds is homozygous (RR and rr).

Generating F2 Generation

  • F1 self-fertilization (crossing two F1 seeds).

  • F1 genotype: Rr (heterozygous) with round phenotype.

    • Cross: Rr x Rr

    • Gametes: R, r

    • F2 Genotype Ratio: 1 (RR): 2 (Rr): 1 (rr)

    • F2 Phenotype Ratio: 3 round: 1 wrinkled.

Solving Genetics Problems

  1. State all possible genotypes and phenotypes of parents.

  2. Determine gametes of P generation.

  3. Set up a Punnett Square with P gametes to determine F1 outcomes.

  4. Solve for genotypic fractions.

  5. Solve for phenotypic percent.

Example #1

  • Cross between a heterozygous pea plant and a homozygous recessive plant:

    • Parents: Rr (round) x rr (wrinkled)

    • Gametes: R, r | r, r

    • Results: F1 has genotypes Rr (50%) and rr (50%); phenotypes 50% round and 50% wrinkled.

Example #2

  • Cross between homozygous purple flower plant and homozygous white flower plant:

    • Parents: PP (purple) x pp (white)

    • F1 Generation: All Pp (100% purple).

Example #3

  • Offspring phenotypes used to determine parent genotypes:

    • Phenotypes Count: Round (5742), Wrinkled (1850)

    • Ratios: 5742:1850 ⇒ 3:1

    • Parent genotypes: Round seeded peas could be Rr or RR; Wrinkled seeded peas must be rr.

Success Criteria

  • Ability to state genotypes and phenotypes of expected offspring.

Definitions

  • Monohybrid Cross: A genetic cross between individuals that only examines a single trait, involving two alleles.

  • Punnett Squares: A diagram used to predict the genotype and phenotype outcomes of a genetic cross by showing all possible combinations of alleles.

  • Homozygous: An organism that has two identical alleles for a specific trait (e.g., RR or rr).

  • Heterozygous: An organism that has two different alleles for a specific trait (e.g., Rr).

  • Law of Segregation: Mendel's principle that states during the formation of gametes, the two alleles for a trait segregate from each other, so that each gamete carries only one allele for each trait.