Monohybrid and Dihybrid Crosses

Week 9: Sections 6.2, 6.3 - Monohybrid and Dihybrid Crosses

Monohybrid Cross

  • Definition: A monohybrid cross investigates a single trait.

  • Gametes examples:

    • Pollen gametes:

    • RY

    • rY

    • ry

Traits Crossed

  1. Seed Shape:

    • Round (dominant) X Wrinkled (recessive)

    • F1 Generation: All round

    • Offspring Ratio:

      • 5474 round

      • 1850 wrinkled

  2. Seed Colour:

    • Yellow (dominant) X Green (recessive)

    • F1 Generation: All yellow

    • Offspring Ratio:

      • 6022 yellow

      • 2001 green

  3. Pod Shape:

    • Inflated (dominant) X Constricted (recessive)

    • F1 Generation: All inflated

    • Offspring Ratio:

      • 882 inflated

      • 299 constricted

  4. Pod Colour:

    • Green (dominant) X Yellow (recessive)

    • F1 Generation: All green

    • Offspring Ratio:

      • 428 green

      • 152 yellow

Punnett Squares

  • Purpose: A Punnett square calculates the potential genotypes and phenotypes of offspring from parental alleles.

Key Concepts

  • Genotype: The set of alleles (e.g., RR, Rr, rr).

  • Phenotype: The physical expression of the genotype.

  • Monohybrid Cross: Examines one trait.

  • Dihybrid Cross: Examines two traits simultaneously.

Probability in Genetics

  • Probability Definition: The likelihood of a random event occurring.

    • Flip a coin: Two possible outcomes (heads or tails).

    • Two flips result in four possible outcomes (HH, HT, TH, TT).

  • For pea plant flower color, if both are heterozygous for purple (dominant):

    1. Determining color: Purple.

    2. Possible gametes: P or p.

    3. Possible offspring genotypes: PP, Pp, or pp.

    4. Probability of white flowers: 25% (pp).

Steps to Create a Punnett Square

  1. Draw the square.

  2. Fill in gametes from parent plants.

  3. Complete rows from side gametes.

  4. Complete columns from top gametes.

  5. Analyze results for genotype and phenotype ratio.

Example with Round Peas (R=round, r=wrinkly):

  • Cross two heterozygous parents (Rr):

  • Possible offspring:

    • RR, Rr, Rr, rr

  • Phenotype ratio: 3 round: 1 wrinkly.

Test Cross

  • Definition: A test cross determines the genotype of an individual with a dominant phenotype by crossing it with a known homozygous recessive.

  • Purpose: To figure out if the dominant phenotype is homozygous (RR) or heterozygous (Rr).

  • Scenario:

    • Cross the unknown round pea (dominant) with a known wrinkled pea (recessive).

    • If all offspring show dominant phenotype: unknown is homozygous (RR).

    • If 50% show recessive: unknown is heterozygous (Rr).

Human Traits and Genetic Probability

  • Many human disease alleles are recessive (e.g., cystic fibrosis:

    • Homozygous recessive: cc (affected).

    • Heterozygous: Cc (carrier, phenotypically healthy).

  • Human traits often involve polygenic inheritance; influenced by multiple genes.

  • Behaviors often misconceived as genetic may also be influenced by environmental and cultural factors.

Dihybrid Cross

  • Definition: A dihybrid cross examines the inheritance of two different traits simultaneously.

Mendel’s Laws of Inheritance

  1. First Law - Segregation:

    • Alleles segregate independently during gamete formation; half gametes carry one allele, half the other.

    • Occurs because diploid germ cells produce haploid gametes, each having one copy of each chromosome.

  2. Second Law - Independent Assortment:

    • The segregation of one pair of alleles is independent of another pair, leading to random combinations in gametes.

Conducting a Dihybrid Cross Punnett Square

  • Cross two parents heterozygous for both traits (e.g., RrYy × RrYy).

Steps to Calculate Outcomes

  1. Determine possible gametes using the FOIL method:

    • First: RY

    • Outer: Ry

    • Inner: rY

    • Last: ry

  2. Fill out the Punnett square similar to a monohybrid cross, analyzing ratios for genotypic and phenotypic outcomes.

Example Probability Calculation

  • Traits: Brown hair (dominant) vs. blonde (recessive) and freckles (dominant) vs. no freckles (recessive).

    • Woman: heterozygous for both traits.

    • Man: homozygous recessive for both traits.

    • Probability of offspring with brown hair and freckles to be computed.

  • Questions on Offspring Probability:

    1. Given specific traits, calculate the ratios and expectations for genotypes and phenotypes.

    2. Example Cross: RRYy (homozygous yellow round peas) with Rryy (heterozygous round and homozygous green).

    3. Determine probability of producing wrinkled green offspring.