Lecture 12 Monohybrid Crosses and the Law of Segregation

Mendelian Genetics

Introduction to Gregor Mendel

  • Gregor Mendel is known as "The Father of Genetics".
  • Before Mendel's work in 1866, knowledge about genetics was minimal.
  • Common opinion held that inheritance followed the concept of "blending" inheritance.
  • Mendel's contribution included experimentation and testable theories of inheritance.
  • His distinctive approach shifted the understanding of heredity in science.

Why Mendel Chose Pea Plants (Pisum)

  • Several key reasons:
    • Availability: Mendel had access to pea plants.
    • Pollination Methods: Pea plants can self-pollinate or cross-pollinate.
    • Prolific Offspring: They produce a high number of offspring.
    • Short Generation Time: Quick turnaround between generations.
    • Manageability: Unlike animals, pea plants remain stationary, facilitating breeding and counting offspring.

Traits Studied by Mendel

  • Mendel examined a total of seven traits, each with two forms:
    • Flower Color: Purple vs. White
    • Plant Height: Tall vs. Short
    • Seed Color: Green vs. Yellow
  • Example used for the explanation: Flower color (Purple and White).

Experimental Rationale

  • Create pure-breeding lines for both purple and white-flowered plants.
    • Pure breeding lines, when selfed, yield offspring with identical appearance.
  • Definition:
    • Phenotype: The observable appearance or characteristic of an organism regarding a trait.
  • Initial crossings:
    • Purple x Purple = Purple
    • White x White = White

The Key Question

  • Inquiry: If true-breeding purple plants are crossed with true-breeding white plants, what are the offspring's phenotypes?
    • Cross: Parental Generation (P) Purple x White
    • Initial Prediction (according to blending hypothesis): Offspring should be pink.
    • Actual Result: All offspring were PURPLE (F1 Generation).

F1 Generation Observations

  • Mendel found that the “whiteness” in the white parent seemed to disappear in the F1 generation.
  • This necessitated further investigation.

F2 Generation Results

  • Inquiry: What happens if the F1 offspring are selfed?
    • Cross: F1 purple x F1 purple
    • Initial Prediction: All offspring would be purple.
    • Actual Outcome: The F2 generation produced offspring with a phenotypic ratio of:
    • 3 Purple: 1 White
    • Expressed as: rac{3}{4} Purple : rac{1}{4} White.

Summary of Monohybrid Cross

  • P Generation: Purple x White
  • F1 Generation: All Purple
  • F1 Self Cross: F1 purple x F1 purple produces F2 Generation: 3 Purple : 1 White

Results Across All Traits

  • Mendel consistently observed the same results across all seven traits studied:
    • F1 offspring resembled one of the parents.
    • Selfing the F1 generation led to consistently obtaining a 3:1 ratio for the trait.

Antagonistic Pairs Examined by Mendel

  • Traits and their Antagonistic Pairs:
    • Seed Color: Yellow vs. Green
    • Seed Shape: Round vs. Wrinkled
    • Flower Color: Purple vs. White
    • Pod Color: Green vs. Yellow
    • Pod Shape: Round vs. Pinched
    • Stem Length: Tall vs. Short
    • Flower Position: Along stem vs. At tip of stem.

Interpretation of Results

  • Mendel's First Law (The Law of Segregation)
    • States that for each trait, there are discrete units of information, which Mendel called “factors” (now known as genes).
    • Each adult plant possesses two copies of a gene for each trait.
    • During gamete formation (eggs and sperm), copies segregate so that each gamete contains one copy.
    • Fertilization combines two gametes, forming a new individual with two copies of the gene for a particular trait.

Important Terminology

  • Genes and Alleles:
    • Genes come in two forms, called alleles.
    • Dominant alleles express the trait when present.
    • Recessive alleles require two copies for expression.
    • The alleles present in an organism are referred to as its genotype.
  • Genotype Definitions:
    • Homozygous: Organism with two identical alleles for a trait.
    • Heterozygous: Organism with two different alleles for a trait.
    • Gene locus: The physical location of a gene on a chromosome.

Final Understanding of Results

  • Summarized Monohybrid Cross:
    • P generation: Purple (PP) x White (pp)
    • F1 generation: All Purple (Pp).

The Punnett Square

  • Example for Purple and White Pea Plants:
    • Cross: PP x pp using a Punnett Square yields:
    • P | P
    • p | Pp
    • p | Pp
    • Result: All F1 offspring are Purple (Pp).

F2 Generation Punnett Square Analysis

  • F1 Generation (Pp) Self Cross:
    • Cross: Pp x Pp using a Punnett Square yields:
    • P | p
    • P | PP
    • p | Pp
    • p | pp
    • Result: F2 generation shows a ratio of 3 Purple (PP, Pp) to 1 White (pp).

Testcross Concept

  • Definition: The genotype of a plant exhibiting the dominant trait can be identified through crossing with a recessive trait.
  • Example Crosses:
    • Cross A: PYY x yy
    • Cross B: Yy x yy
  • Result Analysis:
    • Offspring from Cross A: All yellow
    • Offspring from Cross B: 1:1 Yellow to Green ratio.