BIO102 Ch14a

Biological Science - Chapter 14a: Mendel and the Gene

Introduction to Mendel and Genetics

  • Gregor Mendel: Founder of genetics.
  • Worked with garden peas (Pisum sativum).
  • Maintained detailed notes and careful observations.
  • Conducted controlled breeding experiments to track trait inheritance.

Mendel's Question

  • How are traits passed from parents to offspring?
  • Focus on transmission genetics.
  • Two main hypotheses during Mendel's time:
    • Blending inheritance.
    • Inheritance of acquired characteristics.

Mendelian or Transmission Genetics

  • Heredity: Transmission of traits from parents to offspring.
  • Trait (character): Characteristic of an individual (e.g., height, eye color).
  • Phenotype: Observable character state (e.g., short or tall).
    • Phenotype is partly the observable manifestation of genotype.
  • Genotype: Alleles of a particular gene(s) inherited by an individual.

Pisum sativum as a Model Organism

  • Peas were an ideal model organism for Mendel:
    • Inexpensive and easy to grow.
    • Short generation time.
    • Produce large numbers of seeds.
    • Controllable mating.
    • Many polymorphic traits.

Traits Studied by Mendel

  • Seed coat color/flower color (gray/purple, white/white).
  • Seed color (yellow, green).
  • Seed shape (smooth, wrinkled).
  • Pod color (green, yellow).
  • Pod shape (inflated, pinched).
  • Stem height (tall, short).
  • Flower position (axial, terminal).

Self-fertilization vs. Cross-fertilization

  • Self-fertilization: Pollen fertilizes eggs within the same flower.
  • Cross-fertilization: Pollen from one plant fertilizes eggs of another.
  • Mendel controlled fertilization by collecting pollen from one individual and transferring it to the female organ of another plant after removing the male organs.

True Breeding vs. Hybrid Individuals

  • Mendel worked with pure lines (true breeding).
  • True breeding lines: Individuals producing progeny with the same phenotype over generations.
    • Possess only one type of allele for the trait gene.
  • Hybrid individuals: Produced by crossing two true-breeding individuals with different phenotypes.
    • Hybrids have two different alleles for the trait gene.
    • Can produce progeny with different phenotypes.

Monohybrid Cross

  • Mendel's single-trait cross:
    • F1 generation: All offspring have round seeds.
    • F2 generation: 5474 round seeds : 1850 wrinkled seeds (approximately 3:1 ratio).

Reciprocal Cross

  • Pollen from round-seeded parent to female organ of wrinkle-seeded parent, and vice versa.
  • Results were identical: All progeny had round seeds.
  • The inheritance of seed shape is not affected by whether the genetic determinant comes from a male or female gamete.

Inheritance Patterns

  • Mendel tested multiple traits, all inherited similarly.
    • Examples:
      • Flower color: 705 purple, 224 white (3.15:1 ratio).
      • Seed shape: 5474 round, 1850 wrinkled (2.96:1 ratio).
      • Stem length: 787 tall, 266 dwarf (2.96:1 ratio).

Particulate Factors (Genes)

  • Mendel proposed the existence of particulate factors (genes).
    • Genes: Segments of DNA containing information.
    • Alleles: Alternate forms of a gene, differing in nucleotide sequence, leading to different gene products.
  • Pea plants are diploid and may have two different alleles for any gene.

Dominant and Recessive Alleles

  • F1 progeny receive one allele from each parent.
  • Heterozygous condition: Rr.
  • The allele for wrinkled seeds is recessive to the allele for round seeds.
  • Wrinkled phenotype is masked in the F1 progeny.
  • The allele for round seeds is dominant to the allele for wrinkled seeds.

Genotypes

  • Homozygous: Two copies of the same allele (e.g., RR or rr).
  • Heterozygous: Two different alleles (e.g., Rr).
  • In heterozygotes, the dominant allele masks the recessive allele.

Punnett Squares

  • Used to represent genetic crosses.
  • Example:
    • Cross between two homozygotes (RR x rr) results in all Rr (heterozygous) offspring, all with round seeds.
    • Cross between two heterozygotes (Rr x Rr) results in offspring with genotypes 14RR:12Rr:14rr\frac{1}{4} RR : \frac{1}{2} Rr : \frac{1}{4} rr, and phenotypes 34\frac{3}{4} round : 14\frac{1}{4} wrinkled.

Test Crosses

  • Used to test Mendel's model.
  • Cross a parent of known phenotype but unknown genotype with a homozygous recessive parent (rr).
  • If unknown parent is homozygous dominant (RR), all offspring will have the dominant phenotype (Rr).
  • If unknown parent is heterozygous (Rr), 12\frac{1}{2} offspring will have the dominant phenotype (Rr) and 12\frac{1}{2} will have the recessive phenotype (rr).

Principle of Segregation

  • In anaphase I of meiosis, the two alleles of a gene separate so that each daughter cell inherits only one.
  • Pairs of alleles are separated during meiosis I in the formation of gametes.

Dihybrid Cross

  • Mendel used dihybrid crosses, examining two traits simultaneously.
  • Crossed true-breeding round, yellow (RR YY) with wrinkled, green (rr yy).
  • Tested two hypotheses:
    1. Independent assortment: Alleles of different genes are transmitted independently of each other.
    2. Dependent assortment: The transmission of one allele depends on the transmission of another.

Independent Assortment

  • The genes for seed shape and seed color assort independently because they are located on different chromosomes.
  • Chromosomes have two equally likely ways of lining up before segregation during meiosis. Predicted gamete combinations are RY, ry, Ry, and rY each with a frequency of 14\frac{1}{4}.