topic 13

Topic 13: The Basic Principles of Heredity

Heredity, Historically

  • Prior to the 19th century, the concept of heredity was largely misunderstood.

  • Questions raised about parental contributions to offspring:

    • Equality of Contribution: Do parents contribute equally to their offspring?

    • Traits of Offspring: Will a tall father and a short mother produce a child of average height?

    • Acquired Traits: Can traits acquired through experiences, like a mother’s athletic training, be passed to her child?

    • Generational Traits: Why do certain traits appear to skip generations?

    • Predictability: How can heredity be predicted?

Heredity and Genetics

  • Heredity: Defined as the transmission of genetic information from parent to offspring, adhering to predictable patterns.

  • Genetics: The scientific study of heredity. This includes the analysis of genetic similarities and variations, focusing on differences among individuals, their parents, and larger populations.

Gregor Mendel: Father of Modern Genetics

  • A monk in the 19th century, Mendel is recognized for initiating the scientific study of inheritance through experiments with pea plants.

Common Terminologies

  • Phenotype: The observable physical appearance of an organism.

  • Genotype: The genetic constitution of an organism.

  • True-Breeding Line: A lineage that produces offspring displaying the same phenotype consistently from generation to generation.

  • P Generation: The parent generation that consists of genetically pure individuals.

  • F1 Generation: The first generation of offspring resulting from the P generation.

  • F2 Generation: The second-generation offspring that arise from crosses of F1 individuals or from their self-pollination.

Hybrids and True Breeders

  • Hybrids: Creatures produced from the mating of genetically different parents.

  • First generation (F1) hybrids from true-breeding parents produce offspring (F2) that exhibit a variety of traits.

Mendel’s Principles of Inheritance

  • Mendel identified patterns where parental traits reappear in the offspring. He used seven traits in peas to study inheritance:

    • Flower color: purple vs. white

    • Seed color: yellow vs. green

    • Seed shape: round vs. wrinkled

    • Pod color: yellow vs. green

    • Pod shape: smooth vs. indented

    • Plant height: tall vs. short

    • Flower position: axial vs. terminal (position on the stem).

The Garden Pea (Pisum sativum)

  • Advantages of using pea plants for Mendel's experiments:

    • Easy cultivation and many available varieties.

    • Facilitated controlled pollination due to both male and female parts within the flowers.

    • Peas typically self-pollinate but can be cross-pollinated manually by removing male parts.

Mendel’s Experiments

  • Began by crossing plants of different phenotypes (e.g., tall vs. short).

  • F1 Generation: Uniform in appearance, resembling one parent (e.g., tall plants).

  • F2 Generation Results: A result of 787 tall plants to 277 short plants demonstrating a ratio near to 3:1.

Ratios from Mendel's Experiments

  • Dominant Trait vs. Recessive Trait in F1 and F2 Generations:

    • Stem Height: Tall (T) x Short (t)

    • F1: All T (tall)

    • F2: 787 Tall : 277 Short (Ratio ~ 3:1)

    • Flower Color: Purple (P) x White (p)

    • F1: All P (purple)

    • F2: 705 Purple : 224 White (Ratio ~ 3:1)

    • Other traits (flower position, pod color, pod shape, seed color, seed shape) yielded similar ratios, indicating consistent dominant-recessive relationships.

Mendel’s Conclusions

  1. Different forms of a genetic