Overview
This lecture covers Gregor Mendel’s experiments on heredity using pea plants, his discovery of dominant and recessive traits, and the use of probability rules to predict genetic outcomes.
Mendel’s Experiments and Success
- Mendel used garden peas (Pisum sativum) because they self-fertilize, are true-breeding, and mature quickly.
- He focused on traits with distinct categories (discontinuous variation) rather than blended traits (continuous variation).
- Mendel’s large sample sizes and careful control over fertilization made his results statistically reliable.
Mendelian Crosses and Observations
- Mendel crossed true-breeding plants to produce the first generation (F1) and then allowed F1 plants to self-fertilize to create F2.
- In each case, F1 offspring all showed the dominant trait, while F2 offspring showed a 3:1 ratio of dominant to recessive traits.
- Reciprocal crosses (switching which parent contributed which trait) yielded the same 3:1 ratio, confirming inheritance patterns.
Key Findings in Pea Plants
- Mendel studied seven characteristics, each with two contrasting traits.
- Dominant traits appear in F1; recessive traits disappear in F1 but reappear in about 25% of F2.
- Traits do not blend; they remain separate and can re-emerge in later generations.
Probability in Genetics
- Probability is used to predict the likelihood of specific trait outcomes.
- Empirical probability is calculated from observed events; theoretical probability assumes equal chances of all outcomes.
- In F2, dominant traits appear in 75% and recessive in 25% of offspring, reflecting theoretical probabilities.
The Product Rule and Sum Rule
- Product Rule: The probability of independent events both happening is the product of their individual probabilities (use "and").
- Sum Rule: The probability of one or another mutually exclusive event is the sum of their individual probabilities (use "or").
- These rules are essential for predicting outcomes in genetic crosses, especially with multiple traits.
Key Terms & Definitions
- Model System — An organism chosen for study to generalize findings to other systems.
- Discontinuous Variation — Traits inherited in distinct forms, not blended.
- True-breeding — Organisms that produce offspring identical to themselves when self-fertilized.
- Dominant Trait — A trait that appears unchanged in hybrids.
- Recessive Trait — A trait that disappears in hybrids but can reappear in later generations.
- Product Rule — The probability principle applied when two independent events must both occur.
- Sum Rule — The probability principle used when either of two mutually exclusive events can occur.
Action Items / Next Steps
- Review Mendel’s seven pea plant traits and the resulting F1 and F2 generation ratios.
- Practice predicting genetic outcomes using the sum and product rules.
- Read the next section on the chromosomal basis of heredity.