Overview
This lecture covers Gregor Mendel's groundbreaking pea plant experiments, detailing his discovery of hereditary patterns and the application of probability laws to genetic crosses.
Mendel’s Experiments and Scientific Approach
- Mendel chose pea plants (Pisum sativum) due to their true-breeding nature, short generation time, and ability for controlled crosses.
- He focused on discrete (discontinuous) traits, such as flower color, which either appeared in one form or another, not blended.
- Mendel’s use of large sample sizes ensured statistically reliable results.
Mendelian Crosses and Generational Terms
- Parental generation (P) refers to the original true-breeding plants.
- First filial generation (F1) are offspring of the parental cross; all showed dominant traits.
- F1 plants self-fertilize to produce the F2 generation, where dominant and recessive traits reappear in a roughly 3:1 ratio.
Key Observations and Laws Derived
- Traits in offspring did not blend but appeared as dominant or recessive forms.
- The dominant trait masks the recessive in F1, but the recessive returns in F2.
- Each parent contributes one of two trait copies (alleles) to the offspring.
- The same 3:1 ratio was observed across multiple traits (e.g., flower color, plant height, seed shape).
Probability in Genetics
- Empirical probability is based on observed outcomes; theoretical is based on expected outcomes given equal likelihood.
- Probability of an event = (number of favorable outcomes)/(total possible events).
- Mendel used large populations to calculate reliable trait probabilities.
Product and Sum Rules in Probability
- The product rule: Probability of two independent events both occurring = probability of event A × probability of event B.
- The sum rule: Probability of one of two mutually exclusive events occurring = probability of event A + probability of event B.
- These rules predict the outcomes of genetic crosses, such as dihybrid crosses.
Key Terms & Definitions
- True-breeding — organisms that always produce offspring with the same traits as the parent.
- Discontinuous variation — traits that exist in distinct forms rather than a range.
- Hybridization — mating individuals with different traits.
- Dominant trait — trait that appears unchanged in hybrids and masks the recessive.
- Recessive trait — trait that is hidden in F1 hybrids but reappears in later generations.
- Empirical probability — calculated using actual experimental outcomes.
- Theoretical probability — calculated based on expected outcomes under ideal conditions.
- Product rule — probability of independent events both occurring together.
- Sum rule — probability of one of two mutually exclusive events occurring.
Action Items / Next Steps
- Review Table 12.1 for trait ratios in Mendel's pea hybrids.
- Practice applying product and sum rules to genetic cross problems.
- Prepare for a quiz on Mendelian inheritance and probability rules.