Gregor Mendel and Principles of Inheritance

Gregor Mendel's Theory of Inheritance

• Proposed the existence of heritable factors that pass traits from parents to offspring.
• These factors are now known as genes, which are located on chromosomes.
• At the time, molecular understanding of genetics was limited.

Mendel's Experiments with Pea Plants

• Studied flower color as a trait.
• Crossed a purple flowering plant with a white flowering plant.
• The first generation offspring (F1) all displayed purple flowers.

Generational Terminology

• P Generation: Parent generation.
• F1 Generation: First filial generation, offspring of the P generation.
• After crossing two F1 plants, Mendel observed a phenotypic ratio of 3:1 for purple to white flowers in the second generation (F2).

Key Genetic Concepts

• Alleles: Different versions of a gene.
• Homozygous: An organism with two identical alleles (true breeding).
• Heterozygous: An organism with two different alleles (hybrid).
• Dominant Allele: Determines appearance; represented by a capital letter.
• Recessive Allele: Masked by dominant allele; represented by a lowercase letter.

Genotype vs. Phenotype

• Phenotype: Observable traits of an organism.
• Genotype: Genetic makeup or combination of alleles.

Example of Flower Color Cross

• Purple flower (homozygous): Phenotype is purple, Genotype is BB.
• White flower (homozygous): Phenotype is white, Genotype is bb.
• F1 Generation: Heterozygous plants with a genotype of Bb; phenotype is purple.

F2 Generation Outcomes

• Cross between two heterozygous plants in F1 leads to a genotype ratio of 1 BB : 2 Bb : 1 bb.
• Phenotypic ratio is 3 purple : 1 white.

Using Punnett Squares

• A tool to visualize and predict genetic outcomes of crosses.
• Confirms the 3:1 phenotypic ratio for traits, alongside a 1:2:1 ratio for genotypes among offspring.
• Predictions are probabilistic, indicating likelihoods rather than certainties of the outcomes.

Important Insights

• Alleles are randomly distributed during gamete formation.
• Genes on the same chromosome tend to be inherited together, which Mendel recognized but did not fully address in his early work.

Conclusion

• Mendel's work significantly laid the groundwork for modern genetics.
• The principles of inheritance he established are still fundamental in understanding genetic transmission today.