Overview
This lecture explains how inheritance of traits works, starting with Gregor Mendel's pea plant experiments and introducing basic genetics concepts.
Mendel’s Experiments and Discoveries
- Gregor Mendel studied how traits are passed from parents to offspring using pea plants.
- Crossing a purebred yellow pea with a purebred green pea produced only yellow peas in the first generation.
- The yellow trait was called “dominant” because it appeared in all offspring, while green was “recessive,” hidden in the first generation.
- Self-fertilizing the first-generation hybrids produced both yellow and green peas in the second generation.
Basic Genetic Concepts
- Traits depend on pairs of factors, one from each parent; these are now called alleles.
- Alleles are different versions of a gene.
- Homozygous means both alleles are identical; heterozygous means the alleles are different.
- Genotype is the combination of alleles an organism has.
- Phenotype is the physical trait expressed (e.g., yellow or green pea).
- A dominant allele will mask the effect of a recessive one in the phenotype.
Visualizing Inheritance: The Punnett Square
- The Punnett square is a diagram used to predict allele combinations in offspring.
- Uppercase letters represent dominant alleles (e.g., “Y” for yellow), lowercase for recessive (e.g., “y” for green).
- Only double recessive (“yy”) offspring show the recessive trait (green peas).
- Second-generation crosses of heterozygous (“Yy”) plants yield a 3:1 ratio of dominant to recessive phenotypes.
Multiple Traits and Complexity
- Peas can have other traits (such as round or wrinkled), which can combine with color.
- The Punnett square can be expanded to calculate genotype and phenotype ratios for multiple traits.
- Inheritance in humans and other organisms is more complex, but Mendel’s principles are foundational.
Key Terms & Definitions
- Allele — one of two or more versions of a gene.
- Homozygous — having two identical alleles for a trait.
- Heterozygous — having two different alleles for a trait.
- Genotype — the genetic makeup (allele combination) for a trait.
- Phenotype — the physical expression of a trait.
- Dominant Trait — a trait that appears even when only one allele is present.
- Recessive Trait — a trait that appears only when both alleles are recessive.
- Punnett Square — a chart used to predict genetic outcomes.
Action Items / Next Steps
- Review how to set up and use a Punnett square.
- Practice predicting genotype and phenotype ratios for single and double traits.