Overview
This lecture explains how to perform monohybrid genetic crosses using Punnett squares, covering key terminology, Mendel's laws, and step-by-step worked examples.
Key Concepts in Monohybrid Crosses
- Monohybrid cross studies inheritance of a single trait (e.g., color or height).
- Genes are DNA sections coding for traits; alleles are different versions of a gene.
- Capital letters represent dominant alleles; lowercase letters represent recessive alleles.
- Genotype is the genetic allele combination (e.g., TT, Tt, tt); phenotype is the observable trait.
- Homozygous means two identical alleles (TT or tt); heterozygous means two different alleles (Tt).
- Use "homozygous" and "heterozygous" only when describing genotype.
Mendel's Laws in Crosses
- Law of Dominance: In purebred crosses, only the dominant trait appears in offspring when present.
- Law of Segregation: Allele pairs separate during meiosis; each gamete gets one allele from each pair.
How to Perform a Monohybrid Cross
- Start with P1 (parent) generation; write physical traits (phenotypes) and then genotypes.
- Separate alleles into gametes, indicating meiosis.
- Use a Punnett square to combine gametes and predict offspring.
- Fill in offspring genotypes and calculate proportions or percentages as required.
- Determine offspring phenotypes using dominance rules.
Example 1: True-Breeding Dominant x Recessive
- Cross pure green pod (GG) with pure yellow pod (gg), where green (G) is dominant.
- All F1 offspring are heterozygous (Gg) and display green phenotype (100% green).
Example 2: Heterozygous x Heterozygous
- Cross two heterozygous purple flowers (Bb x Bb), where purple (B) is dominant.
- F1 ratio: Genotype - 1 BB : 2 Bb : 1 bb; Phenotype - 3 purple : 1 white.
- 75% offspring are purple (BB or Bb), 25% are white (bb).
Key Terms & Definitions
- Gene — section of DNA coding for a trait.
- Allele — different versions/forms of a gene.
- Genotype — combination of alleles inherited (e.g., TT, Tt, tt).
- Phenotype — observable trait or appearance.
- Homozygous — two identical alleles (TT or tt).
- Heterozygous — two different alleles (Tt).
- Dominant — allele expressed when present (represented by uppercase).
- Recessive — allele only expressed with two copies (lowercase).
- Gamete — reproductive cell carrying one allele from each gene pair.
- Punnett Square — grid used to predict genetic cross outcomes.
- True Breeding — organism homozygous for a trait.
Action Items / Next Steps
- Practice drawing and filling Punnett squares for provided genotype crosses.
- Memorize key terminology and Mendel's laws.
- Review example problems on monohybrid crosses and predict genotype/phenotype ratios.