Overview
This lecture covers the foundational principles of heredity from Mendel's experiments, including genetic terminology, Mendelian laws, Punnett squares, probability in genetics, chi-square analysis, and pedigree interpretation.
Mendel's Experiments and Principles
- Gregor Mendel used pea plants (Pisum sativum) for genetic studies due to their clear traits and easy manipulation.
- He used statistical analysis with large sample sizes, leading to reliable genetic ratios.
- Mendel's monohybrid cross revealed the principle of segregation: individuals possess two alleles per trait, which segregate during gamete formation.
- The monohybrid cross also demonstrated dominance: one allele can mask expression of another.
- Dihybrid crosses showed the principle of independent assortment: genes for different traits segregate independently during gamete formation.
Essential Genetic Terms and Concepts
- Genes encode characteristics; alleles are alternative forms of a gene.
- The locus is the specific chromosome location of a gene/allele.
- Genotype is the set of alleles an individual has; phenotype is the observable trait.
- Homozygous: two identical alleles; heterozygous: two different alleles.
- Dominant alleles mask recessive ones in the phenotype.
Understanding Genetic Ratios
- In a monohybrid F2 generation: genotypic ratio is 1 homozygous dominant : 2 heterozygous : 1 homozygous recessive; phenotypic ratio is 3 dominant : 1 recessive.
- Test crosses determine genotype of an individual showing the dominant phenotype.
Chromosomes, Meiosis, and Segregation
- Alleles reside on homologous chromosomes.
- During meiosis, alleles segregate into haploid gametes, explaining Mendel’s laws at the cellular level.
- Independent assortment results from random alignment of chromosomes during metaphase I of meiosis.
Punnett Squares and Probability Tools
- Punnett squares visualize possible offspring genotypes from parental crosses.
- Multiplication rule: probability of independent events occurring together (e.g., chance of two coin tosses).
- Addition rule: probability of either of two mutually exclusive events occurring.
Chi-Square Analysis in Genetics
- Chi-square tests compare observed vs. expected genetic ratios to determine if deviations are due to chance.
- Degrees of freedom = number of categories - 1.
- P-value > 0.05 = difference due to chance; P-value < 0.05 = statistically significant difference.
Pedigree Analysis
- Pedigrees are diagrams tracing inheritance of traits within families.
- Symbols: squares = males, circles = females, shaded = affected, half-shaded/dotted = carrier.
- Autosomal recessive traits appear only if both alleles are recessive; autosomal dominant traits appear with one affected allele.
- Inbreeding increases the likelihood of recessive genetic disorders.
Key Terms & Definitions
- Gene — an inherited factor determining a characteristic.
- Allele — an alternative form of a gene.
- Locus — specific location of a gene on a chromosome.
- Genotype — combination of alleles an individual has.
- Phenotype — observable trait or appearance.
- Homozygous — two identical alleles.
- Heterozygous — two different alleles.
- Monohybrid cross — cross involving one genetic trait.
- Dihybrid cross — cross involving two traits.
- Punnett square — grid predicting genotypes from a cross.
- Chi-square test — statistical test for deviation from expected genetic ratios.
- Pedigree — family tree diagram for inheritance patterns.
Action Items / Next Steps
- Review Punnett square practice problems and genetic ratio tables.
- Practice chi-square calculations using observed and expected values.
- Read next chapter on chromosomes and advanced genetic linkage.