Overview
This lecture reviews genetic problem-solving, covering Mendelian and non-Mendelian inheritance, Punnett square analysis, and pedigree interpretation essential for biology studies.
Mendelian Genetics: One-Trait Crosses
- Monohybrid crosses involve a single gene with two alleles (dominant/recessive).
- Each offspring receives one allele from each parent.
- Dominant allele (e.g., H) results in the dominant trait (hair); recessive (h) only shows if both alleles are recessive.
- Genotype ratio for crossing hh (hairless) x Hh (heterozygous): 1 Hh : 1 hh.
- Phenotype ratio for same cross: 1 with hair : 1 hairless.
Mendelian Genetics: Two-Trait (Dihybrid) Crosses
- Dihybrid crosses analyze inheritance of two traits simultaneously.
- Heterozygous for both traits example: HhSs x HhSs.
- Use FOIL method to determine gamete combinations (HS, Hs, hS, hs).
- Classic dihybrid phenotype ratio: 9:3:3:1 when crossing two double heterozygotes.
Non-Mendelian Inheritance Patterns
- Incomplete dominance: Heterozygote has intermediate phenotype (e.g., RR=red, rr=white, Rr=pink); genotype ratio: 1 RR: 2 Rr: 1 rr, phenotype: 1 red: 2 pink: 1 white.
- Codominance: Both alleles are fully expressed (e.g., BB=black, WW=white, BW=black/white speckled); genotype and phenotype ratios: 1:1 for cross of BB x BW.
- Multiple alleles: More than two allele forms exist (e.g., blood types: A, B, AB, O); type O occurs in 25% of offspring from heterozygous A x heterozygous B cross.
- Sex-linked traits: Usually carried on X chromosome; males (XY) express X-linked recessive disorders with one recessive allele; females (XX) need two.
Pedigree Analysis
- Pedigrees track inheritance of traits in families using standardized symbols (circles for females, squares for males).
- Shaded shapes indicate individuals with the trait of interest.
- Autosomal recessive traits require two recessive alleles (ee).
- Genotypes can often be inferred by working backward from affected individuals.
- If a pedigree does not fit expected inheritance for sex-linked traits, it’s likely autosomal.
Key Terms & Definitions
- Allele — A variant form of a gene.
- Genotype — The genetic makeup (allele combination) of an organism.
- Phenotype — Observable traits resulting from genotype.
- Homozygous — Two identical alleles for a trait.
- Heterozygous — Two different alleles for a trait.
- Monohybrid Cross — Genetic cross involving one trait.
- Dihybrid Cross — Genetic cross involving two traits.
- Incomplete Dominance — Heterozygote displays blended phenotype.
- Codominance — Both alleles are fully expressed in heterozygotes.
- Multiple Alleles — More than two possible alleles in a population.
- Sex-linked Trait — Trait determined by genes located on sex chromosomes.
- Pedigree — Diagram showing inheritance of traits in a family.
Action Items / Next Steps
- Practice drawing and analyzing Punnett squares for various inheritance patterns.
- Review and complete problems on pedigree interpretation.
- Watch the full genetics series videos for additional explanations.