Mendelian & Non-Mendelian Genetics

Mendel's Framework

• Segregation: allele pairs separate into gametes. Independent assortment: genes on different chromosomes sort independently — the cellular basis is random homolog orientation at metaphase I.
• Monohybrid $Aa \times Aa$ gives 1:2:1 genotypes and 3:1 phenotypes; a test cross against $aa$ reveals an unknown genotype. For dihybrids, skip the 16-box grid and multiply single-gene probabilities: $P(\text{tall and wrinkled}) = \frac{3}{4} \times \frac{1}{4} = \frac{3}{16}$ Each fertilization is independent of previous children.

Beyond Mendel

• Incomplete dominance blends (red and white make pink, 1:2:1 phenotypes); codominance expresses both alleles fully (AB blood shows both antigens).
• ABO: $I^A$ and $I^B$ are codominant, both dominant over $i$; a type O child needs an $i$ from each parent, the core of exclusion problems. Type O donates universally; AB receives universally.
• Pleiotropy: one gene, many effects (sickle cell). Epistasis: one locus masks another (yellow Labradors). Polygenic traits vary continuously. Mitochondrial DNA passes maternally — an affected mother transmits to all children. X-linked recessive traits appear far more often in hemizygous males.

Key Takeaways

• The product rule solves dihybrid questions fastest.
• Distinguish blended (incomplete dominance) from dual expression (codominance).
• ABO paternity logic: type AB parents cannot have type O children.
• Maternal-only transmission signals mitochondrial inheritance.