Heredity
What you must understand
Mendelian genetics rests on segregation and independent assortment of alleles. Know how to set up Punnett squares and predict genotype and phenotype ratios. Distinguish genotype (alleles) from phenotype (observable trait).
Non-Mendelian patterns include incomplete dominance (blended phenotype), codominance (both expressed), multiple alleles (ABO blood type), polygenic traits (continuous variation), and sex-linked traits (more common in males for X-linked recessive).
Meiosis explains heredity: homologous chromosomes separate in meiosis I, sister chromatids in meiosis II. Crossing over and independent assortment generate variation. Chromosomal errors (nondisjunction) cause aneuploidy.
Traps
- Incomplete dominance and codominance are not the same: blending vs. both traits shown distinctly.
- A recessive allele is not "weaker"; it is simply masked in heterozygotes.
- Sex-linked recessive traits appear more often in males because they have one X.
- Independent assortment applies to genes on different chromosomes; linked genes do not assort independently.
Rule of thumb
For dihybrid crosses of unlinked genes, multiply individual probabilities (product rule) rather than drawing a 16-square grid.
Worked example
Two heterozygotes (Aa × Aa) for a simple dominant trait: offspring are 1 AA : 2 Aa : 1 aa, giving a 3:1 phenotypic ratio.