Unit 5: Heredity — Quick Review

Meiosis in one breath

• $2n \to 4$ haploid gametes, each genetically unique.
• Meiosis I separates homologs (reductional). Meiosis II separates sister chromatids (equational, mitosis-like).
• Three sources of variation: crossing over (prophase I), independent assortment (metaphase I), random fertilization.
• Nondisjunction → aneuploidy (e.g., trisomy 21).

Mendel's two laws

• Segregation: alleles separate; each gamete gets one. Mechanism = anaphase I.
• Independent assortment: genes on different chromosomes segregate independently. Mechanism = random metaphase-I orientation.
• Monohybrid cross ($Aa \times Aa$): 3:1 phenotypic, 1:2:1 genotypic.
• Dihybrid cross ($AaBb \times AaBb$): 9:3:3:1 — deviation means linkage or non-Mendelian.
• Product rule for independent events (AND); sum rule for mutually exclusive (OR).

Non-Mendelian patterns — one-line diagnostics

• Incomplete dominance → intermediate (pink). Codominance → both expressed (AB, roan).
• Multiple alleles → >2 in population (ABO).
• Pleiotropy → one gene, many traits (sickle-cell).
• Polygenic → continuous trait (skin color, height).
• Epistasis → one gene masks another (Lab coat color).
• Sex-linked recessive → hits males much more often.
• Mitochondrial → from mom only, to ALL kids.

Pedigree logic (four-step checklist)

1. Two unaffected parents → affected child = recessive.
2. Every affected child has affected parent = dominant.
3. Skips generations in males = X-linked recessive.
4. Passes only through mothers to all kids = mitochondrial.

Chi-square essentials

$\chi^2 = \sum \frac{(O - E)^2}{E}$

• $df$ = #categories $- 1$.
• Use $p = 0.05$.
• $\chi^2 >$ critical → reject the expected ratio.
• $\chi^2 <$ critical → fail to reject (data consistent with prediction).

💡 Exam Tip: Compute $E$ as (total offspring) × (expected fraction). Ratios like 3:1 must be scaled by total count before plugging in.

Key Terms

• Allele — alternative version of a gene.
• Homozygous / heterozygous — two identical / two different alleles.
• Genotype / phenotype — genetic makeup / observable trait.
• Tetrad — paired homologs (4 chromatids) at synapsis.
• Chiasma — crossing-over contact point.
• Recombination frequency — fraction of recombinant offspring, in centimorgans.
• Hemizygous — males for X-linked genes (one copy).
• Aneuploidy / polyploidy — abnormal chromosome count / extra whole sets.

Must-Know for the Exam

• State when in meiosis ploidy changes (anaphase I).
• Give three sources of genetic variation in meiosis.
• Convert a 3:1 or 9:3:3:1 ratio to expected counts for any offspring number.
• Tell autosomal recessive from X-linked recessive from a pedigree.
• Set up and interpret a chi-square test against the critical table (given).
• Explain why linked genes violate Mendel's second law — and how crossing over partially rescues it.
• Distinguish incomplete dominance, codominance, and multiple alleles by offspring phenotype.
• Identify mitochondrial inheritance by its maternal-only, every-child pattern.

💡 Exam Tip: The AP grader's favorite FRQ move is to give you ambiguous data and ask you to evaluate two inheritance hypotheses. Always state what each hypothesis would predict, compare to the data, and explicitly accept or reject.