Designer Genes Practice Tests

classic, evolutionary, and molecular genetics at the Division C level: inheritance, gene regulation, biotechnology, and population genetics.

Event scope 2027 (division c)

• Classic genetics, evolutionary genetics, AND molecular genetics - broader than Division B Heredity
• Expect pedigree analysis, molecular mechanisms, data analysis, and multi-concept problems

Mendelian genetics

• Law of segregation: alleles separate into separate gametes; law of independent assortment: genes on different chromosomes sort independently
• Monohybrid cross Aa × Aa → 3:1 phenotype; dihybrid AaBb × AaBb → 9:3:3:1
• Testcross (with homozygous recessive) reveals an unknown genotype
• Probability: product rule (AND, independent events) and sum rule (OR, mutually exclusive)
• Chi-square test for goodness of fit: χ² = Σ(observed - expected)²/expected; reject null if χ² exceeds critical value

Extensions of mendelian

• Incomplete dominance (red × white → pink) vs. codominance (both alleles expressed, AB blood, roan cattle)
• Multiple alleles: ABO blood (I^A, I^B codominant; i recessive); H substance required
• Epistasis: one gene masks another (9:3:4, 12:3:1, 9:7 ratios)
• Lethal alleles (2:1 surviving ratios); pleiotropy (one gene, many effects, e.g., sickle cell)
• Polygenic traits + environment = continuous variation (skin color, height)
• Linkage: genes on the same chromosome inherited together; crossing over breaks linkage
• Gene mapping: 1 map unit (centimorgan) = 1% recombination frequency; map distances approximately add
• Sex-linked: X-linked recessive (males affected more), X-linked dominant, Y-linked

Pedigrees

• Autosomal recessive: skips generations, carriers; unaffected parents can have affected children
• Autosomal dominant: every affected child has an affected parent; no skipping
• X-linked recessive: affected fathers pass to daughters (carriers) not sons; carrier mothers → affected sons
• Mitochondrial: maternal inheritance only

Molecular genetics

• DNA: antiparallel double helix, A=T (2 H-bonds), G≡C (3 H-bonds); semiconservative replication
• Replication enzymes: helicase (unzip), primase (RNA primer), DNA polymerase (5'→3' synthesis), ligase (join Okazaki fragments)
• Central dogma: DNA → (transcription, RNA polymerase) → mRNA → (translation, ribosome) → protein
• Genetic code: triplet codons, 64 codons, AUG start, stop codons UAA/UAG/UGA; degenerate but unambiguous
• Mutations: point (silent, missense, nonsense) vs. frameshift (insertion/deletion not multiples of 3)
• Gene regulation: lac operon (inducible, allolactose removes repressor), trp operon (repressible), eukaryotic enhancers/transcription factors, epigenetics (methylation, histone modification)

Biotechnology

• PCR: cycles of denaturation, annealing, extension; amplifies DNA exponentially
• Gel electrophoresis: DNA negative → runs to + electrode; small fragments move fastest/farthest
• Restriction enzymes cut at palindromic sites; sticky ends; recombinant plasmids
• CRISPR-Cas9: guide RNA + nuclease for targeted cuts; uses: editing, knockouts
• Applications: DNA fingerprinting (paternity, forensics), GMOs, gene therapy

Population and evolutionary genetics

• Hardy-Weinberg equilibrium: p² + 2pq + q² = 1 (p + q = 1); assumes no selection, mutation, migration, drift, or nonrandom mating
• q² = recessive homozygote frequency; q = √(affected frequency); carriers = 2pq
• Selection types: directional, stabilizing, disruptive; heterozygote advantage (sickle cell vs. malaria)
• Genetic drift (bottleneck, founder effect) vs. gene flow; fitness = reproductive success

Common mistakes

• Taking q² as allele frequency instead of taking the square root first
• Confusing codominance (both phenotypes visible) with incomplete dominance (blend)
• Frameshift vs. nonsense mutation classification
• Assuming linked genes assort independently (only unlinked ones do)
• Reading gels backward (largest fragments at the wells)