Heredity Practice Tests

classic and molecular genetics for the 2027 Division B season: inheritance patterns, Punnett squares, pedigrees, chromosomes, and the basics of DNA.

Event scope 2027 (division b)

• Answer questions, solve problems, and analyze data on classic AND molecular genetics
• Division C counterpart is Designer Genes (adds evolutionary genetics and deeper biotech)

Genetics vocabulary

• Gene: DNA segment coding a trait; allele: version of a gene
• Genotype (alleles, e.g., Aa) vs. phenotype (expressed trait)
• Homozygous (AA/aa) vs. heterozygous (Aa); dominant vs. recessive alleles
• P (parental) → F₁ (first filial) → F₂ generations; true-breeding = homozygous

Mendelian inheritance

• Law of segregation: two alleles separate into different gametes during meiosis
• Law of dominance: one allele can mask another in heterozygotes
• Law of independent assortment: genes on different chromosomes sort into gametes independently
• Punnett squares: monohybrid Aa × Aa → 3:1 phenotype, 1:2:1 genotype
• Dihybrid AaBb × AaBb → 9:3:3:1 phenotypic ratio
• Probability: product rule for AND, sum rule for OR; coin-flip logic applies to alleles
• Testcross with homozygous recessive reveals unknown genotype

Beyond simple mendelian

• Incomplete dominance: red × white snapdragon → pink (blend)
• Codominance: both alleles show (roan cattle, AB blood type)
• Multiple alleles: ABO blood groups (I^A, I^B dominant, i recessive; A and B codominant)
• Sex-linked traits: on X chromosome; males (XY) express recessive X alleles with no second copy
• X-linked recessive examples: red-green colorblindness, hemophilia, Duchenne muscular dystrophy
• Carrier: heterozygous individual who shows no symptoms but can pass recessive allele

Pedigree analysis

• Squares = male, circles = female; shaded = affected; half-shaded = carrier
• Autosomal recessive clues: skips generations, affected kids from unaffected (carrier) parents
• Autosomal dominant clues: every affected person has an affected parent; no carriers
• X-linked recessive clues: more males affected; affected fathers DO NOT pass to sons (get Y instead)

Chromosomes and karyotypes

• Humans: 46 chromosomes, 23 pairs (22 autosomes + 1 sex pair); XX female, XY male
• Nondisjunction (failure of chromosomes to separate in meiosis): trisomy or monosomy
• Down syndrome: trisomy 21; Turner syndrome: XO (female); Klinefelter: XXY (male)
• Mutations can be chromosomal (deletion, duplication, inversion, translocation) or gene-level

Molecular genetics basics

• DNA double helix: A pairs with T (2 H-bonds), G pairs with C (3 H-bonds); sugar-phosphate backbone
• DNA → mRNA (transcription) → protein (translation at ribosome); codons of 3 bases
• Mutations: substitution (point), insertion/deletion (frameshift); effects range from silent to severe
• Meiosis makes gametes (haploid n); fertilization restores diploid 2n

Common genetic disorders to know

• Autosomal recessive: cystic fibrosis, sickle cell disease, PKU, Tay-Sachs
• Autosomal dominant: Huntington's disease, achondroplasia
• X-linked recessive: hemophilia, colorblindness, Duchenne muscular dystrophy
• Chromosomal: Down (21), Turner (XO), Klinefelter (XXY)

Common mistakes

• Assigning X-linked alleles to Y (Y carries almost no X-linked alleles; fathers pass X to daughters only)
• Treating carriers as affected for recessive disorders
• Mixing up genotype ratio (1:2:1) with phenotype ratio (3:1) in monohybrid crosses
• Confusing incomplete dominance (blend) with codominance (both show)
• Forgetting meiosis (not mitosis) produces gametes