Genetics & Evolution Cheat Sheet (NEET)

Mendel's Laws

• Law of Dominance, Segregation (2 alleles separate in gametes), Independent Assortment (only for unlinked genes).
• Monohybrid ratio 3:1 (F₂ phenotype), 1:2:1 (genotype). Dihybrid 9:3:3:1.
• Test cross: cross with homozygous recessive; ratio 1:1 (heterozygote) or 3:1 (dihybrid Hets).

Deviations

• Incomplete dominance (snapdragon): 1:2:1 = phenotype = genotype.
• Codominance (ABO blood group, MN): both alleles expressed.
• Multiple alleles: IA, IB, i.
• Polygenic (skin colour): continuous variation.
• Pleiotropy (PKU, sickle-cell): 1 gene, multiple traits.

Chromosomal Theory

• Linkage: genes on same chromosome inherited together; recombination frequency ≤ 50% (map units).
• Sex determination: XX-XY (humans), ZW (birds), XX-XO (grasshopper), haplo-diploid (bees).

Sex-Linked Inheritance

• X-linked recessive: haemophilia, colour blindness, DMD. More common in males.
• Y-linked: hairy pinna.
• Pedigree symbols: square = male, circle = female, filled = affected.

Mutations

• Point mutations: silent, missense, nonsense.
• Frameshift: insertion/deletion (not multiple of 3).
• Sickle cell: GAG→GTG, Glu→Val on β-chain.
• Down (Trisomy 21), Klinefelter (XXY), Turner (XO), Edwards (Trisomy 18), Patau (Trisomy 13).

Molecular Basis

• DNA structure: double helix, antiparallel, Chargaff (A=T, G=C), 3.4 Å between bases, 10 bp per turn.
• Replication semiconservative (Meselson–Stahl); leading vs lagging strand; Okazaki fragments.
• Transcription: RNA pol II (mRNA), III (tRNA), I (rRNA). Introns spliced.
• Translation: 3 codons = 3 amino acids; AUG start, UAA/UAG/UGA stop; 61 sense codons.
• Lac operon (inducible), Trp (repressible).

Evolution

• Darwin — natural selection; Lamarck — inheritance of acquired characters.
• Hardy–Weinberg: p² + 2pq + q² = 1; conditions — no mutation, migration, selection, drift, random mating.
• Homologous (analogy in origin), analogous (function). Adaptive radiation (Darwin's finches).

Practice MCQs

Q1. Ratio in dihybrid F₂ phenotype (Mendel) is A) 9:3:3:1 B) 1:2:1 C) 3:1 D) 9:7

Q2. A colour-blind mother and normal father have children. What is expected in sons? A) All normal B) All colour-blind C) 50% colour-blind D) 25% affected

Q3. Trisomy of chromosome 21 causes A) Turner B) Klinefelter C) Down D) Edwards

Q4. If allele frequency of q = 0.1, frequency of heterozygotes is A) 0.01 B) 0.09 C) 0.18 D) 0.81

Q5. Codominance is seen in A) Snapdragon flower B) ABO blood C) Pea plant D) Skin colour

Q6. Sickle-cell anemia is due to substitution at which position of β-chain? A) 4 B) 6 C) 10 D) 24

Q7. A test cross gives 1:1 ratio, so parent under test is A) Homozygous dominant B) Homozygous recessive C) Heterozygous D) Sterile

Q8. Sex chromosomes in a normal human male are A) XX B) XY C) XO D) XXY

Q9. In lac operon, presence of lactose does what? A) Represses transcription B) Inactivates operator C) Binds repressor and inactivates it D) Increases repressor

Q10. Chromosomal disorder XXY is A) Down B) Turner C) Klinefelter D) Patau

Answers: 1-A (Classic Mendelian ratio.), 2-B (Sons get X from mother → all affected.), 3-C (Down syndrome.), 4-C (2pq = 2×0.9×0.1 = 0.18.), 5-B (Both A and B expressed together (AB).), 6-B (Position 6: Glu → Val.), 7-C (1:1 confirms heterozygote.), 8-B (XY.), 9-C (Allolactose binds repressor.), 10-C (Klinefelter syndrome.)

MCQ Metadata