Biology Formula Sheet — Class 12
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Biology Formula Sheet
16 formulas across 5 chapters — with variables explained and exam tips where needed.
Ch 4Principles of Inheritance and Variation(5 formulas)
Monohybrid cross — F₂ phenotypic ratio
Dominant : Recessive = 3 : 1
Genotypic ratio: TT : Tt : tt = 1 : 2 : 1
Dihybrid cross — F₂ phenotypic ratio
9 : 3 : 3 : 1 (two dominant : one dom + one rec : other dom + rec : both recessive)
Requires genes on different chromosomes (Independent Assortment)
Incomplete dominance — F₂ ratio
Phenotypic ratio = 1 : 2 : 1 (same as genotypic; blending apparent)
Example: red × white → pink in snapdragon; no complete dominance
Test cross ratio
1 : 1 (F₁ heterozygous × homozygous recessive parent)
Used to determine whether an organism showing dominant phenotype is TT or Tt
Codominance (ABO blood groups)
I^A I^B genotype → AB blood group (both A and B antigens expressed)
Both alleles expressed simultaneously with no blending; distinct from incomplete dominance
Ch 5Molecular Basis of Inheritance(3 formulas)
Chargaff's Rule
A = T | G = C | A+G = T+C (Purines = Pyrimidines)
Most important equation in genetics — appears every year
DNA base percentage
If A% is given: T = A%, G = C = (100 − 2A)/2 %
Number of H-bonds in DNA
H-bonds = 2×(A=T pairs) + 3×(G≡C pairs)
Ch 6Evolution(1 formulas)
Hardy-Weinberg Principle
p² + 2pq + q² = 1 and p + q = 1
p = frequency of dominant allele, q = frequency of recessive allele
Ch 11Organisms and Populations(3 formulas)
Exponential (J-shaped) growth
dN/dt = rN or N_t = N₀ × e^(rt)
r = intrinsic rate of natural increase, N = population size, t = time
Occurs when resources are unlimited; produces J-shaped curve
Logistic (S-shaped) growth
dN/dt = rN × (K − N)/K
K = carrying capacity; (K−N)/K = environmental resistance factor
S-shaped (sigmoid) curve; population growth slows and stabilises at K
Population growth rate (r)
r = b − d
b = birth rate, d = death rate
r > 0: growing; r = 0: stable; r < 0: declining population
Ch 12Ecosystem(4 formulas)
Lindemann's 10% law
Energy available at next trophic level = 10% of energy at current trophic level
90% lost as heat, respiration, decomposition — limits food chains to 3–4 trophic levels
Gross Primary Productivity (GPP)
GPP = NPP + R_plants
R_plants = energy used by producers in respiration
Total rate of energy fixed by producers via photosynthesis
Net Primary Productivity (NPP)
NPP = GPP − R_plants
Energy available to consumers; measured in g/m²/yr or kcal/m²/yr
Secondary Productivity
Secondary P = Ingested energy − Respiration − Fecal losses
Rate of energy storage by consumers (herbivores, carnivores)