CBSE · Class 10 · Biology · Chapter 8

Heredity — Formula Sheet

Board Formulas

9 formulas

1.Monohybrid Cross F₂ Phenotypic Ratio★
$T t \times T t \Rightarrow F_{2} = 3 Tall : 1 Dwarf$
$T$ : Dominant allele (tallness) · $t$ : Recessive allele (dwarfness)
F₁ heterozygotes (Tt × Tt) yield 75% dominant phenotype and 25% recessive phenotype in F₂.
2.Monohybrid Genotypic Ratio
$1 T T : 2 T t : 1 tt$
1 homozygous dominant : 2 heterozygous : 1 homozygous recessive.
3.Dihybrid Cross F₂ Phenotypic Ratio★
$R r Y y \times R r Y y \Rightarrow F_{2} = 9 : 3 : 3 : 1$
9 (both dominant) : 3 (first dominant, second recessive) : 3 (first recessive, second dominant) : 1 (both recessive).
4.Human Sex Determination★
$Father: X Y \times Mother: X X \Rightarrow 50% X X (girl) + 50% X Y (boy)$
Sex of the child is decided by the FATHER (Y chromosome from father → boy; X from father → girl).
5.Human Chromosome Number
$Somatic: 2 n = 46 (44 autosomes + 2 sex chromosomes)$
Gametes are haploid: n = 23 (22 autosomes + 1 sex chromosome). Fertilisation restores 2n = 46.
6.DNA — Base Pairing Rule
$A = T (2 H-bonds), G \equiv C (3 H-bonds)$
Adenine always pairs with Thymine; Guanine always with Cytosine. Ensures accurate replication.
7.DNA Nucleotide
$Nucleotide = Deoxyribose (C_{5} H_{10} O_{4}) + Phosphate (P O_{4}^{3 -}) + Base (A, T, G, C)$
DNA = double helix of nucleotides. Genes are stretches of DNA that code for proteins.
8.Test Cross
$Unknown (T_) \times tt \Rightarrow all tall \Rightarrow T T, else 1 : 1 \Rightarrow T t$
Cross an unknown dominant with a homozygous recessive. Offspring ratio reveals whether the unknown is TT (all tall) or Tt (1 tall : 1 dwarf).
9.Probability Rule (Independent Traits)
$P (A \cap B) = P (A) \times P (B)$
Probability of two independent traits appearing together = product of individual probabilities. Basis of the 9:3:3:1 ratio.