CBSE · Class 12 · Biology · Chapter 4

Principles of Inheritance and Variation — Formula Sheet

Board Formulas

10 formulas

1.Monohybrid Phenotypic Ratio★
$F_{2} ratio = 3 : 1$
Cross Tt × Tt: dominant phenotype : recessive phenotype.
2.Monohybrid Genotypic Ratio
$F_{2} genotypic ratio = 1 T T : 2 T t : 1 tt$
Only visible when a test cross is made — Tt × tt gives 1:1 tall to dwarf.
3.Dihybrid Phenotypic Ratio★
$F_{2} ratio = 9 : 3 : 3 : 1$
Two independently segregating pairs. 9 dominant-dominant : 3 dominant-recessive : 3 recessive-dominant : 1 recessive-recessive.
4.Test Cross Ratio
$A a \times aa \to 1 : 1 (dominant : recessive)$
Used to expose the genotype of an individual showing the dominant phenotype.
5.Incomplete Dominance★
$F_{2} ratio = 1 : 2 : 1 (RR : Rr : rr)$
Phenotypic ratio equals genotypic ratio (e.g. Mirabilis: red : pink : white).
6.ABO Blood Group Alleles
$I^{A}, I^{B}, i; I^{A} = I^{B} > i$
Multiple alleles; Iᴬ and Iᴮ are codominant; both dominant over i.
7.Sex Determination★
$Human: XX (female), XY (male); Birds: ZW (female), ZZ (male)$
Grasshopper follows XX/XO. In humans the father is always the sex-determining parent.
8.Chromosome Numbers (Human)
$Diploid 2 n = 46; Autosomes = 44; Sex chromosomes = 2$
22 pairs autosomes + 1 pair sex chromosomes.
9.Down Syndrome Karyotype
$Trisomy of chromosome 21 : 2 n + 1 = 47$
Extra copy of chromosome 21 due to non-disjunction during meiosis.
10.Klinefelter and Turner Karyotypes
$Klinefelter: 47, XXY; Turner: 45, XO$
Klinefelter is male with an extra X; Turner is female with a missing X (sterile).