Schooling · Class 12 · Biology · Principles of Inheritance and Variation · Chapter on a page
CBSE Class 12 · Biology · NCERT Biology
Principles of Inheritance and Variation: chapter on a page
A cut of Shishya's notes for this chapter: the first paragraph of the introduction and the key ideas, formulas and facts, common mistakes and quick revision lists, word for word. Nothing is added or reworded; the worked examples are left out (they are on the chapter page).
About the notes: Written by Shishya's AI on 27 Sept 2026 from the chapter's title and class level, in Shishya's own words — not a copy or summary of the textbook. Read the official chapter for the book's own text, activities and exercises.
This note's current text has no applied fact-check on record.
Mind map
Drawn from the same notes by a fixed rule (no AI): each label is the item's own words — its bold term, its label before a colon, a short formula as written, or its first words.
The mind map as a list
- Key ideas
- Mendel's Law of Dominance
- Law of Segregation
- Law of Independent Assortment
- Chromosomal Theory of Inheritance
- Linkage and Recombination
- Sex Determination
- Deviations from Mendelian Ratios
- Formulas and facts
- Genotypic ratio in a monohybrid…
- Phenotypic ratio in a monohybrid…
- Phenotypic ratio in a dihybrid cross…
- Test cross
- Recombination frequency
- ABO blood group inheritance
- Sex-linked inheritance pattern
- Worked examples
- Monohybrid cross predicting F₂ ratio
- Determining blood group possibilities
- Sex-linked inheritance of colour…
- Watch out for
- Confusing genotype (genetic makeup…
- Assuming a 3
- Forgetting that linked genes do not…
- Thinking all daughters of a…
- Mixing up homozygous (two identical…
What this chapter is about
This chapter explores how biological traits pass from parents to offspring and why offspring are not exact copies of their parents. You will study the foundational work of Gregor Mendel, who experimented with garden pea plants and discovered the basic laws governing inheritance. His observations, made in the 1860s, form the bedrock of classical genetics.
Key ideas
- Mendel's Law of Dominance: When two contrasting alleles are present, one (dominant) expresses itself while the other (recessive) remains hidden in the phenotype of the heterozygote.
- Law of Segregation: During gamete formation, the two alleles of a gene separate so that each gamete receives only one allele; this occurs during anaphase I of meiosis when homologous chromosomes move to opposite poles.
- Law of Independent Assortment: Alleles of different genes located on different chromosomes assort independently during gamete formation, producing new allele combinations in offspring.
- Chromosomal Theory of Inheritance: Genes are located on chromosomes; the behaviour of chromosomes during meiosis (separation and independent assortment) parallels Mendel's laws.
- Linkage and Recombination: Genes on the same chromosome tend to be inherited together (linkage), but crossing over during meiosis can separate them, creating recombinant offspring at a frequency proportional to the distance between genes.
- Sex Determination: In humans, females are XX and males are XY; the Y chromosome carries the SRY gene that triggers male development. Sex-linked traits are often carried on the X chromosome.
- Deviations from Mendelian Ratios: Incomplete dominance (heterozygote intermediate), co-dominance (both alleles express equally, as in ABO blood groups), and multiple alleles (more than two allelic forms in a population) modify expected phenotypic ratios.
- Genetic Disorders: Some conditions follow Mendelian inheritance: autosomal recessive (phenylketonuria, sickle-cell anaemia), autosomal dominant (Huntington disease), and sex-linked recessive (haemophilia, colour blindness).
Formulas and facts to remember
- Genotypic ratio in a monohybrid cross (F₂): 1 homozygous dominant : 2 heterozygous : 1 homozygous recessive (1 : 2 : 1).
- Phenotypic ratio in a monohybrid cross (F₂): 3 dominant phenotype : 1 recessive phenotype (3 : 1).
- Phenotypic ratio in a dihybrid cross (F₂): 9 : 3 : 3 : 1 when genes assort independently.
- Test cross: Crossing an individual of dominant phenotype with a homozygous recessive individual reveals whether the dominant individual is homozygous or heterozygous.
- Recombination frequency = (Number of recombinant offspring / Total offspring) × 100 %; a value of 50 % indicates independent assortment (genes on different chromosomes or far apart).
- ABO blood group inheritance: Three alleles (Iᴬ, Iᴮ, i); Iᴬ and Iᴮ are co-dominant to each other and both are dominant over i. Genotypes: Iᴬ Iᴬ or Iᴬ i → Type A; Iᴮ Iᴮ or Iᴮ i → Type B; Iᴬ Iᴮ → Type AB; ii → Type O.
- Sex-linked inheritance pattern: A trait carried on the X chromosome (e.g., colour blindness) passes from a carrier mother to half her sons, who then express the trait because they have only one X chromosome.
- Pedigree symbols: Square = male, circle = female; filled symbol = affected individual; horizontal line = mating, vertical line = offspring.
Common mistakes
- Confusing genotype (genetic makeup, e.g., Rr) with phenotype (observable trait, e.g., round seeds) → genotype is the allele combination; phenotype is what you see or measure.
- Assuming a 3 : 1 ratio always applies → this ratio holds only when one allele is completely dominant; incomplete dominance gives 1 : 2 : 1 phenotypic ratio.
- Forgetting that linked genes do not assort independently → genes on the same chromosome travel together unless crossing over separates them.
- Thinking all daughters of a colour-blind father will be colour blind → daughters receive one X from each parent; if mother is not a carrier, daughters are carriers, not affected.
- Mixing up homozygous (two identical alleles) and heterozygous (two different alleles) → careful reading of genotype symbols prevents this error.
Quick revision
- Mendel's three laws: Dominance, Segregation, Independent Assortment.
- Monohybrid F₂ phenotypic ratio: 3 : 1; dihybrid F₂ ratio: 9 : 3 : 3 : 1.
- A test cross reveals whether a dominant-phenotype individual is homozygous or heterozygous.
- Co-dominance: both alleles express equally (ABO blood groups); incomplete dominance: heterozygote shows intermediate phenotype.
- Sex-linked recessive traits appear more often in males because they have only one X chromosome.
- Recombination frequency measures how often linked genes separate; 50 % indicates independent assortment.