CBSE Class 10 · Science · NCERT Science
Heredity: 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 26 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
- A gene is a unit of heredity
- Alleles are different forms of the…
- A dominant allele expresses itself…
- Genotype refers to the genetic…
- Mendel's Law of Segregation
- In humans, sex is determined by sex…
- Variations arise from the shuffling…
- Formulas and facts
- Homozygous
- Monohybrid cross phenotypic ratio…
- Monohybrid cross genotypic ratio (F₂…
- Probability of a child being male =…
- Mendel chose pea plants because they…
- Acquired traits (e.g., muscles built…
- A Punnett square is a grid used to…
- Worked examples
- Monohybrid cross in pea plants
- Sex determination in humans
- Identifying genotype from offspring…
- Watch out for
- Thinking that a dominant trait is…
- Believing the mother determines the…
- Confusing genotype with phenotype
- Assuming acquired characteristics…
- Adding ratios instead of multiplying…
What this chapter is about
Heredity is the biological process by which parents pass traits to their offspring. This chapter explains how characteristics such as eye colour, height and blood group are transmitted from one generation to the next through genes located on chromosomes. You will learn the basic rules of inheritance first discovered by Gregor Mendel through his experiments on pea plants.
Key ideas
- A gene is a unit of heredity: a segment of DNA on a chromosome that codes for a specific protein and hence a specific trait.
- Alleles are different forms of the same gene; an organism inherits one allele from each parent, giving it a pair for every gene.
- A dominant allele expresses itself even when only one copy is present; a recessive allele expresses itself only when both copies are the same.
- Genotype refers to the genetic makeup (e.g., Tt), while phenotype refers to the observable trait (e.g., tall plant).
- Mendel's Law of Segregation: the two alleles of a gene separate during gamete formation, so each gamete carries only one allele.
- In humans, sex is determined by sex chromosomes: females have XX and males have XY; the father's gamete decides the sex of the child.
- Variations arise from the shuffling of genes during sexual reproduction and from occasional mutations in DNA.
Formulas and facts to remember
- Homozygous: both alleles identical (TT or tt). Heterozygous: alleles different (Tt).
- Monohybrid cross phenotypic ratio (F₂ generation): 3 dominant : 1 recessive.
- Monohybrid cross genotypic ratio (F₂ generation): 1 homozygous dominant : 2 heterozygous : 1 homozygous recessive.
- Probability of a child being male = 1/2; probability of being female = 1/2, because the father contributes either X or Y with equal chance.
- Mendel chose pea plants because they have clearly contrasting traits, short life cycle, and allow both self-pollination and cross-pollination.
- Acquired traits (e.g., muscles built by exercise) are not inherited because they do not change the DNA in reproductive cells.
- A Punnett square is a grid used to predict genotype and phenotype ratios of offspring from a genetic cross.
Common mistakes
- Thinking that a dominant trait is more common in a population → Dominant only means it masks the recessive allele in heterozygotes; frequency depends on allele distribution.
- Believing the mother determines the sex of a child → The father's sperm (X or Y) determines sex; the mother always contributes X.
- Confusing genotype with phenotype → Genotype is the allele combination (e.g., Tt); phenotype is the visible trait (e.g., tall).
- Assuming acquired characteristics are inherited → Only changes in DNA of germ cells pass to offspring; traits gained during life do not.
- Adding ratios instead of multiplying probabilities → For independent events (like successive births), multiply the individual probabilities.
Quick revision
- Gene = unit of heredity on a chromosome; alleles = different versions of a gene.
- Dominant allele masks recessive; recessive shows only when homozygous.
- Monohybrid cross F₂ ratio: 3 dominant phenotype : 1 recessive phenotype.
- Sex in humans: XX = female, XY = male; father's gamete decides sex.
- Variations come from genetic recombination in sexual reproduction and from mutations.
- Acquired traits are not heritable because they do not alter germ-cell DNA.