Heredity Notes
Heredity is the passing of traits (characters) from parents to offspring. This chapter explains how variation arises, Mendel's experiments with pea plants, the rules of inheritance (dominant and recessive traits, independent inheritance), and how sex is determined in human beings.
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- 12 practice questions
- Aligned to CBSE 2025–26 syllabus
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Use this map for a first read, then return to the checks and formulas during revision.
- 1.Inherited Traits and Variation
- 2.Mendel's Experiments — Monohybrid Cross
- 3.Dihybrid Cross and Independent Inheritance
- 4.Sex Determination in Human Beings
- 1 key formulas
- 2 concept checks
Inherited Traits and Variation
Offspring look similar to their parents because they inherit the parents' genes. Sexually reproducing organisms get one set of genes from each parent, so offspring show a combination of features from both.
Each trait is influenced by both a paternal and a maternal contribution. Because two versions of each gene are present, more than one version can be passed on — this creates variation. Some variations are inherited from parents; new variations also arise by changes (mutations) in the DNA.
Only variations in the germ cells (gametes) can be passed to the next generation. Changes in body (somatic) cells during an individual's life, such as building muscle by exercise, are not inherited.
Mendel's Experiments — Monohybrid Cross
Gregor Mendel worked with garden pea plants that had clear contrasting traits — tall vs short, round vs wrinkled seeds, and so on. He first obtained pure-breeding parents.
When he crossed a pure tall plant (TT) with a pure short plant (tt), all the first-generation (F₁) plants were tall. The short character had not been lost; it was simply not expressed. Tall is dominant, short is recessive.
When the F₁ tall plants (Tt) were self-pollinated, the second generation (F₂) showed both tall and short plants in a ratio of about 3 : 1. The genotype ratio was 1 TT : 2 Tt : 1 tt. This shows that the two factors for a trait separate during gamete formation and pair again at fertilisation.
Dihybrid Cross and Independent Inheritance
Mendel also followed two pairs of traits at once — for example seed shape (round R / wrinkled r) and seed colour (yellow Y / green y). He crossed pure round-yellow (RRYY) with pure wrinkled-green (rryy).
All F₁ seeds were round and yellow. When the F₁ (RrYy) plants were self-pollinated, the F₂ showed four types of seeds — round-yellow, round-green, wrinkled-yellow, wrinkled-green — in a ratio of about 9 : 3 : 3 : 1.
The appearance of new combinations (round-green and wrinkled-yellow) shows that the trait for seed shape and the trait for seed colour are inherited independently of each other.
Sex Determination in Human Beings
Human beings have 23 pairs of chromosomes. One pair is the sex chromosomes: females have two X chromosomes (XX), and males have one X and one Y chromosome (XY).
All eggs made by the mother carry one X chromosome. Sperms are of two kinds: half carry an X and half carry a Y. If an X-carrying sperm fertilises the egg, the child is a girl (XX); if a Y-carrying sperm fertilises the egg, the child is a boy (XY).
So the sex of the child is determined by the father's sperm, not by the mother. On average, equal numbers of boys and girls are expected.
Heredity: revision sheet
Close the notes and explain each idea in your own words. Then check the formulas and attempt the examples before opening their solutions.
Recall checklist
- Inherited Traits and Variation
- Mendel's Experiments — Monohybrid Cross
- Dihybrid Cross and Independent Inheritance
- Sex Determination in Human Beings
For each topic: state the rule, explain when it applies, give an example and identify one mistake to avoid. Return to any topic you cannot explain without looking.
Formula recap (1)
Mendel's Experiments — Monohybrid Cross
Explain your method
Write the given information, choose a method, show the steps, then check units and whether the answer makes sense.
Check worked solution 1
Check worked solution 2
Turn mistakes into revision
Record the question, the step that went wrong and the corrected reason. Try it again tomorrow without the solution, then revisit it after a week. A correct guess still needs an explanation.
Practice and Revision
Test your understanding with quick chapter-level practice.
Chapter Q&A
How do Mendel's experiments show that traits may be dominant or recessive?
When Mendel crossed pure tall and pure short pea plants, all F₁ plants were tall — the short trait seemed to vanish. But in F₂, short plants reappeared. So the short factor was present in F₁ but not expressed: tall is dominant and short is recessive.
How do Mendel's experiments show that traits are inherited independently?
In a dihybrid cross the F₂ generation contained parental combinations and also new combinations (e.g. round-green, wrinkled-yellow) in a 9 : 3 : 3 : 1 ratio, which is only possible if each pair of traits is passed on independently of the other.
A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits — A or O — is dominant?
Yes. The daughter has O, so she received an O factor from each parent, meaning the father is AO. He shows blood group A even though he carries O, so A is dominant over O.
How is the sex of a newborn determined in human beings?
By the sex chromosome contributed by the father's sperm. An X-bearing sperm plus the egg's X gives a girl (XX); a Y-bearing sperm plus the egg's X gives a boy (XY).