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Heredity – Class 10 Science notes, important questions and MCQs

NCERT Class 10 Science, chapter 8. Notes, key terms, important questions with answers and MCQs.

Chapter overview

Heredity is the passing of traits from parents to offspring through genes. Mendel's pea experiments showed how traits are inherited, and sex chromosomes decide sex in humans.

Mendel's experiments

Monohybrid cross: tall (TT) × short (tt) → F1 all tall (Tt); F2 ratio 3 tall : 1 short (genotype 1 TT : 2 Tt : 1 tt).
Dihybrid cross: F2 ratio 9 : 3 : 3 : 1, showing traits are inherited independently.
The trait that shows in F1 is dominant; the hidden one is recessive.

Genes and sex determination

Genes are sections of DNA on chromosomes; each trait is controlled by a pair of genes (one from each parent).
Humans: female XX, male XY.
The father's sperm (X or Y) decides the sex of the child.

Key terms

Gene: a unit of heredity, a section of DNA
Dominant trait: a trait that appears even with one copy of the gene
Recessive trait: a trait that appears only with two copies
Genotype: the gene combination (e.g. Tt); phenotype is the visible trait

Formulas

Monohybrid F2 = 3 : 1 (phenotype), 1 : 2 : 1 (genotype)
Dihybrid F2 = 9 : 3 : 3 : 1

Important questions with answers

How is the sex of a child decided in humans?2 marks

A mother always gives an X chromosome. If the father's sperm carries X, the child is a girl (XX); if it carries Y, the child is a boy (XY). So the father's chromosome decides the sex.

Mendel crossed tall pea plants (TT) with short ones (tt). What will F₁ and F₂ look like?3 marks

F₁: all tall (Tt), because tall is dominant. F₂ (selfing F₁): TT : Tt : tt = 1 : 2 : 1 genotypes, so tall : short = 3 : 1 in appearance.

Explain with a dihybrid cross how Mendel showed that traits are inherited independently.5 marks

Mendel crossed round-yellow seed plants (RRYY) with wrinkled-green ones (rryy). F₁ plants were all round-yellow (RrYy). On self-pollinating F₁, the F₂ showed four types: round-yellow 9, round-green 3, wrinkled-yellow 3, wrinkled-green 1. New combinations (round-green and wrinkled-yellow) appeared, which is possible only if seed shape and seed colour are inherited independently of each other.

Quick revision

Mendel used garden pea plants.
Tt shows the dominant trait.
Y chromosome comes only from the father.

Exam tips

Use a Punnett square for every cross question.
The chance of a boy or girl is 50 : 50 in every pregnancy.

Practice MCQs

Mendel worked on
1. maize
2. garden pea
3. wheat
4. rose
Show answer
Answer: (b) garden pea. Pisum sativum.
Human males have sex chromosomes
1. XY
2. XX
3. YY
4. XO
Show answer
Answer: (a) XY. XY.
The visible trait is called the
1. genotype
2. phenotype
3. gene
4. allele pair
Show answer
Answer: (b) phenotype. Phenotype.
In a monohybrid cross, the F2 phenotype ratio is
1. 3 : 1
2. 1 : 1
3. 9 : 3 : 3 : 1
4. 1 : 2 : 1
Show answer
Answer: (a) 3 : 1. 3 dominant : 1 recessive.
A cross Tt × tt gives tall and short plants in the ratio
1. 3 : 1
2. 1 : 2 : 1
3. 1 : 1
4. all tall
Show answer
Answer: (c) 1 : 1. Half Tt, half tt.
The sex of a child is decided by the
1. father's sperm
2. mother's egg
3. both equally
4. environment only
Show answer
Answer: (a) father's sperm. Sperm carries X or Y.
In a dihybrid cross, how many of 16 F2 plants show both recessive traits?
1. 9
2. 3
3. 1
4. 4
Show answer
Answer: (c) 1. Only the 1 in 9 : 3 : 3 : 1.
A tall plant crossed with a short plant gives half tall and half short offspring. The tall parent was
1. TT
2. Tt
3. tt
4. cannot say
Show answer
Answer: (b) Tt. Tt × tt gives 1 : 1.

All chapters of Class 10 Science

Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.