Which one of the following is an acquired trait that will NOT be passed on to the children?

Large muscles built by years of weight training

When pure tall (TT) and pure short (tt) pea plants are crossed, all the F1 plants are tall. What does this show?

Tallness is dominant; the short trait is not lost

Tallness is recessive and shortness is dominant

The gene for shortness is destroyed in the F1 plants

Tallness is an acquired trait of the parent plants

In the F2 of the cross RRYY × rryy, which seed types are new combinations that were not present in either parent?

Round yellow and wrinkled green

Wrinkled green and wrinkled yellow

A tall pea plant is crossed with a short plant and produces 52 tall and 48 short offspring. What is the genotype of the tall parent?

Tt, because short offspring need a t from each parent

tt, because the offspring are half short

Either TT or Tt, since the data cannot decide

In human beings, which parent’s gamete determines the sex of the child?

The father’s sperm, which carries either X or Y

The mother’s egg, which carries either X or Y

Both gametes equally, with a 3 : 1 chance

The father’s sperm, which carries either X or Y

The mother’s egg, which carries two X chromosomes

Which statement about sex determination in human beings is NOT correct?

The egg of the mother may carry a Y chromosome

A sperm carries either an X or a Y chromosome

A girl receives one X from each parent

A boy receives his Y chromosome from his father

Which mode of reproduction produces the greatest variation among offspring?

Sexual reproduction, which combines genes of two parents

Vegetative propagation by cuttings

In undisturbed layers of rock, which statement about fossils is correct?

Fossils found in deeper layers are generally older

Fossils found near the surface are generally older

Fossils in all layers are of the same age

Heredity

This chapter answers the Class 10 Life Sciences item on heredity in the RRB general science syllabus, the notices asking for General Science and Life Science up to Class 10. It is used in the Level-1 (Group D) and NTPC general science papers and in the Technician and ALP basic science sections. It teaches variation and acquired versus inherited traits, Mendel’s monohybrid and dihybrid crosses with their ratios and worked counts, dominance, test cross, and sex determination in humans. Evolution content has been cut back in the current Class 10 book, so it gets a short, clearly marked section.

Inheritance and variation

Heredity is the passing of traits from parents to offspring; variation is the difference between individuals. Both arise from DNA. A gene is a segment of DNA that codes for a protein and so controls a trait; genes lie on chromosomes. Human body cells have 23 pairs (46) of chromosomes: 22 pairs of autosomes and one pair of sex chromosomes. Each parent gives one set of 23 in the gamete, so one member of every pair comes from the mother and one from the father, and the pair number is restored at fertilisation.

Variation is small in asexual reproduction (only errors in DNA copying) but large in sexual reproduction, where two parents’ genes are shuffled in new combinations. Variation is useful for the species: if the environment changes, some varieties may survive.

Traits are inherited (eye colour, free or attached earlobes) or acquired during life (muscles built by exercise, a scar, weight lost). Acquired traits change only body cells, not the DNA in the germ cells, so they are not passed on.

⚠️ Exam trap

A trait gained in a lifetime is not inherited: a wrestler’s muscles or a bodybuilder’s weight do not pass to children, and cutting the tails of generations of mice does not give tailless mice. Only changes in the DNA of gametes are inherited.

Mendel and the monohybrid cross

Gregor Mendel worked on the garden pea (Pisum sativum) because it has easily seen contrasting traits (tall or short, round or wrinkled seed, yellow or green seed), a short life cycle, and flowers that self-pollinate but can be cross-pollinated by hand. Mendel counted the offspring of each cross, which gave his work its power.

In a monohybrid cross one pair of contrasting traits is followed. A pure tall plant (TT) crossed with a pure short plant (tt) gives an F1 generation that is all tall (Tt); the short trait seems to vanish. Self-pollinating the F1 gives F2 with tall and short plants in the ratio 3 : 1. So the trait that shows in F1 is dominant (T), the hidden trait is recessive (t), and it is not lost, it reappears in F2.

Punnett square for Tt × Tt
F1 gametes	T	t
T	TT (tall)	Tt (tall)
t	Tt (tall)	tt (short)

From the square: phenotypic ratio (appearance) 3 tall : 1 short; genotypic ratio (gene makeup) 1 TT : 2 Tt : 1 tt. TT and tt are homozygous, Tt is heterozygous; the two forms of a gene (T and t) are alleles. Worked example: of 1,600 F2 plants, tall = ¾ × 1,600 = 1,200 and short = ¼ × 1,600 = 400. A test cross of a tall plant with a short one (tt) shows its genotype: Tt × tt gives tall : short in 1 : 1, while TT × tt gives all tall.

A further point: among the tall F2 plants, only 1 in 3 is pure (TT) and 2 in 3 are Tt, because the tall class is 1 TT + 2 Tt; so a randomly chosen tall F2 plant has probability 1/3 of being TT, which is why self-pollination of F2 tall plants can again throw up short plants.

⚠️ Exam trap

3 : 1 is the phenotypic ratio; the genotypic ratio in the same cross is 1 : 2 : 1. Also the F1 is not a mixture of tall and short: it is uniformly tall, because Tt looks like TT.

Reading crosses quickly

Mendel’s idea explains every ratio: each plant carries two alleles of a gene, but a gamete carries only one of them, chosen by chance, and the offspring get one allele from each parent. A gene works by making a protein, so a tall plant is tall because its gene makes an efficient enzyme for a growth hormone, while the recessive allele makes a less efficient one. For a one-line answer in the exam, use the table below and remember that a recessive trait shows only when both alleles are recessive.

Common monohybrid crosses
Cross	Offspring genotypes	Offspring phenotypes
TT × tt	All Tt	All tall
Tt × Tt	1 TT : 2 Tt : 1 tt	3 tall : 1 short
Tt × tt	1 Tt : 1 tt	1 tall : 1 short
TT × Tt	1 TT : 1 Tt	All tall
tt × tt	All tt	All short

🧠 Memory trick

Quick test: if the offspring include any recessive (short) plants from two tall parents, both parents must be Tt. A recessive plant is always homozygous (tt), so it must have received one t from each parent.

The dihybrid cross and independent inheritance

In a dihybrid cross two pairs of traits are followed together. Cross a pure round yellow-seeded plant (RRYY) with a pure wrinkled green-seeded plant (rryy): all F1 plants are round and yellow (RrYy), so round (R) and yellow (Y) are dominant. Self-pollinating the F1 gives, in F2, four types in the ratio 9 : 3 : 3 : 1.

F2 seed type	Fraction	Out of 1,600
Round yellow	9/16	900
Round green (new)	3/16	300
Wrinkled yellow (new)	3/16	300
Wrinkled green	1/16	100

The two parental types (round yellow, wrinkled green) appear, but so do two new combinations, round green and wrinkled yellow. This shows that the traits are inherited independently of each other: each pair separates into the gametes without regard to the other. Each trait still shows a 3 : 1 ratio on its own (round 12 : wrinkled 4; yellow 12 : green 4).

🧠 Memory trick

Dihybrid memory hook: 9 : 3 : 3 : 1 adds up to 16; the 9 is both dominant traits together, the 1 is both recessive, and the two 3s are the new combinations. Probability method: P(round) × P(yellow) = ¾ × ¾ = 9/16.

Sex determination

Of the 23 pairs of human chromosomes, 22 are matched pairs (autosomes). The 23rd is the pair of sex chromosomes: XX in a woman, XY in a man. Every egg carries one X. A sperm carries either X or Y, in equal numbers. So the father’s sperm decides the sex of the child: an X sperm fertilising the egg gives XX, a girl; a Y sperm gives XY, a boy. The chance of a boy or a girl at each birth is 1/2, and each birth is independent of the earlier ones.

Sex of the child
Egg ↓ / Sperm →	X	Y
X (egg)	XX (girl)	XY (boy)

Worked example: a couple has three daughters. The probability that the next child is a girl is still 1/2, because the sperm is X or Y with equal chance each time. The probability that their next two children are both girls is 1/2 × 1/2 = 1/4. In some animals, other factors decide sex: in some reptiles (turtles) the temperature of the eggs during incubation, and in snails the sex can change.

⚠️ Exam trap

The mother always gives X, so she cannot decide the sex. The commonest false statement is that the mother is responsible for the birth of a daughter or a son.

Evolution: a low-priority topic

ℹ️ Note

In the current NCERT Class 10 science book the chapter is titled Heredity and the evolution content has been cut back to a minimum, so it is treated here as a formative-only, low-priority topic (it may still appear in general-science papers). Treat it as low priority and learn only the short ideas below.

Natural selection (Darwin): variations that help survival are favoured, and the population slowly changes over generations. Geographical isolation and genetic drift (chance changes in gene frequency) lead to new species (speciation).
Homologous organs have the same basic structure but may do different work (forelimb of a human and of a frog); analogous organs do the same work with different origin (wing of an insect and of a bird).
Fossils are preserved remains of past organisms; the deeper layer is older, and the age is also found from ratios of isotopes. Archaeopteryx, with feathers and reptile-like teeth, links reptiles and birds.

Only the basic definitions are likely to be asked. For the numerical and ratio questions of this chapter, the monohybrid and dihybrid ratios and the 1/2 chance of a boy or girl are the heart of the matter.

Key takeaways

Genes on chromosomes carry traits; humans have 23 pairs (22 autosomes + 1 sex pair); acquired traits are not inherited.
Monohybrid cross TT × tt: F1 all Tt (tall); F2 phenotypes 3 : 1 and genotypes 1 : 2 : 1; in 1,600 plants, 1,200 tall and 400 short.
A test cross Tt × tt gives 1 : 1, which tells the tall parent is heterozygous; TT × tt gives all tall.
The dihybrid cross RRYY × rryy gives 9 : 3 : 3 : 1 in F2, with new combinations (round green, wrinkled yellow): traits are inherited independently.
In humans, woman XX and man XY; the father’s sperm (X or Y) decides the child’s sex, with probability 1/2 each birth.
Evolution is low priority now that the Class 10 chapter is titled Heredity: know natural selection, homologous versus analogous organs and fossils in brief.

Practice questions (17)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

How many pairs of chromosomes are present in a normal human body cell?
1. 22
2. 23
3. 44
4. 46
Show answer
Answer: B — 23
A human body cell has 23 pairs, that is 46 chromosomes: 22 pairs of autosomes and one pair of sex chromosomes. The figure 46 is the total number of chromosomes and not the number of pairs, which is the usual slip.
Which one of the following is an acquired trait that will NOT be passed on to the children?
1. Colour of the eyes
2. Attached earlobes
3. Large muscles built by years of weight training
4. Blood group of the person
Show answer
Answer: C — Large muscles built by years of weight training
Muscles built by exercise change only body cells and not the DNA in the gametes, so they are not inherited. Eye colour, earlobe shape and blood group are all controlled by genes and are inherited.
When pure tall (TT) and pure short (tt) pea plants are crossed, all the F1 plants are tall. What does this show?
1. Tallness is dominant; the short trait is not lost
2. Tallness is recessive and shortness is dominant
3. The gene for shortness is destroyed in the F1 plants
4. Tallness is an acquired trait of the parent plants
Show answer
Answer: A — Tallness is dominant; the short trait is not lost
The trait that shows in the F1 is dominant, so tall is dominant. The F1 plants are Tt and still carry the t allele, which reappears as short plants in the F2. The idea that the short gene is destroyed is the tempting error, but it comes back in the next generation.
The F1 plants of a TT × tt cross are self-pollinated and give 1,600 F2 plants. How many short plants are expected?
1. 1,200
2. 800
3. 100
4. 400
Show answer
Answer: D — 400
The F2 ratio is 3 tall : 1 short, so short plants are ¼ × 1,600 = 400. The figure 1,200 is the number of tall plants (¾), and 800 would follow from a 1 : 1 ratio, which belongs to the test cross.
A heterozygous tall pea plant (Tt) is crossed with a short plant (tt). What fraction of the offspring is expected to be short?
1. One half
2. One quarter
3. Three quarters
4. None, all will be tall
Show answer
Answer: A — One half
The gametes are T or t from the first parent and only t from the second, giving Tt and tt in equal numbers, so half are short. One quarter is the figure for Tt × Tt, which is the different cross that gives tall and short as 3 : 1.
In the F2 generation of a monohybrid cross between pure TT and pure tt parents, what is the genotypic ratio?
1. 3 : 1
2. 1 : 2 : 1
3. 1 : 1
4. 9 : 3 : 3 : 1
Show answer
Answer: B — 1 : 2 : 1
The Punnett square gives 1 TT : 2 Tt : 1 tt, the genotypic ratio. The ratio 3 : 1 is the phenotypic ratio of the same cross (tall to short), which is the usual confusion, and 9 : 3 : 3 : 1 belongs to a dihybrid cross.
Pure round yellow (RRYY) and pure wrinkled green (rryy) pea plants are crossed, and the F1 is self-pollinated to give 1,600 F2 plants. How many wrinkled yellow plants are expected?
1. 900
2. 100
3. 300
4. 500
Show answer
Answer: C — 300
Wrinkled yellow is one of the two 3/16 classes: 3/16 × 1,600 = 300. The number 900 is the round yellow class (9/16) and 100 is wrinkled green (1/16), the other classes of the 9 : 3 : 3 : 1 ratio.
In the F2 of the cross RRYY × rryy, which seed types are new combinations that were not present in either parent?
1. Round yellow and wrinkled green
2. Round yellow and round green
3. Wrinkled green and wrinkled yellow
4. Round green and wrinkled yellow
Show answer
Answer: D — Round green and wrinkled yellow
The parents were round yellow and wrinkled green, so the new combinations are round green and wrinkled yellow. Their appearance shows that seed shape and seed colour are inherited independently. The pair made of round yellow and wrinkled green is the set of parental types, which are not new.
Consider these statements. P: A plant with genotype Tt is heterozygous and looks tall. Q: The recessive trait appears in the F1 generation of a cross between pure parents. R: TT and Tt plants have the same phenotype. S: Genotype means the visible appearance of the plant. Which statements are correct?
1. P and R only
2. Q and S only
3. P, Q and R only
4. R and S only
Show answer
Answer: A — P and R only
P and R are correct: Tt is heterozygous and, with T dominant, looks the same as TT. Q is wrong because the recessive trait stays hidden in F1 and appears only in F2. S is wrong because the visible appearance is the phenotype, while the genotype is the gene makeup.
A tall pea plant is crossed with a short plant and produces 52 tall and 48 short offspring. What is the genotype of the tall parent?
1. TT, because tall is dominant
2. Tt, because short offspring need a t from each parent
3. tt, because the offspring are half short
4. Either TT or Tt, since the data cannot decide
Show answer
Answer: B — Tt, because short offspring need a t from each parent
The short parent is tt and gives only t, so every short offspring (tt) must have received a t from the tall parent, which is therefore Tt. A TT parent would give only tall offspring, and the nearly 1 : 1 ratio confirms Tt × tt.
In human beings, which parent’s gamete determines the sex of the child?
1. The mother’s egg, which carries either X or Y
2. Both gametes equally, with a 3 : 1 chance
3. The father’s sperm, which carries either X or Y
4. The mother’s egg, which carries two X chromosomes
Show answer
Answer: C — The father’s sperm, which carries either X or Y
Every egg has one X, but a sperm carries X or Y, so the sperm decides: X gives a girl (XX) and Y gives a boy (XY). The mother’s egg can never carry Y, and it carries one X, not two.
What is the combination of sex chromosomes in a normal human male?
1. XX
2. YY
3. XO
4. XY
Show answer
Answer: D — XY
A man has one X and one Y, and a woman has two X chromosomes. YY does not occur in humans, and XO is not the normal male pattern.
A couple already has one son. Assuming each child is equally likely to be a boy or a girl, what is the probability that their next two children are both daughters?
1. 1/2
2. 1/4
3. 1/8
4. 1/3
Show answer
Answer: B — 1/4
Each birth is independent, and the son already born has no effect: 1/2 × 1/2 = 1/4. The value 1/8 wrongly counts the existing son as a third event, and 1/2 is the chance for a single child.
Which statement about sex determination in human beings is NOT correct?
1. The egg of the mother may carry a Y chromosome
2. A sperm carries either an X or a Y chromosome
3. A girl receives one X from each parent
4. A boy receives his Y chromosome from his father
Show answer
Answer: A — The egg of the mother may carry a Y chromosome
The egg always carries an X, never a Y, so the statement about a Y in the egg is false. A girl is XX with one X from each parent, and a boy gets his Y only from his father, as only sperms can carry it.
Which mode of reproduction produces the greatest variation among offspring?
1. Binary fission in Amoeba
2. Budding in Hydra
3. Sexual reproduction, which combines genes of two parents
4. Vegetative propagation by cuttings
Show answer
Answer: C — Sexual reproduction, which combines genes of two parents
Sexual reproduction mixes the genes of two parents, and gives new combinations. Fission, budding and cuttings copy a single parent, so the offspring are nearly identical, with only small DNA-copying errors.
The wing of an insect and the wing of a bird do the same work but have different origins. Such organs are called:
1. Homologous organs
2. Vestigial organs
3. Analogous organs
4. Fossil organs
Show answer
Answer: C — Analogous organs
Organs with the same function but different origin are analogous. Homologous organs share the same basic structure and origin even if the work differs, as in the forelimbs of a human and a frog.
In undisturbed layers of rock, which statement about fossils is correct?
1. Fossils found in deeper layers are generally older
2. Fossils found near the surface are generally older
3. Fossils in all layers are of the same age
4. Fossils can never be dated
Show answer
Answer: A — Fossils found in deeper layers are generally older
In undisturbed layers the deeper layers were laid down earlier, so fossils lying deeper are older. Age can also be found from the ratios of isotopes in the fossil material, so the claim that fossils cannot be dated is false.