Assertion (A): When a heterozygous tall pea plant (Tt) is crossed with a dwarf pea plant (tt), all the offspring are tall. Reason (R): The tall allele is dominant over the dwarf allele.

Both A and R are true and R is the correct explanation of A.

Both A and R are true but R is not the correct explanation of A.

Mendel performed a dihybrid cross between pure round‑yellow and wrinkled‑green pea plants. In the F2 generation he obtained four phenotypic combinations in the ratio 9:3:3:1. How does this ratio demonstrate independent assortment of the two pairs of traits? Explain with the help of a Punnett square.

The 9:3:3:1 ratio shows that the two traits are inherited independently. During gamete formation, the alleles for seed shape and seed colour segregate and recombine at random, producing four types of gametes in equal proportions (RY, Ry, rY, ry). When these gametes fuse, the Punnett square yields nine round‑yellow, three round‑green, three wrinkled‑yellow and one wrinkled‑green. This confirms that the alleles of different genes do not influence each other’s segregation, which is the basis of Mendel’s Law of Independent Assortment.

In garden pea, round seeds (R) are dominant over wrinkled (r). A heterozygous round-seeded plant is crossed with a pure wrinkled-seeded plant. Work out the cross and state the genotypic and phenotypic ratios of the offspring.

Cross: Rr × rr. Gametes: R, r and r. Offspring: Rr (round) and rr (wrinkled) in 1:1 ratio. Genotypic ratio: 1 Rr : 1 rr, phenotypic ratio: 1 round : 1 wrinkled.

In humans, brown eye colour (B) is dominant over blue eye colour (b). A brown-eyed man marries a blue-eyed woman and they have three children: two brown-eyed and one blue-eyed. Determine the genotype of the man. Support your answer with a Punnett square.

The blue-eyed woman is homozygous recessive (bb). The blue-eyed child must have inherited one b allele from each parent, so the father must possess at least one b allele. Since the father is brown-eyed, he must be heterozygous (Bb). A Punnett square of Bb (father) × bb (mother) gives a 1:1 ratio of brown (Bb) to blue (bb) offspring, matching the observed distribution. If the father were BB, all children would be brown-eyed.

What is a dihybrid cross? Explain how Mendel’s experiment with round yellow and wrinkled green pea seeds illustrates the law of independent assortment.

A dihybrid cross involves two pairs of contrasting traits. Mendel crossed pure round yellow (RRYY) with wrinkled green (rryy) peas. The F1 were all round yellow (RrYy). On selfing F1, the F2 generation showed four phenotypes in a 9:3:3:1 ratio (round yellow, round green, wrinkled yellow, wrinkled green). This shows that alleles for seed shape and seed colour are inherited independently of each other, demonstrating the law of independent assortment.

In pea plants, tall (T) is dominant over dwarf (t). A farmer crossed a homozygous tall plant (TT) with a dwarf plant (tt). The F1 progeny was self-pollinated to obtain the F2 generation. Predict the phenotypic and genotypic ratios in the F2 progeny. Illustrate with a Punnett square.

Initial cross: TT (tall) × tt (dwarf) → F1 all Tt (tall). Self-pollination of F1: Tt × Tt. Gametes: T and t from each parent. Punnett square: - T from female, T from male: TT (tall) - T from female, t from male: Tt (tall) - t from female, T from male: Tt (tall) - t from female, t from male: tt (dwarf) Thus, genotypic ratio: 1 TT : 2 Tt : 1 tt; phenotypic ratio: 3 tall : 1 dwarf.

Explain the mechanism of sex determination in humans with a neat labelled cross. Why is the father responsible for determining the sex of the child? What is the scientific significance of the Y chromosome in this process?

Humans have 23 pairs of chromosomes, including one pair of sex chromosomes. Females possess two X chromosomes (XX), while males have one X and one Y chromosome (XY). During gametogenesis, females produce eggs all containing one X chromosome. Males produce sperm: half contain an X chromosome and half a Y chromosome. The cross: Parents: Female (XX) × Male (XY) Gametes: X (from female) and X or Y (from male) Offspring: XX (female) if an X-sperm fertilizes the egg; XY (male) if a Y-sperm fertilizes the egg. The ratio is 1:1. The father is responsible because the mother always contributes an X; the father's sperm determines the sex by contributing either X or Y. The Y chromosome carries the SRY gene, which triggers the development of male characteristics. Absence of the Y (i.e., XX) leads to female development. Thus, the Y chromosome is the key determinant of maleness.

Different organisms have different mechanisms of sex determination. In humans, the XY system determines sex. In birds, the ZW system is present where males are ZZ and females are ZW. In honeybees, sex is determined by the number of sets of chromosomes; males are haploid, and females are diploid. In some reptiles like turtles, the temperature during egg incubation decides the sex of the offspring. (1) In birds, which sex chromosome combination produces a male? How does this differ from humans? (2) Explain how a male honeybee is produced and why it cannot have a father. (3) In a turtle species, eggs incubated at 25°C produce only males, while at 30°C produce only females. Compare this with genetic sex determination and discuss one advantage of such a system in evolution.

Birds: male ZZ vs human male XY. Male honeybees are haploid from unfertilized eggs, so no father. Temperature-dependent sex determination allows environmental influence on sex ratio.

Class 10 Science Chapter 8: Heredity — Important Questions & Sample Paper

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Chapter 8, Heredity, explores how traits pass from one generation to the next. It begins with Mendel’s pea plant experiments, introducing monohybrid crosses (single trait like stem height) and dihybrid crosses (two traits like seed shape and colour). These reveal fundamental laws: dominance, segregation, and independent assortment. Students learn to construct Punnett squares, predict phenotypic and genotypic ratios, and analyse inheritance patterns. The chapter also explains sex determination in humans—how the father’s sperm (X or Y) decides the child’s sex, and the role of the Y chromosome. Moving beyond individual inheritance, it connects to evolution by showing how variations arising from sexual reproduction accumulate over generations, potentially leading to speciation. Typical exam questions include: determining genotypes from given phenotypes, drawing Punnett squares for mono- and dihybrid crosses, calculating progeny ratios, and explaining mechanisms like independent assortment or sex determination. Some questions integrate evolution, asking how variations fuel natural selection. This chapter requires logical reasoning and clear diagrammatic skills. Our question paper generator offers targeted practice, helping teachers create assessments that mirror CBSE patterns and strengthen students’ conceptual understanding and problem-solving ability.

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Science — Heredity

Class 10Time: 3 hrsMax Marks: 80

SECTION A

1.
The ABO system of human blood groups is an example of multiple alleles. How many different alleles of the ABO gene exist in the population?
(a) 2(b) 3(c) 4(d) 5
1
2.
In a dihybrid cross, an F1 pea plant with genotype RrYy (round yellow) is self-pollinated. Among the F2 offspring with round green seeds, what fraction is homozygous for both traits (RRyy)?
(a) 1/16(b) 1/4(c) 1/3(d) 2/3
1
3.
A pea plant with genotype RrYy (round, yellow seeds) is test-crossed with a plant of genotype rryy (wrinkled, green). What proportion of the offspring will be wrinkled and green?
(a) 1/2(b) 1/4(c) 1/8(d) 3/16
1

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In a CBSE exam, this chapter typically contributes questions across the following types. The last column shows how many original questions of each type we have ready in our bank for this chapter:

Question type	Marks each	In our bank
Multiple Choice (MCQ)	1 mark	13
Assertion–Reason	1 mark	6
Short Answer	2 marks	8
Short Answer	3 marks	6
Long Answer	5 marks	5
Case Study	4 marks	6

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Q1. The ABO system of human blood groups is an example of multiple alleles. How many different alleles of the ABO gene exist in the population?
1 mark
Multiple Choice (MCQ)
(A) 2(B) 3(C) 4(D) 5
▸ Answer▾ Answer
3
Q2. In a dihybrid cross, an F1 pea plant with genotype RrYy (round yellow) is self-pollinated. Among the F2 offspring with round green seeds, what fraction is homozygous for both traits (RRyy)?
1 mark
Multiple Choice (MCQ)
(A) 1/16(B) 1/4(C) 1/3(D) 2/3
▸ Answer▾ Answer
1/3
Q3. A pea plant with genotype RrYy (round, yellow seeds) is test-crossed with a plant of genotype rryy (wrinkled, green). What proportion of the offspring will be wrinkled and green?
1 mark
Multiple Choice (MCQ)
(A) 1/2(B) 1/4(C) 1/8(D) 3/16
▸ Answer▾ Answer
1/4
Q4. A cross that studies the inheritance of a single contrasting trait is best described as a:
1 mark
Multiple Choice (MCQ)
(A) Dihybrid cross(B) Reciprocal cross(C) Monohybrid cross(D) Test cross
▸ Answer▾ Answer
Monohybrid cross
Q5. Assertion (A): When a heterozygous tall pea plant (Tt) is crossed with a dwarf pea plant (tt), all the offspring are tall. Reason (R): The tall allele is dominant over the dwarf allele.
1 mark
Assertion–Reason
(A) Both A and R are true and R is the correct explanation of A.(B) Both A and R are true but R is not the correct explanation of A.(C) A is true but R is false.(D) A is false but R is true.
▸ Answer▾ Answer
A is false but R is true.
Q6. Mendel performed a dihybrid cross between pure round‑yellow and wrinkled‑green pea plants. In the F2 generation he obtained four phenotypic combinations in the ratio 9:3:3:1. How does this ratio demonstrate independent assortment of the two pairs of traits? Explain with the help of a Punnett square.
2 marks
Short Answer
▸ Answer▾ Answer
The 9:3:3:1 ratio shows that the two traits are inherited independently. During gamete formation, the alleles for seed shape and seed colour segregate and recombine at random, producing four types of gametes in equal proportions (RY, Ry, rY, ry). When these gametes fuse, the Punnett square yields nine round‑yellow, three round‑green, three wrinkled‑yellow and one wrinkled‑green. This confirms that the alleles of different genes do not influence each other’s segregation, which is the basis of Mendel’s Law of Independent Assortment.
Q7. In garden pea, round seeds (R) are dominant over wrinkled (r). A heterozygous round-seeded plant is crossed with a pure wrinkled-seeded plant. Work out the cross and state the genotypic and phenotypic ratios of the offspring.
2 marks
Short Answer
▸ Answer▾ Answer
Cross: Rr × rr. Gametes: R, r and r. Offspring: Rr (round) and rr (wrinkled) in 1:1 ratio. Genotypic ratio: 1 Rr : 1 rr, phenotypic ratio: 1 round : 1 wrinkled.
Q8. In humans, brown eye colour (B) is dominant over blue eye colour (b). A brown-eyed man marries a blue-eyed woman and they have three children: two brown-eyed and one blue-eyed. Determine the genotype of the man. Support your answer with a Punnett square.
3 marks
Short Answer
▸ Answer▾ Answer
The blue-eyed woman is homozygous recessive (bb). The blue-eyed child must have inherited one b allele from each parent, so the father must possess at least one b allele. Since the father is brown-eyed, he must be heterozygous (Bb). A Punnett square of Bb (father) × bb (mother) gives a 1:1 ratio of brown (Bb) to blue (bb) offspring, matching the observed distribution. If the father were BB, all children would be brown-eyed.
Q9. What is a dihybrid cross? Explain how Mendel’s experiment with round yellow and wrinkled green pea seeds illustrates the law of independent assortment.
3 marks
Short Answer
▸ Answer▾ Answer
A dihybrid cross involves two pairs of contrasting traits. Mendel crossed pure round yellow (RRYY) with wrinkled green (rryy) peas. The F1 were all round yellow (RrYy). On selfing F1, the F2 generation showed four phenotypes in a 9:3:3:1 ratio (round yellow, round green, wrinkled yellow, wrinkled green). This shows that alleles for seed shape and seed colour are inherited independently of each other, demonstrating the law of independent assortment.
Q10. In pea plants, tall (T) is dominant over dwarf (t). A farmer crossed a homozygous tall plant (TT) with a dwarf plant (tt). The F1 progeny was self-pollinated to obtain the F2 generation. Predict the phenotypic and genotypic ratios in the F2 progeny. Illustrate with a Punnett square.
5 marks
Long Answer
▸ Answer▾ Answer
Initial cross: TT (tall) × tt (dwarf) → F1 all Tt (tall). Self-pollination of F1: Tt × Tt. Gametes: T and t from each parent. Punnett square: - T from female, T from male: TT (tall) - T from female, t from male: Tt (tall) - t from female, T from male: Tt (tall) - t from female, t from male: tt (dwarf) Thus, genotypic ratio: 1 TT : 2 Tt : 1 tt; phenotypic ratio: 3 tall : 1 dwarf.
Q11. Explain the mechanism of sex determination in humans with a neat labelled cross. Why is the father responsible for determining the sex of the child? What is the scientific significance of the Y chromosome in this process?
5 marks
Long Answer
▸ Answer▾ Answer
Humans have 23 pairs of chromosomes, including one pair of sex chromosomes. Females possess two X chromosomes (XX), while males have one X and one Y chromosome (XY). During gametogenesis, females produce eggs all containing one X chromosome. Males produce sperm: half contain an X chromosome and half a Y chromosome. The cross: Parents: Female (XX) × Male (XY) Gametes: X (from female) and X or Y (from male) Offspring: XX (female) if an X-sperm fertilizes the egg; XY (male) if a Y-sperm fertilizes the egg. The ratio is 1:1. The father is responsible because the mother always contributes an X; the father's sperm determines the sex by contributing either X or Y. The Y chromosome carries the SRY gene, which triggers the development of male characteristics. Absence of the Y (i.e., XX) leads to female development. Thus, the Y chromosome is the key determinant of maleness.
Q12. Different organisms have different mechanisms of sex determination. In humans, the XY system determines sex. In birds, the ZW system is present where males are ZZ and females are ZW. In honeybees, sex is determined by the number of sets of chromosomes; males are haploid, and females are diploid. In some reptiles like turtles, the temperature during egg incubation decides the sex of the offspring.
4 marks
Case Study
1. (i) In birds, which sex chromosome combination produces a male? How does this differ from humans?1 mark
2. (ii) Explain how a male honeybee is produced and why it cannot have a father.1 mark
3. (iii) In a turtle species, eggs incubated at 25°C produce only males, while at 30°C produce only females. Compare this with genetic sex determination and discuss one advantage of such a system in evolution.2 marks
▸ Answer▾ Answer
Birds: male ZZ vs human male XY. Male honeybees are haploid from unfertilized eggs, so no father. Temperature-dependent sex determination allows environmental influence on sex ratio.

Frequently asked questions

What is the difference between a monohybrid and a dihybrid cross in Mendel's experiments?

A monohybrid cross involves one pair of contrasting traits (e.g., tall vs. dwarf pea plants), studying the inheritance of a single gene. A dihybrid cross involves two pairs of contrasting traits (e.g., round yellow vs. wrinkled green seeds), demonstrating how two genes segregate independently. In the F2 generation, monohybrid cross gives a 3:1 phenotypic ratio, while dihybrid cross gives a 9:3:3:1 ratio.

How do we determine the genotype of a parent from a given cross of offspring?

By analysing the phenotypes of the offspring. For example, if a brown-eyed man (dominant) and a blue-eyed woman (recessive) have a blue-eyed child, the man must be heterozygous (Bb). If all children were brown-eyed, he might be homozygous dominant (BB). Using a Punnett square confirms the possible genotypic combinations.

How does sex determination in humans work?

Humans have 23 pairs of chromosomes. Females are XX, producing only eggs with X chromosome. Males are XY, producing sperm with either X or Y. The sex of the child is determined by the sperm: X-bearing sperm + X egg = female (XX); Y-bearing sperm + X egg = male (XY). Thus, the father’s sperm determines the sex.

Why are variations important, and how do they lead to speciation?

Variations are differences among individuals of a species, arising from genetic recombination during sexual reproduction and mutations. These variations provide survival advantages under changing environments. Accumulation of advantageous variations over many generations, especially when populations become isolated, can lead to the formation of new species (speciation). For example, Darwin’s finches on the Galápagos Islands evolved different beak shapes adapted to different food sources.

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