Heredity — Class 10 Science practice questions

Question 1

In humans, a newborn girl is found to have the sex chromosome combination XX in all her body cells. One X chromosome in this combination was inherited from her father and one from her mother. Which of the following statements correctly explains how this girl received an X chromosome from her father?

Accepted Answer

Option (A) is correct.

Explanation: In humans, males have the sex chromosome combination XY in their body cells. During sperm formation (meiosis), the X and Y chromosomes separate into different sperms — half the sperms produced carry the X chromosome and half carry the Y chromosome. When the X-bearing sperm from the father fertilised the mother's egg (which always carries an X chromosome), the resulting zygote received one X from the father and one X from the mother, giving the XX combination that develops into a girl. Options (B) and (D) incorrectly describe the father's sex chromosome constitution, and Option (C) incorrectly states that both X and Y chromosomes are present together in every sperm, which contradicts the separation of sex chromosomes during meiosis.

Question 2

During gamete formation in humans, a mother produces eggs and a father produces sperms. Which of the following statements correctly describes the sex chromosomes present in the gametes of each parent?

Accepted Answer

Option (A) is correct.

Explanation: In humans, females have the sex chromosome constitution XX and males have XY. During gamete formation (meiosis), each gamete receives only one sex chromosome. Since both chromosomes in a female are X, every egg produced carries only the X chromosome. In a male, one sex chromosome is X and the other is Y, so half the sperms carry the X chromosome and the other half carry the Y chromosome. Thus, all eggs carry X while sperms carry either X or Y.

Question 3

Assertion (A): In a cross between a tall pea plant and a short pea plant, all plants in the first generation (F1) are tall.
Reason (R): The trait that appears in the F1 generation is called the dominant trait, and it masks the expression of the other trait called the recessive trait.

Accepted Answer

Option (A) is correct.

Explanation: When a tall pea plant (TT) is crossed with a short pea plant (tt), all F1 offspring inherit one T allele and one t allele (Tt). Since tall (T) is dominant over short (t), all F1 plants appear tall — Assertion A is true. The Reason correctly defines that the trait expressed in F1 is the dominant trait and that it masks the unexpressed recessive trait — Reason R is also true. Furthermore, R directly explains why all F1 plants are tall: tallness is dominant and masks the recessive shortness in the Tt genotype. Therefore, R is the correct explanation of A.

Question 4

In humans, a couple has three daughters. The husband claims that his wife is responsible for producing only girl children. A biology student disagrees and explains that the sex of a child is determined by the father, not the mother. Which of the following correctly justifies the student's explanation?

Accepted Answer

Option (A) is correct.

Explanation: In humans, females have the sex chromosome constitution XX and males have XY. During oogenesis, every egg produced by the mother carries only one X chromosome — the mother has no Y chromosome to contribute, so she cannot produce two types of eggs with respect to sex chromosomes. During spermatogenesis, the father produces two types of sperms in equal proportion: 50% carrying an X chromosome and 50% carrying a Y chromosome. If an X-bearing sperm fertilises the egg, the resulting zygote is XX (female); if a Y-bearing sperm fertilises the egg, the zygote is XY (male). Since the mother always contributes X and it is the father's sperm that determines whether the zygote receives X or Y, the sex of the child is determined entirely by the father. Option (B) is incorrect because not all sperms carry Y — half carry X and half carry Y. Option (C) is incorrect because the mother produces only one type of egg (X-bearing), not two types. Option (D) is incorrect because only the father produces two types of gametes with respect to sex chromosomes; the mother produces only one type.

Question 5

Assertion (A): In a cross between tall and short pea plants, all plants in the F1 generation are tall.
Reason (R): The trait for tallness is dominant over the trait for shortness, so the recessive short trait is masked and does not express itself in the F1 generation.

Accepted Answer

Option (a) is correct.

Explanation: The Assertion is true — when Mendel crossed pure-breeding tall (TT) pea plants with pure-breeding short (tt) plants, all F₁ offspring inherit one T allele and one t allele (Tt), and all appear tall. The Reason is also true and correctly explains the Assertion: tallness is dominant over shortness, so in the heterozygous Tt condition the dominant T allele is expressed while the recessive t allele is completely masked, resulting in a uniformly tall F₁ generation. Since R directly states the principle of dominance that causes the observation described in A, R is the correct explanation of A.

Question 6

In humans, sex chromosomes are inherited from both parents. A biology student prepares a table listing the sex chromosome composition of body cells and gametes in males and females. Which of the following rows in the student's table is completely correct?

Accepted Answer

Option (A) is correct.

Explanation: In humans, females have XX sex chromosomes in their body cells, and males have XY sex chromosomes in their body cells. During meiosis (gamete formation), each parent contributes one chromosome of each homologous pair. Since both chromosomes in a female are X, every egg produced carries only one X chromosome — making all female gametes identical in sex chromosome composition (X only). In a male, one sex chromosome is X and the other is Y; meiosis separates these, so half the sperms carry X and the other half carry Y, in equal proportion. Option (A) correctly states all four entries: Female body cells = XX, Female gametes = X only, Male body cells = XY, Male gametes = X and Y in equal proportion.

Question 7

A human female produces eggs and a human male produces sperms. Which of the following statements correctly describes the sex chromosome present in the egg and in each type of sperm produced by the male?

Accepted Answer

Option (A) is correct.

Explanation: A human female has the sex chromosome constitution XX in her body cells. During meiosis (egg formation), each egg receives only one sex chromosome, which is always X — so every egg carries one X chromosome. A human male has the sex chromosome constitution XY. During spermatogenesis, the X and Y chromosomes segregate into different sperms, producing two types: 50% carrying X and 50% carrying Y. The sex of the offspring is therefore determined by the type of sperm that fertilises the egg — an X-bearing sperm produces a female (XX) and a Y-bearing sperm produces a male (XY).

Question 8

A genetics student notes that in humans, females have the sex chromosome combination XX, while males have XY. During the formation of sperms, a male produces two types of sperms in equal proportion. Which of the following statements correctly identifies the two types of sperms produced by a human male and explains which type determines the birth of a male child?

Accepted Answer

Option (A) is correct.

Explanation: Human males have the genotype XY. During meiosis (spermatogenesis), the X and Y chromosomes segregate into separate sperms, producing two types in equal proportion — 50% carrying the X chromosome and 50% carrying the Y chromosome. All eggs produced by females (XX) carry only the X chromosome. When a Y-carrying sperm fertilises the egg, the resulting zygote is XY, giving a male child; when an X-carrying sperm fertilises the egg, the zygote is XX, giving a female child. Therefore, it is the sperm carrying the Y chromosome that determines the birth of a male child, and both types of sperms are produced in equal numbers.

Question 9

Assertion (A): When Mendel crossed tall pea plants with dwarf pea plants, all plants in the F1 generation were tall.
Reason (R): The trait for tallness is dominant over dwarfness, so the dominant allele masks the expression of the recessive allele in a heterozygous organism.

Accepted Answer

Option (a) is correct.

The Assertion is true: in Mendel's monohybrid cross between pure tall (TT) and pure dwarf (tt) plants, all F₁ offspring are heterozygous (Tt) and therefore all exhibit the tall phenotype. The Reason is also true: tallness (T) is dominant over dwarfness (t), meaning that in a heterozygous (Tt) organism the dominant allele T masks the expression of the recessive allele t, producing the tall phenotype. The Reason correctly and directly explains why every F₁ plant was tall — because the dominant T allele suppresses the expression of t whenever both are present together. Thus R is the correct explanation of A.

Question 10

Assertion (A): In Mendel's experiment, when tall pea plants were crossed with dwarf pea plants, all plants in the first filial (F1) generation were tall.
Reason (R): The trait for tallness is dominant over the trait for dwarfness, so the recessive dwarf trait is masked and does not appear in the F1 generation.

Accepted Answer

Option (A) is correct.

The Assertion is true: when Mendel crossed pure tall (TT) plants with pure dwarf (tt) plants, all F₁ offspring had the genotype Tt and showed the tall phenotype, so all F₁ plants were indeed tall. The Reason is also true: tallness is dominant over dwarfness, and in the heterozygous Tt condition the dominant T allele masks the recessive t allele, so the dwarf trait does not appear in the F₁ generation. The Reason is the correct explanation of the Assertion, because it is precisely the dominance of the tall allele over the dwarf allele that causes all F₁ plants to be tall despite carrying the recessive t allele.

Question 11

Assertion (A): In a cross between a tall pea plant and a short pea plant, all plants in the F1 generation are tall.
Reason (R): The trait for tallness is dominant over shortness, so the dominant allele masks the expression of the recessive allele in a heterozygous condition.

Accepted Answer

Option (A) is correct.

The Assertion is true: when a homozygous tall plant (TT) is crossed with a homozygous short plant (tt), all F₁ offspring are heterozygous (Tt) and therefore tall, as Mendel demonstrated in his monohybrid cross. The Reason is also true: tallness is dominant over shortness, and the dominant allele T masks the expression of the recessive allele t whenever both are present together in the heterozygous condition (Tt). The Reason directly and correctly explains the Assertion, since it is precisely this dominance relationship that causes all F₁ plants to appear tall despite carrying the recessive allele for shortness.

Question 12

Assertion (A): When Mendel crossed tall pea plants with dwarf pea plants, all plants in the first filial generation (F1) were tall.
Reason (R): The trait that appears in the F1 generation is called the dominant trait, and it masks the expression of the other trait called the recessive trait.

Accepted Answer

Option (a) is correct.

The Assertion is true: when Mendel crossed pure tall (TT) pea plants with pure dwarf (tt) pea plants, all F₁ offspring received the genotype Tt and expressed the tall phenotype, so all F₁ plants were indeed tall. The Reason is also true: the trait that expresses itself in the F₁ generation is called the dominant trait, and it masks the expression of the other trait, which is called the recessive trait. The Reason directly and correctly explains the Assertion — all F₁ plants appeared tall precisely because tallness is the dominant trait and it masked the recessive dwarf trait in the heterozygous (Tt) condition.

Question 13

Assertion (A): When Mendel crossed tall pea plants with dwarf pea plants, all plants in the first generation (F1) were tall.
Reason (R): The trait that appears in the F1 generation is called the dominant trait, and it masks the expression of the other trait called the recessive trait.

Accepted Answer

Option (A) is correct.

The Assertion is true: in Mendel's monohybrid cross between pure tall (TT) and pure dwarf (tt) pea plants, all F₁ offspring were tall (Tt), because tallness is dominant over dwarfness. The Reason is also true: the trait that appears in the F₁ generation is called the dominant trait, and it masks the expression of the other trait, which is called the recessive trait. The Reason is the correct explanation of the Assertion, because it is precisely this principle of dominance — the masking of the recessive allele (t) by the dominant allele (T) — that causes all F₁ plants to appear tall even though each plant carries the allele for dwarfness.

Question 14

A human male has 46 chromosomes in each body cell — 44 autosomes and 2 sex chromosomes (XY). During sperm formation, the sex chromosomes separate into different sperms. A particular sperm is found to carry only autosomes and no sex chromosome other than X. When this sperm fertilises a normal egg, what will be the sex chromosome composition of the resulting zygote and the sex of the child?

Accepted Answer

Option (A) is correct.

Explanation: During sperm formation (meiosis), the sex chromosomes X and Y separate into different sperms — half the sperms carry X and half carry Y. The sperm described carries only autosomes and the X chromosome, making it an X-bearing sperm. A normal egg is always X-bearing, because females have XX sex chromosomes and meiosis produces eggs each carrying one X chromosome. When this X-bearing sperm fertilises the normal X-bearing egg, the resulting zygote receives one X from the sperm and one X from the egg, giving an XX sex chromosome composition. An XX individual develops as a female. ∴ The zygote is XX and the child will be a girl.

Question 15

A biology teacher states: 'In humans, the mother always contributes the same type of sex chromosome to every child she produces, whereas the father contributes either of two different types.' Which of the following correctly explains this statement?

Accepted Answer

Option (A) is correct.

Explanation: In humans, females have the sex chromosome constitution XX. During egg formation (oogenesis), meiosis produces eggs that each carry only one X chromosome — so every egg produced by the mother contains an X chromosome, and the mother always contributes the same type of sex chromosome to every child. Males have the sex chromosome constitution XY. During sperm formation (spermatogenesis), meiosis produces two types of sperm in equal proportions — 50% carrying an X chromosome and 50% carrying a Y chromosome. When an X-bearing sperm fertilises the egg, the child is XX (female); when a Y-bearing sperm fertilises the egg, the child is XY (male). It is therefore the father's sperm — not the mother's egg — that determines the sex of the child. Option (A) states this correctly. Options (B), (C), and (D) incorrectly assign chromosome constitutions or incorrectly identify which parent determines sex.

Question 16

In humans, every body cell contains 46 chromosomes — 44 autosomes and 2 sex chromosomes. A biology student states that the sex of a newborn is solely determined by the type of sex chromosome present in the fertilising sperm, not in the egg. Which of the following correctly justifies the student's statement?

Accepted Answer

Option (A) is correct.

Explanation: In human females (XX), meiosis produces eggs that carry only the X chromosome — there is no variation in the sex chromosome of the egg. In human males (XY), meiosis produces two types of sperm in equal proportions: 50% carry the X chromosome and 50% carry the Y chromosome. When an X-bearing sperm fertilises the egg, the zygote is XX (female); when a Y-bearing sperm fertilises the egg, the zygote is XY (male). Since the egg always contributes only X, it is exclusively the type of sex chromosome present in the fertilising sperm that determines the sex of the newborn. This directly justifies the student's statement. Option (B) is incorrect because males produce both X-bearing and Y-bearing sperm, not only Y-bearing sperm. Option (C) is incorrect because the mother's eggs carry only X, so the mother does not determine the sex. Option (D) is incorrect because eggs never carry the Y chromosome.

Question 17

Assertion (A): In a cross between a tall pea plant and a short pea plant, all plants in the F1 generation are tall.
Reason (R): The trait for tallness is dominant over shortness, so the dominant allele masks the expression of the recessive allele in a heterozygous organism.

Accepted Answer

Option (A) is correct.

The Assertion is true: in Mendel's monohybrid cross between a pure tall plant (TT) and a pure short plant (tt), all F₁ offspring are heterozygous (Tt) and appear tall. The Reason is also true: tallness is dominant over shortness, and in a heterozygous organism (Tt), the dominant allele (T) masks the expression of the recessive allele (t) — this is Mendel's principle of dominance. The Reason directly and correctly explains why all F₁ plants are tall, since it is precisely this masking of the recessive allele by the dominant allele that produces a uniform tall F₁ generation. Therefore, both A and R are true and R is the correct explanation of A.

Question 18

Assertion (A): When Mendel crossed tall pea plants with dwarf pea plants, all plants in the first filial (F1) generation were tall.
Reason (R): The trait that appears in the F1 generation is called the dominant trait, and it masks the expression of the other trait, which is called the recessive trait.

Accepted Answer

Option (A) is correct.

The Assertion is true: when Mendel crossed pure tall (TT) pea plants with pure dwarf (tt) pea plants, all F₁ offspring were tall (Tt), since the tall allele dominates over the dwarf allele. The Reason is also true: the trait expressed in the F₁ generation is called the dominant trait, and it masks the expression of the other trait, which is called the recessive trait. The Reason is the correct explanation of the Assertion, because it is precisely this dominance of the tall allele over the dwarf allele that causes all F₁ plants to appear tall while the dwarf trait remains hidden.

Question 19

Assertion (A): In a monohybrid cross between a tall pea plant and a short pea plant, all plants in the F1 generation are tall.
Reason (R): When two contrasting traits are present together in an organism, only one trait expresses itself and is called the dominant trait, while the other remains hidden and is called the recessive trait.

Accepted Answer

Option (A) is correct.

The Assertion is true: when Mendel crossed pure tall pea plants with dwarf pea plants, all F₁ offspring were tall, because the allele for tallness completely masks the allele for dwarfness. The Reason is also true and correctly states that in a monohybrid cross, the trait expressed in F₁ is the dominant trait while the hidden trait is the recessive trait. The Reason directly and correctly explains the Assertion — all F₁ plants appeared tall precisely because tallness is the dominant trait and dwarfness, being recessive, remained hidden. Hence, R is the correct explanation of A.

Question 20

Consider the following statements about sex chromosomes and sex determination in humans:

(i) Every egg produced by a human female carries one X chromosome as its sex chromosome.
(ii) A human male produces two types of sperms — 50% carrying X chromosome and 50% carrying Y chromosome.
(iii) If a Y-chromosome-carrying sperm fertilises the egg, the resulting child will be a girl.
(iv) The sex of a child is determined by the type of sex chromosome inherited from the mother.

Which of the following combinations lists ONLY the correct statements?

Accepted Answer

Option (A) is correct.

Explanation: Human females have XX sex chromosomes, so during egg formation every egg receives exactly one X chromosome — statement (i) is correct. Human males have XY sex chromosomes, so spermatogenesis produces two types of sperms in equal proportion — 50% carrying X and 50% carrying Y — statement (ii) is correct. Statement (iii) is incorrect because a Y-bearing sperm fertilising an X-bearing egg produces an XY zygote, which develops into a boy, not a girl. Statement (iv) is incorrect because the mother always contributes only an X chromosome; it is the type of sperm from the father that determines the sex of the child. Therefore, only statements (i) and (ii) are correct.

Heredity: Class 10 Science Practice Questions

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