CBSE Class 10 Science Heredity VBQs Set 02

Accumulation of Variation During Reproduction

VSA

Question. How many pairs of chromosomes are present in human beings ?
Answer: There are 23 pairs of chromosomes present in human beings.

Question. All the variations in a species do not have equal chances of survival. Why?
Answer: Variations do not have equal chances of survival because survival depends on the nature of the variations and their interaction with the environment. Only those variations that are advantageous to the organism in its specific environment are selected by nature.

SA I 

Question. Justify the statement ‘‘Sex of the children will be determined by what they inherit from their father’’.
Answer: In humans, females have two X chromosomes (XX), meaning all eggs carry an X chromosome. Males have one X and one Y chromosome (XY), meaning 50% of sperms carry an X chromosome and 50% carry a Y chromosome. If an X-carrying sperm fertilizes the egg, the child is a girl (XX). If a Y-carrying sperm fertilizes the egg, the child is a boy (XY). Thus, the father's contribution determines the sex.

Question. ‘‘Sex chromosomes in human males and females are XY and XX respectively. Statistical probability of getting either a male or a female child is 50%. Justify this statement giving reason.
Answer: A female produces only one type of gamete (ovum) containing the X chromosome. A male produces two types of gametes (sperms) in equal proportions: one containing the X chromosome and the other containing the Y chromosome. Since the fusion of an X or Y sperm with the ovum is a matter of chance and both types of sperms are produced in 50:50 ratio, the statistical probability of a child being male or female is exactly 50%.

Question. What is variation? List two main reasons that may lead to variation in a population.
Answer: Variation refers to the differences in traits or characteristics among individuals of the same species. Two main reasons for variation are:
1. Errors during DNA copying/replication.
2. Recombination of genes during sexual reproduction (crossing over during gamete formation).

SA II 

Question. (a) Name the two types of gametes produced by men.
(b) Does a male child inherit X chromosome from his father? Justify.
(c) How many types of gametes are produced by a human female?
Answer: (a) The two types of gametes are X-carrying sperm and Y-carrying sperm.
(b) No, a male child does not inherit an X chromosome from his father. A male child (XY) must inherit the Y chromosome from his father and the X chromosome from his mother.
(c) A human female produces only one type of gamete, which always carries an X chromosome.

LA 

Question. Sex of an individual is determined by different factors in various species. Some animals rely entirely on the environmental cues, while in some other animals the individuals scan change their sex during their life time indicating that sex of some species is not genetically determined. However, in human beings, the sex of an individual is largely determined genetically.
(a) In what way are the sex chromosomes ‘X’ and ‘Y’ different in size? Name the mismatched pair of sex chromosome in humans.
(b) Write the number of pair/pairs of sex chromosomes present in human beings. In which one of the parent (male/female) perfect pair/pairs of sex chromosomes are present?
(c) Citing two examples, justify the statement “Sex of an individual is not always determined genetically”.
Answer: (a) The 'X' chromosome is larger in size compared to the 'Y' chromosome, which is a short, small chromosome. The mismatched pair in humans is the XY pair found in males.
(b) Humans have 1 pair of sex chromosomes. A perfect pair (XX) is present in the female parent.
(c) In some animals, sex is determined by environmental factors. For example, in some reptiles like turtles, the temperature at which fertilized eggs are kept determines the sex. In snails, individuals can change their sex, indicating it is not genetically fixed.
 

Question. (a) ‘‘Sexual reproduction gives rise to more viable variations than asexual reproduction.’’ Justify this statement and explain how the viable variations affect the evolution of those organisms that reproduce sexually.
(b) Does genetic combination of mothers play a significant role in determining the sex of a newborn ? Give reason to justify your answer.
Answer: (a) Sexual reproduction involves the fusion of gametes from two different individuals, leading to the shuffling of genetic material through crossing over and independent assortment. This creates a vast array of new genetic combinations. These viable variations are the raw material for evolution, as they allow species to adapt to changing environments, increasing their survival chances.
(b) No, the mother's genetic combination does not play a significant role because she is homogametic (XX) and always contributes an X chromosome. The sex of the newborn depends entirely on whether the father contributes an X or a Y chromosome.

MCQ

Question. A cross between pea plant with white flowers (vv) and pea plant with violet flowers (VV) resulted in F2 progeny in which ratio of violet (VV) and white (vv) flowers will be
(a) 1 : 1
(b) 2 : 1
(c) 3 : 1
(d) 1 : 3.
Answer: (c) 3 : 1

Question. Assertion (A) : In humans, if gene (B) is responsible for black eyes and gene (b) responsible for brown eyes, then the colour of eyes of the progeny having gene combination Bb, bb or BB will be black only.
Reason (R) : The black colour of the eyes is a dominant trait.
(a) Both (A) and (R) are true and (R) is the correct explanation of (A).
(b) Both (A) and (R) are true and (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: (d) (A) is false but (R) is true.

Question. Which one of the given statements is incorrect?
(a) DNA has the complete information for a particular characteristic.
(b) DNA is the molecule responsible for the inheritance of characters from parents to offsprings.
(c) Change in information will produce a different protein.
(d) Characteristics will remain the same even if protein changes.
Answer: (d) Characteristics will remain the same even if protein changes.

Question. Consider the following two statements:
(i) The trait that expresses itself in F1 generation.
(ii) The trait that keeps on passing from one generation to another.
The appropriate terms for the statements (i) and (ii) respectively are
(a) Recessive trait ; Dominant trait
(b) Dominant trait ; Recessive trait
(c) Dominant trait ; Inherited trait
(d) Recessive trait ; Inherited trait
Answer: (c) Dominant trait ; Inherited trait

Question. Assertion (A) : Human population show a great deal of variations in traits.
Reason (R) : All variations in a species have equal chances of surviving in the environment in which they live.
(a) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A)
(b) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A)
(c) Assertion (A) is true, but Reason (R) is False.
(d) Assertion (A) is false, but Reason (R) is true.
Answer: (c) Assertion (A) is true, but Reason (R) is False.

Question. Assertion (A) : The sex of a child in human beings will be determined by the type of chromosome he/she inherits from the father.
Reason (R) : A child who inherits ‘X’ chromosome from his father would be a girl (XX), while a child who inherits a ‘Y’ chromosome from the father would be a boy (XY).
(a) Both (A) and (R) are true and reason (R) is the correct explanation of the assertion (A).
(b) Both (A) and (R) are true, but reason (R) is not the correct explanation of the assertion (A).
(c) (A) is true, but (R) is false.
(d) (A) is false, but (R) is true.
Answer: (a) Both (A) and (R) are true and reason (R) is the correct explanation of the assertion (A).

VSA 

Question. What is heredity?
Answer: Heredity is the transmission of characters or traits from parents to their offspring through genes.

Question. A Mendelian experiment consisted of breeding pea plants bearing violet flowers with pea plants bearing white flowers. What will be the result in F1 progeny?
Answer: All plants in the F1 progeny will bear violet flowers because violet is the dominant trait.

Question. Name the information source for making proteins in the cells.
Answer: Deoxyribonucleic acid (DNA) is the information source for making proteins in the cells.

Question. What is a gene?
Answer: A gene is a segment of DNA on a chromosome that provides information for one protein and acts as the functional unit of heredity.

Question. Why is the progeny always tall when a tall pea plant is crossed with a short pea plant?
Answer: The progeny is always tall because the 'tall' trait is dominant over the 'short' trait, and in the F1 generation, only the dominant trait expresses itself.

SA I 

Question. (i) In a cross between violet flowered plants and white flowered plants, state the characteristics of the plants obtained in the F1 progeny.
(ii) If the plants of F1 progeny are self-pollinated, then what would be observed in the plants of F2 progeny?
(iii) If 100 plants are produced in F2 progeny, then how many plants will show the recessive trait?
Answer: (i) All plants in the F1 progeny will have violet flowers.
(ii) In the F2 progeny, both violet and white flowered plants will appear in the ratio of 3:1.
(iii) 25 plants will show the recessive trait (white flowers).

SA II 

Question. How do Mendel’s experiments show that the traits are inherited independently ? Explain.
Answer: Mendel performed a dihybrid cross using plants with two pairs of contrasting characters (e.g., Round Yellow seeds vs Wrinkled Green seeds). In the F2 generation, he obtained four types of plants: parental combinations and new combinations (Round Green and Wrinkled Yellow). The appearance of these new combinations shows that the inheritance of one pair of traits (shape) is independent of the inheritance of the other pair (color).

Question. A cross was made between green-stemmed tomato plants denoted by (GG) and purple-stemmed tomato plants denoted as (gg) to obtain F1 progeny.
(a) What colour of the stem would you expect in their F1 progeny and why?
(b) Give the percentage of purple-stemmed plants if F1 plants are allowed to self-pollinate to produce F2 progeny.
(c) Write the ratio between GG and gg plants in the F2 progeny.
Answer: (a) Green stem, because green (G) is the dominant trait over purple (g).
(b) 25% purple-stemmed plants.
(c) The ratio between GG and gg plants is 1:1.

Question. (a) Why did Mendel carry out an experiment to study inheritance of two traits in garden pea?
(b) What were his findings with respect to inheritance of traits in F1 and F2 generation?
(c) State the ratio obtained in the F2 generation in the above mentioned experiment.
Answer: (a) Mendel carried out dihybrid crosses to see if different traits are inherited independently or together.
(b) In F1, only dominant traits were expressed. In F2, both parental and new recombinant combinations appeared.
(c) The phenotypic ratio in F2 is 9 : 3 : 3 : 1.

Question. A green stemmed rose plant denoted by GG and a brown stemmed rose plant denoted by gg are allowed to undergo a cross with each other.
(a) List your observations regarding :
(i) Colour of stem in their F1 progeny
(ii) Percentage of brown stemmed plants in F2 progeny if plants are self pollinated.
(iii) Ratio of GG and Gg in the F2 progeny.
Answer: (a) (i) All F1 plants will have green stems. (ii) 25% of F2 plants will be brown stemmed. (iii) The ratio of GG : Gg is 1 : 2.

Question. (a) Why is the F1 progeny always of tall plants when a tall plant is crossed with a short pea plant?
(b) How is F2 progeny obtained by self-pollination of F1 progeny different from F1 progeny? Give reason for this observation.
Answer: (a) It is because the tall trait is dominant and masks the expression of the short trait in the heterozygous F1 condition.
(b) F2 progeny contains both tall and short plants in a 3:1 ratio, whereas F1 had only tall plants. This is because the recessive 'short' alleles were hidden in F1 and segregated during gamete formation of F1 plants.

Question. Name the plant Mendel used for his experiment. What type of progeny was obtained by Mendel in \( F_1 \) and \( F_2 \) generations when he crossed the tall and short plants? Write the ratio he obtained in \( F_2 \) generation plants.
Answer: Mendel used the garden pea plant (Pisum sativum). In the \( F_1 \) generation, all plants were tall. In the \( F_2 \) generation, both tall and short plants were obtained. The phenotypic ratio in the \( F_2 \) generation was \( 3:1 \) (3 tall : 1 short).

Question. How did Mendel explain that it is possible that a trait is inherited but not expressed in an organism?
OR
With the help of an example justify the following statement: “A trait may be inherited, but may not be expressed.”
OR
“It is possible that a trait is inherited but may not be expressed.” Give a suitable example to justify this statement.
Answer: Mendel explained this through his monohybrid cross. When a pure tall (\( TT \)) plant is crossed with a pure dwarf (\( tt \)) plant, the \( F_1 \) offspring are all tall (\( Tt \)). Here, the dwarf trait is inherited from the parent but is not expressed because it is recessive and is masked by the dominant tall trait. This dwarf trait reappears in the \( F_2 \) generation when the plants are self-pollinated.

Question. In one of his experiments with pea plants Mendel observed that when a pure tall pea plant is crossed with a pure dwarf pea plant, in the first generation, \( F_1 \) only tall plants appear.
(a) What happens to the traits of the dwarf plants in this case?
(b) When the \( F_1 \) generation plants were self fertilized, he observed that in the plants of second generation \( F_2 \), both tall plants and dwarf plants were present. Why it happened? Explain briefly.
Answer: (a) The dwarf trait is inherited but remains hidden (unexpressed) in the \( F_1 \) generation because it is a recessive trait.
(b) This happened due to the Law of Segregation. The \( F_1 \) tall plants (\( Tt \)) produce two types of gametes (\( T \) and \( t \)). When these self-fertilize, the recessive alleles (\( t \)) can combine (\( tt \)), allowing the dwarf phenotype to reappear in the \( F_2 \) generation.

Question. How did Mendel interpret his result to show that traits may be dominant or recessive? Describe briefly.
Answer: Mendel observed that when he crossed two plants with contrasting traits, the \( F_1 \) generation showed only one of the traits. He called this the "dominant" trait. When he self-pollinated the \( F_1 \) plants, the trait that had disappeared in \( F_1 \) reappeared in the \( F_2 \) generation in 25% of the offspring. He called this the "recessive" trait. This showed that the \( F_1 \) plants carried the factors for both traits, but only the dominant one was visible.

Question. In a monohybrid cross between tall pea plants (\( TT \)) and short pea plants (\( tt \)), a scientist obtained only tall pea plants (\( Tt \)) in the \( F_1 \) generation. However, on selfing the \( F_1 \) generation pea plants, he obtained both tall and short plants in \( F_2 \) generation. On the basis of above observations with other angiosperms also, can the scientist arrive at a law? If yes, explain the law. If not, give justification for your answer.
Answer: Yes, the scientist can arrive at the "Law of Segregation". It states that during the formation of gametes, the two alleles of a gene pair separate (segregate) from each other so that each gamete receives only one allele. This ensures that the recessive trait, which was masked in the \( F_1 \) generation, has a chance to pair up and express itself in the \( F_2 \) generation.

Question. List two differences in tabular form between dominant trait and recessive traits. What percentage/proportion of the plants in the \( F_2 \) generation/progeny were round, in Mendel’s cross between round and wrinkled pea plants?
Answer:

• Dominant Trait: The trait that expresses itself in both homozygous and heterozygous conditions (e.g., \( TT \) or \( Tt \)).


• Recessive Trait: The trait that expresses itself only in the homozygous condition (e.g., \( tt \)).

In a cross between round and wrinkled pea plants, 75% (or 3/4) of the plants in the \( F_2 \) generation were round.

Question. Explain Mendel’s experiment with peas on inheritance of characters considering only one visible contrasting character.
Answer: Mendel selected pure-breeding tall (\( TT \)) and dwarf (\( tt \)) plants. Upon crossing them, the \( F_1 \) generation (\( Tt \)) were all tall. Upon self-pollinating the \( F_1 \) plants, the \( F_2 \) generation showed tall and dwarf plants in a \( 3:1 \) ratio. This experiment demonstrated that traits are determined by factors (genes) which segregate during gamete formation and that one trait (tallness) is dominant over the other (dwarfness).

Question. A cross was made between pure breeding pea plants, one with round and green seeds and the other with wrinkled and yellow seeds.
(a) Write the phenotype of \( F_1 \) progeny. Give reason for your answer.
(b) Write the different types of \( F_2 \) progeny obtained along with their ratio when \( F_1 \) progeny was selfed.
Answer: (a) Phenotype of \( F_1 \) progeny: Round and Yellow seeds. Reason: Round (\( R \)) and Yellow (\( Y \)) are dominant traits over wrinkled (\( r \)) and green (\( y \)).
(b) The \( F_2 \) progeny will have four types in the ratio \( 9:3:3:1 \):
1. Round Yellow (9)
2. Round Green (3)
3. Wrinkled Yellow (3)
4. Wrinkled Green (1)

Question. (a) Mendel crossed tall pea plants with dwarf pea plants in his experiment. Write his observations giving reasons on the \( F_1 \) and \( F_2 \) generations.
(b) List any two contrasting characters other than height that Mendel used in his experiments in pea plants.
Answer: (a) In \( F_1 \), all plants were tall because tallness is dominant. In \( F_2 \), plants were tall and dwarf in a \( 3:1 \) ratio because the traits segregate independently during gamete formation and recombine in the next generation.
(b) Two other contrasting characters: Flower color (Violet/White) and Seed shape (Round/Wrinkled).

LA 

The most obvious outcome of the reproductive process is the generation of individuals of similar design, but in sexual reproduction they may not be exactly alike. The resemblances as well as differences are marked. The rules of heredity determine the process by which traits and characteristics are reliably inherited. Many experiments have been done to study the rules of inheritance.

Question. (i) Why an offspring of human being is not a true copy of his parents in sexual reproduction?
(ii) While performing experiments on inheritance in plants, what is the difference between \( F_1 \) and \( F_2 \) generation?
(iii) Why do we say that variations are useful for the survival of a species over time?
OR
(iii) Study Mendel’s cross between two plants with a pair of contrasting characters.
\( RRYY \) × \( rryy \)
Round Yellow × Wrinkled Green
He observed 4 types of combinations in \( F_2 \) generation. Which of these were new combinations? Why do new features which are not present in the parents, appear in \( F_2 \) generation?
Answer: (i) Offspring are not true copies because sexual reproduction involves the fusion of gametes from two different parents and genetic recombination (crossing over) occurs during meiosis.
(ii) In the \( F_1 \) generation, all offspring express only the dominant trait. In the \( F_2 \) generation, both dominant and recessive traits appear in a specific ratio (e.g., \( 3:1 \)).
(iii) Variations are useful because they allow some individuals in a population to adapt and survive environmental changes, preventing the extinction of the species.
OR
(iii) The new combinations were Round Green and Wrinkled Yellow. New features appear because of the "Law of Independent Assortment," where different pairs of traits separate independently of each other during gamete formation, creating new genetic combinations.

Question. In some families, either rural or urban, females are tortured for giving birth to a female child. They do not seem to understand the scientific reason behind the birth of a boy or a girl. Infact the mother is not responsible for the sex of the child and it has been genetically proved that the sex of a newborn is determined by what the child inherits from the father.
(a) State the basis on which the sex of a newborn baby is determined in humans.
(b) Why is the pair of sex chromosomes called a mismatched pair in males?
(c) How is the original number of chromosomes present in the parents restored in the progeny?
OR
(c) Explain by giving two examples of the organisms in which the sex is not genetically determined.
Answer: (a) Sex is determined by the sex chromosome inherited from the father (an \( X \) chromosome results in a girl, a \( Y \) chromosome results in a boy).
(b) In males, the pair is \( XY \). The \( X \) chromosome is normal-sized, while the \( Y \) chromosome is much smaller and carries different genes, making them a mismatched pair.
(c) The original number is restored because gametes are haploid (contain half the chromosomes). During fertilization, the fusion of a male and female gamete forms a diploid zygote with the full set of chromosomes.
OR
(c) 1. Snails: They can change their sex during their lifetime. 2. Turtles: The incubation temperature of the eggs determines whether the offspring will be male or female.

Question. A student crossed pea plants having round and yellow seeds with pea plants having wrinkled and green seeds. He found that only one type of seeds were produced in the \( F_1 \) generation. When these \( F_1 \) generation pea plants were self-pollinated with each other, then in addition to the seed type of \( F_1 \) generation, some new types of seed combinations were also obtained in the \( F_2 \) generation.
(a) Mention the dominant traits observed in \( F_1 \) generation.
(b) What are the new possible combinations of seeds likely to be observed in \( F_2 \) generation ?
(c) Give reason why the traits which were not visible in the seeds of \( F_1 \) generation reappeared in the seeds of \( F_2 \) generation. Write the ratio of different types of seeds obtained in \( F_2 \) generation in this case.
OR
(c) What is meant by the terms (I) dominant, and (II) recessive traits ? Explain.
Answer: (a) The dominant traits are Round shape and Yellow color.
(b) The new combinations are Round Green and Wrinkled Yellow.
(c) The traits reappeared because of the Law of Independent Assortment. The phenotypic ratio is \( 9:3:3:1 \).
OR
(c) (I) Dominant trait: The trait that expresses itself even in the presence of an alternative allele (e.g., \( T \) in \( Tt \)). (II) Recessive trait: The trait that is masked by the dominant trait and only expresses itself when two identical alleles are present (e.g., \( t \) in \( tt \)).

The mechanism by which the sex of an individual is determined is called sex-determination. In human beings, sex of a newborn is genetically determined, whereas in some others it is not. There are 46 (23 pairs) chromosomes in human beings. Out of these, 44 (22 pairs) control the body characters and 2 (one pair) are known as sex chromosomes. The sex chromosomes are of two types – X chromosome and Y chromosome. At the time of fertilisation, depending upon which type of male gamete fuses with the female gamete, the sex of the newborn child is decided.

Question. (a) Why is a pair of sex chromosomes in human beings called a mismatched pair in terms of type and size?
(b) If the gametes always have half the number of chromosomes, then how is the original number of chromosomes restored in the organism?
(c) Name two animals whose sex is not genetically determined. Explain the process of their sex determination.
OR
(c) With the help of a flowchart only, show how sex is genetically determined in human beings.
Answer: (a) In males, the sex chromosomes are \( XY \). The \( X \) is larger while the \( Y \) is smaller, hence they are "mismatched" compared to the matching \( XX \) in females.
(b) Gametes are formed through meiosis, making them haploid (\( n \)). When a sperm (\( n \)) fertilizes an egg (\( n \)), the resulting zygote is diploid (\( 2n \)), restoring the original number.
(c) Snails (sex can change) and Crocodiles (temperature-dependent sex determination).
OR
(c) Flowchart:
Male (\( XY \)) × Female (\( XX \))
Gametes: (\( X \)), (\( Y \)) × (\( X \)), (\( X \))
Combinations:
\( X + X \)
\( \implies \) \( XX \) (Female)
\( Y + X \)
\( \implies \) \( XY \) (Male)

Mendel blended his knowledge of Science and Mathematics to keep the count of the individual exhibiting a particular trait in each generation. He observed a number of contrasting visible characters controlled in pea plants in a field. He conducted many experiments to arrive at the laws of inheritance.

Question. (a) What do the \( F_1 \) progeny of tall plants with round seeds and short plants with wrinkled seeds look like?
(b) Name the recessive traits in above case.
(c) Mention the type of the new combinations of plants obtained in \( F_2 \) progeny along with their ratio, if \( F_1 \) progeny was allowed to self pollinate.
OR
(c) If 1600 plants were obtained in \( F_2 \) progeny, write the number of plants having traits :
(i) Tall with round seeds
(ii) Short with wrinkled seeds
Write the conclusion of the above experiment.
Answer: (a) All \( F_1 \) progeny will be tall with round seeds.
(b) Shortness and wrinkled seed shape.
(c) New combinations: Tall Wrinkled and Short Round. Ratio: \( 9:3:3:1 \).
OR
(c) (i) Tall with round seeds: \( \frac{9}{16} \times 1600 = 900 \). (ii) Short with wrinkled seeds: \( \frac{1}{16} \times 1600 = 100 \). Conclusion: The traits for seed shape and plant height are inherited independently.

Question. How do Mendel’s experiments show that
(a) traits may be dominant or recessive?
(b) inheritance of two traits is independent of each other?
Answer: (a) Mendel showed this through a monohybrid cross where only the dominant trait (e.g., Tall) appeared in \( F_1 \), but the recessive trait (e.g., Short) reappeared in \( F_2 \).
(b) He showed this through a dihybrid cross where new combinations of traits (recombinants) appeared in the \( F_2 \) generation, proving that alleles for different traits assort independently.

Question. (a) Why did Mendel choose garden pea for his experiments ? Write two reasons.
(b) List two contrasting visible characters of garden pea Mendel used for his experiment.
(c) Explain in brief how Mendel interpreted his results to show that the traits may be dominant or recessive.
Answer: (a) 1. Peas have distinct contrasting characters. 2. They have a short life cycle and are easy to grow/cross-pollinate.
(b) 1. Seed color (Yellow/Green). 2. Seed shape (Round/Wrinkled).
(c) Mendel noted that in a cross between two pure-bred plants with contrasting traits, only one trait was expressed in the \( F_1 \) generation. This was the dominant trait. The other trait, which remained hidden in \( F_1 \) but reappeared in \( F_2 \), was the recessive trait.