Science · Textbook solutions

Heredity

Every solved example, exercise, and miscellaneous question, in the order the textbook teaches them. Plus every board question asked from this chapter. · 10 questions

8.1 Accumulation of Variation During Reproduction

2 q

Questions

Practice · 2

IT 8.1 Q1 $If a trait A exists in 10% of a population of an asexually reproducing species and a trait B exists in 60% of the same population, which trait is likely to have arisen earlier?$
IT 8.1 Q2 $How does the creation of variations in a species promote survival?$

8.2 Heredity

4 q

Questions

Practice · 4

IT 8.2 Q1 $How do Mendel's experiments show that traits may be dominant or recessive?$
IT 8.2 Q2 $How do Mendel's experiments show that traits are inherited independently?$
IT 8.2 Q3 $A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits - blood group A or O - is dominant? Why or why not?$
IT 8.2 Q4 $How is the sex of the child determined in human beings?$

Exercises

4 q

Ex 8 Q1 $A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers. The progeny all bore violet flowers, but almost half of them were short. This suggests that the genetic make-up of the tall parent can be depicted as$
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Ex 8 Q2 $A study found that children with light-coloured eyes are likely to have parents with light-coloured eyes. On this basis, can we say anything about whether the light eye colour trait is dominant or recessive? Why or why not?$
Ex 8 Q3 $Outline a project which aims to find the dominant coat colour in dogs.$
Ex 8 Q4 $How is the equal genetic contribution of male and female parents ensured in the progeny?$

What the board has asked from this chapter

Questions per paper · 67 papers · 2022 to 2026

2026

19 q

1 $Choose the correct statements with reference to chromosomes : (i) carry hereditary information from parents to next generation. (ii) are thread-like structures located inside the nucleus of an animal cell. (iii) always exist in pairs in human gametes. (iv) are involved in the process of cell division.$
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Expression of Traits
1 $\text{F}_2$
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Inherited Traits and Mendel's Experiments
4 $Identify the traits that cannot be transmitted from parents to the off-springs : (i) Swimming technique (ii) Blood group (iii) Eye colour (iv) Dancing skills Options :$
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Inherited Traits and Mendel's Experiments
5 $When a human egg is fertilized by a sperm having ‘Y’ chromosome, the zygote has the following combination of chromosomes :$
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Sex Determination
6 $\mathrm{F_1}$
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Inherited Traits and Mendel's Experiments
6 $Sex is determined by different factors in various species. However, in human beings, it is determined genetically. Which amongst the following option(s) is/are correct for human beings ? (i) Gamete carrying X chromosome from female parent. (ii) Gamete carrying X chromosome from male parent. (iii) Gamete carrying Y chromosome from male parent.$
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Sex Determination
Question No. 8 & 9 consists of two statements – Assertion (A) and Reason (R). Answer these questions selecting the appropriate option from (A), (B), (C) and (D) given below :
8 $Assertion (A) : Bacteria that can withstand heat have better chances of survival in a heat wave. Reason (R) : Accumulation of variations in a species increases the chances of its survival in changing environment.$
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Accumulation of Variation
13 $\mathrm{F_1}$
Inherited Traits and Mendel's Experiments
13 $\mathrm{F_2}$
Inherited Traits and Mendel's Experiments
14 $\mathrm{F_1}$
Inherited Traits and Mendel's Experiments
13 $\mathrm{F_2}$
Inherited Traits and Mendel's Experiments
13 $(a) A couple are parents to 4 daughters in a sequence, and do not have any son. Does this indicate that the husband does not produced Y-chromosome bearing sperms ? Explain. (b) What are the chances of this couple bearing yet another daughter ? Show with the help of a cross.$
Sex Determination
14 $(a) Giving two examples, justify the statement that the sex of the offspring in all animals is not always genetically determined. (b) Differentiate between the sex chromosomes of male and female human beings.$
Sex Determination

\mathrm{F_1}

Parents	$\mathrm{F_1}$	$\mathrm{F_2}$
A pea plant with red flowers was crossed with a pea plant with white flowers.	30 seeds were sown and all bore plants with red flowers.	44 seeds were sown, 33 plants bore red flowers and 11 plants bore white flowers.

Inherited Traits and Mendel's Experiments

14 $\mathrm{F_2}$
Inherited Traits and Mendel's Experiments
Mendel took garden pea plants with different characteristics, such as height to study the inheritance pattern of factors (genes). He crossed tall pea plant with short pea plant and obtained all the tall plants in the $\mathrm{F_1}$ generation.
15 (a) $\mathrm{F_1}$
Inherited Traits and Mendel's Experiments
Mendel took garden pea plants with different characteristics, such as height to study the inheritance pattern of factors (genes). He crossed tall pea plant with short pea plant and obtained all the tall plants in the $\mathrm{F_1}$ generation.
15 (b) $\mathrm{F_2}$
Inherited Traits and Mendel's Experiments
Mendel took garden pea plants with different characteristics, such as height to study the inheritance pattern of factors (genes). He crossed tall pea plant with short pea plant and obtained all the tall plants in the $\mathrm{F_1}$ generation.
15 (c) (i) $Write one difference between dominant and recessive trait.$
Inherited Traits and Mendel's Experiments
Mendel took garden pea plants with different characteristics, such as height to study the inheritance pattern of factors (genes). He crossed tall pea plant with short pea plant and obtained all the tall plants in the $\mathrm{F_1}$ generation.
15 (c) (ii) $\mathrm{F_1}$
Inherited Traits and Mendel's Experiments

2025

25 q

8 $\mathrm{F_2}$
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Inherited Traits and Mendel's Experiments
8 $\mathrm{F_1}$
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Inherited Traits and Mendel's Experiments
9 $\mathrm{F_1}$
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Inherited Traits and Mendel's Experiments
11 $\mathrm{F_1}$
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Inherited Traits and Mendel's Experiments
For Questions number 17 to 20, two statements are given — one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer to these questions from the codes (A), (B), (C) and (D) as given below.
18 $Assertion (A) : A human child bears all the basic features of human beings. Reason (R) : It looks exactly like its parents, showing very little variations.$
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Accumulation of Variation
For Questions number 17 to 20, two statements are given — one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer to these questions from the codes (A), (B), (C) and (D) as given below.
18 $Assertion (A) : A mango seed will germinate to form a mango tree. Reason (R) : Heredity determines the process by which traits and characteristics are reliably inherited from parents to offspring.$
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Inherited Traits and Mendel's Experiments
Question Nos. 17 to 20 consist of two statements – Assertion (A) and Reason (R). Answer these questions selecting the appropriate option (a), (b), (c) and (d) as given below :
19 $Assertion (A) : In reptiles, the temperature at which the fertilized eggs are kept decides the sex of the offsprings. Reason (R) : Sex is not genetically determined in some animals.$
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Sex Determination
23 (a) $“Proteins control the expression of various characters.” Explain this statement by taking an example of “tallness” as a characteristic in plants.$
Expression of Traits
23 (b) $Explain the mechanism of inheritance used by sexually reproducing organisms to ensure the stability of DNA of the species.$
Expression of Traits
24 $(a) Explain how the proteins control the ‘characteristics’ in an organism with the help of an example of ‘short height’ trait in pea plant. (b) Name the information source of making proteins in a cell.$
Expression of Traits
24 $(a) Explain how the proteins control the ‘characteristics’ in an organism with the help of an example of ‘tallness’ trait in pea plant. (b) Name the section of DNA that controls the ‘characteristics’ in an organism.$
Expression of Traits
24 $\mathrm{F_1}$
Inherited Traits and Mendel's Experiments
24 $“Sex of the children is determined by type of sex chromosome which they inherit from their father.” Justify the statement.$
Sex Determination
24 $Explain with the help of a flow chart that in human beings father is responsible for the sex (male or female) of the child.$
Sex Determination
30 $(a) How many chromosomes are present in human beings ? Out of these how many are sex chromosomes ? (b) Explain how, in sexually reproducing organisms, the number of chromosomes in the progeny is maintained.$
Sex Determination
30 $\mathrm{F_1}$
Inherited Traits and Mendel's Experiments
30 $\mathrm{F_1}$
Inherited Traits and Mendel's Experiments
30 $The lowest part of the ear called earlobe, is closely attached to the side of the head in some of us (Figure ‘X’), and not in others, called free earlobe (Figure ‘Y’). Attached and free earlobes are two variants found in human populations. The gene for free earlobe is dominant over attached earlobes. (a) A man with attached earlobes marries a woman having free earlobes. 50% of their children have free earlobes and 50% have attached earlobes. Explain the inheritance of this trait and write the trait combinations of the progeny. (b) Write the gene combinations of the father and the mother in the above case.$
Inherited Traits and Mendel's Experiments
30 $(a) “In human beings the genes inherited from the parents decide whether the newborn individual is male or female.” Explain this statement with the help of a flow diagram. (b) “Some animals rely on environmental cues for sex determination.” Justify this statement giving an example.$
Sex Determination
30 $\mathrm{F_1}$
Inherited Traits and Mendel's Experiments
32 $\mathrm{F_1}$
Inherited Traits and Mendel's Experiments
In human beings, there are 23 pairs of chromosomes. Out of these 23 pairs of chromosomes (i.e. 46 chromosomes), 22 pairs of chromosomes are called autosomes and one pair of chromosomes. i.e. two chromosomes are called sex chromosomes. The sex chromosomes are of two types – ‘X’ chromosomes and ‘Y’ chromosomes. The sex of a child (i.e. progeny), is decided at the time of fertilisation. In other words, at the time of zygote formation the sex chromosomes inherited from the parents of a child decide whether the new born will be a boy or a girl.
38 (a) $What are chromosomes ?$
Sex Determination
In human beings, there are 23 pairs of chromosomes. Out of these 23 pairs of chromosomes (i.e. 46 chromosomes), 22 pairs of chromosomes are called autosomes and one pair of chromosomes. i.e. two chromosomes are called sex chromosomes. The sex chromosomes are of two types – ‘X’ chromosomes and ‘Y’ chromosomes. The sex of a child (i.e. progeny), is decided at the time of fertilisation. In other words, at the time of zygote formation the sex chromosomes inherited from the parents of a child decide whether the new born will be a boy or a girl.
38 (b) $Why is the pair of sex chromosomes in human males called mismatched pair ?$
Sex Determination
In human beings, there are 23 pairs of chromosomes. Out of these 23 pairs of chromosomes (i.e. 46 chromosomes), 22 pairs of chromosomes are called autosomes and one pair of chromosomes. i.e. two chromosomes are called sex chromosomes. The sex chromosomes are of two types – ‘X’ chromosomes and ‘Y’ chromosomes. The sex of a child (i.e. progeny), is decided at the time of fertilisation. In other words, at the time of zygote formation the sex chromosomes inherited from the parents of a child decide whether the new born will be a boy or a girl.
38 (c) (i) $Show with the help of a flow chart that the statistical probability of getting a boy or a girl is 50 : 50.$
Sex Determination
In human beings, there are 23 pairs of chromosomes. Out of these 23 pairs of chromosomes (i.e. 46 chromosomes), 22 pairs of chromosomes are called autosomes and one pair of chromosomes. i.e. two chromosomes are called sex chromosomes. The sex chromosomes are of two types – ‘X’ chromosomes and ‘Y’ chromosomes. The sex of a child (i.e. progeny), is decided at the time of fertilisation. In other words, at the time of zygote formation the sex chromosomes inherited from the parents of a child decide whether the new born will be a boy or a girl.
38 (c) (ii) $Mention two examples of animals where sex is not determined genetically like human beings. Describe in brief the method of sex determination in these animals.$
Sex Determination

Seeds	Round yellow	Round green	Wrinkled yellow	Wrinkled green
Number	800	275	268	90

2024

24 q

8 $Consider the following statements : (i) The sex of a child is determined by what it inherits from the mother. (ii) The sex of a child is determined by what it inherits from the father. (iii) The probability of having a male child is more than that of a female child. (iv) The sex of a child is determined at the time of fertilisation when male and female gametes fuse to form a zygote. The correct statements are :$
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Sex Determination
8 $Which one of the following statements is NOT true ?$
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Expression of Traits
9 $Chromosomes : (i) carry hereditary information from parents to the next generation. (ii) are thread like structures located inside the nucleus of an animal cell. (iii) always exist in pairs in human reproductive cells. (iv) are involved in the process of cell division. The correct statements are :$
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Expression of Traits
10 $\mathrm{F_2}$
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Inherited Traits and Mendel's Experiments
12 $Sex determination depends upon the environment in$
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Sex Determination
12 $A cross made between two pea plants produces 50% tall and 50% short pea plants. The gene combination of the parental pea plants must be$
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Inherited Traits and Mendel's Experiments
12 $A cross between two tall pea plants resulted in offsprings having a few dwarf plants. The gene-combination of the parental plants must be$
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Inherited Traits and Mendel's Experiments
For Questions number 17 to 20, two statements are given — one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer to these questions from the codes (A), (B), (C) and (D) as given below.
18 $Assertion (A) : Sex of the children will be determined by what they inherit from their mother. Reason (R) : Women have XX sex chromosomes.$
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Sex Determination
These questions consist of two statements – Assertion (A) and Reason (R). Answer these questions selecting the appropriate option given below :
18 $Assertion (A) : Human female has a perfect pair of sex chromosome. Reason (R) : Sex chromosome contributed by the human male in the zygote decides the sex of a child.$
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Sex Determination
Q. 17 to 20 consists of two statements – Assertion (A) and Reason (R), answer these questions selecting the appropriate option given below :
19 $Assertion (A) : In human beings, males have ‘XX’ sex chromosomes and females have ‘XY’ sex chromosomes. Reason (R) : Sex of the child is determined at the time of fertilisation when male and female gamete fuse to form a zygote.$
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Sex Determination
23 (a) $List any two pairs of visible contrasting characters of garden pea plants used by Mendel for his experiments stating the dominant and recessive characters in each pair.$
Inherited Traits and Mendel's Experiments
23 $\mathrm{F_1}$
Inherited Traits and Mendel's Experiments
23 (b) $In human beings, the probability of getting a male or a female child is 50%. Explain with the help of a flow diagram only.$
Sex Determination
24 $Explain how equal genetic contribution of male and female parents is ensured in the progeny.$
Expression of Traits
24 $\times$
Inherited Traits and Mendel's Experiments
30 $How is the sex of a newborn individual determined in different species of animals ? Give three examples to support your answer.$
Sex Determination
30 $\mathrm{F_1}$
Inherited Traits and Mendel's Experiments
30 $\mathrm{F_1}$
Inherited Traits and Mendel's Experiments
30 $\mathrm{F_2}$
Inherited Traits and Mendel's Experiments
30 $\mathrm{F_1}$
Inherited Traits and Mendel's Experiments
Mendel worked out the rules of heredity by working on garden pea using a number of visible contrasting characters. He conducted several experiments by making a cross with one or two pairs of contrasting characters of pea plant. On the basis of his observations he gave some interpretations which helped to study the mechanism of inheritance.
38 (i) $\mathrm{F_1}$
Inherited Traits and Mendel's Experiments
Mendel worked out the rules of heredity by working on garden pea using a number of visible contrasting characters. He conducted several experiments by making a cross with one or two pairs of contrasting characters of pea plant. On the basis of his observations he gave some interpretations which helped to study the mechanism of inheritance.
38 (ii) $Write one difference between dominant and recessive trait.$
Inherited Traits and Mendel's Experiments
Mendel worked out the rules of heredity by working on garden pea using a number of visible contrasting characters. He conducted several experiments by making a cross with one or two pairs of contrasting characters of pea plant. On the basis of his observations he gave some interpretations which helped to study the mechanism of inheritance.
38 (iii) (A) $\times$
Inherited Traits and Mendel's Experiments
Mendel worked out the rules of heredity by working on garden pea using a number of visible contrasting characters. He conducted several experiments by making a cross with one or two pairs of contrasting characters of pea plant. On the basis of his observations he gave some interpretations which helped to study the mechanism of inheritance.
38 (iii) (B) $Justify the statement : “It is possible that a trait is inherited but may not be expressed.”$
Inherited Traits and Mendel's Experiments

2023

27 q

9 $\mathrm{F_2}$
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Inherited Traits and Mendel's Experiments
10 $The statement that correctly describes the characteristic(s) of a gene is :$
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Expression of Traits
10 $\mathrm{F_2}$
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Inherited Traits and Mendel's Experiments
10 $Which one of the given statements is incorrect :$
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Expression of Traits
10 $\mathrm{F_1}$
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Inherited Traits and Mendel's Experiments
10 $Which pair of sex chromosome will determine a male ?$
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Sex Determination
12 $When an ‘X’ bearing sperm fertilises the egg, the resulting zygote has the following combination of chromosomes :$
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Sex Determination
For questions number 17 to 20, two statements are given — one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below :
18 $Assertion (A) : Human populations show a great deal of variations in traits. Reason (R) : All variations in a species have equal chances of surviving in the environment in which they live.$
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Accumulation of Variation
Q. Nos. 17 to 20 are Assertion – Reason based questions. These consist of two statements – Assertion (A) and Reason (R). Answer these questions selecting the appropriate option given below :
18 $Assertion (A) : Genes inherited from the parents decide the sex of a child. Reason (R) : X chromosome in a male child is inherited from his father.$
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Sex Determination
Q. No. 17 to 20 are Assertion – Reasoning based questions. These consist of two statements – Assertion (A) and Reason (R). Answer these questions selecting the appropriate option given below.
18 $Assertion (A) : In humans, if gene (B) is responsible for black eyes and gene (b) is responsible for brown eyes, then the colour of eyes of the progeny having gene combination Bb, bb or BB will be black only. Reason (R) : The black colour of the eyes is a dominant trait.$
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Inherited Traits and Mendel's Experiments
23 $\mathrm{F_1}$
Inherited Traits and Mendel's Experiments
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. In fact 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.
38 (a) $State the basis on which the sex of a newborn baby is determined in humans.$
Sex Determination
All human chromosomes are not paired. Most human chromosomes have a maternal and a paternal copy, and we have 22 such pairs. But one pair called the sex chromosomes, is odd in not always being a perfect pair. Women have a perfect pair of sex chromosomes. But men have a mismatched pair in which one is normal sized while the other is a short one.
38 (a) $In humans, how many chromosomes are present in a Zygote and in each gamete ?$
Sex Determination
In order to trace the inheritance of traits Mendel crossed pea plants having one contrasting character or a pair of contrasting characters. When he crossed pea plants having round and yellow seeds with pea plants having wrinkled and green seeds, he observed that no plants with wrinkled and green seeds were obtained in the $\mathrm{F_1}$ generation. When the $\mathrm{F_1}$ generation pea plants were cross-bred by self-pollination, the $\mathrm{F_2}$ generation had seeds with different combinations of shape and colour also.
38 (a) $Write any two pairs of contrasting characteristics of pea plant used by Mendel other than those mentioned above.$
Inherited Traits and Mendel's Experiments
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.
38 (i) $Why an offspring of human being is not a true copy of his parents in sexual reproduction ?$
Accumulation of Variation
All human chromosomes are not paired. Most human chromosomes have a maternal and a paternal copy, and we have 22 such pairs. But one pair called the sex chromosomes, is odd in not always being a perfect pair. Women have a perfect pair of sex chromosomes. But men have a mismatched pair in which one is normal sized while the other is a short one.
38 (b) $A few reptiles rely entirely on environmental cues for sex determination. Comment.$
Sex Determination
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. In fact 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.
38 (b) $Why is the pair of sex chromosomes called a mismatched pair in males ?$
Sex Determination
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.
38 (ii) $\mathrm{F_1}$
Inherited Traits and Mendel's Experiments
All human chromosomes are not paired. Most human chromosomes have a maternal and a paternal copy, and we have 22 such pairs. But one pair called the sex chromosomes, is odd in not always being a perfect pair. Women have a perfect pair of sex chromosomes. But men have a mismatched pair in which one is normal sized while the other is a short one.
38 (c) $“The sex of a child is a matter of chance and none of the parents are considered to be responsible for it.” Justify it through flow chart only.$
Sex Determination
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. In fact 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.
38 (c) $How is the original number of chromosomes present in the parents restored in the progeny ?$
Sex Determination
In order to trace the inheritance of traits Mendel crossed pea plants having one contrasting character or a pair of contrasting characters. When he crossed pea plants having round and yellow seeds with pea plants having wrinkled and green seeds, he observed that no plants with wrinkled and green seeds were obtained in the $\mathrm{F_1}$ generation. When the $\mathrm{F_1}$ generation pea plants were cross-bred by self-pollination, the $\mathrm{F_2}$ generation had seeds with different combinations of shape and colour also.
38 (b) $Differentiate between dominant and recessive traits.$
Inherited Traits and Mendel's Experiments
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.
38 (iii) (A) $Why do we say that variations are useful for the survival of a species over time ?$
Accumulation of Variation
All human chromosomes are not paired. Most human chromosomes have a maternal and a paternal copy, and we have 22 such pairs. But one pair called the sex chromosomes, is odd in not always being a perfect pair. Women have a perfect pair of sex chromosomes. But men have a mismatched pair in which one is normal sized while the other is a short one.
38 (c) OR $Why do all the gametes formed in human females have an X chromosome ?$
Sex Determination
In order to trace the inheritance of traits Mendel crossed pea plants having one contrasting character or a pair of contrasting characters. When he crossed pea plants having round and yellow seeds with pea plants having wrinkled and green seeds, he observed that no plants with wrinkled and green seeds were obtained in the $\mathrm{F_1}$ generation. When the $\mathrm{F_1}$ generation pea plants were cross-bred by self-pollination, the $\mathrm{F_2}$ generation had seeds with different combinations of shape and colour also.
38 (c) $\mathrm{F_2}$
Inherited Traits and Mendel's Experiments
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. In fact 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.
38 (c) OR $Explain by giving two examples of the organisms in which the sex is not genetically determined.$
Sex Determination
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.
38 (iii) (B) $\times$
Inherited Traits and Mendel's Experiments
In order to trace the inheritance of traits Mendel crossed pea plants having one contrasting character or a pair of contrasting characters. When he crossed pea plants having round and yellow seeds with pea plants having wrinkled and green seeds, he observed that no plants with wrinkled and green seeds were obtained in the $\mathrm{F_1}$ generation. When the $\mathrm{F_1}$ generation pea plants were cross-bred by self-pollination, the $\mathrm{F_2}$ generation had seeds with different combinations of shape and colour also.
38 (c) OR $\mathrm{F_1}$
Inherited Traits and Mendel's Experiments

2022

20 q

3 $Using height (tallness / dwarfness) of a plant as an example, show that genes control the characteristics or traits in an organism.$
Expression of Traits
3 OR $In a cross between red coloured and white coloured flowers, when plants with red coloured flowers of F1 generation were self pollinated, plants of F2 generation were obtained in which 75% of plants were with red flowers and 25% plants were with white flowers. Explain the inheritance of traits in the above cross with the help of a flow chart only along with the ratio of plants obtained.$
Inherited Traits and Mendel's Experiments
5 (a) $What is variation ? List two main reasons that may lead to variation in a population.$
Accumulation of Variation
5 (b) $\mathrm{F_1}$
Inherited Traits and Mendel's Experiments
6 (a) $\mathrm{F_1}$
Inherited Traits and Mendel's Experiments
6 (b) $Justify the statement “Sex of the children will be determined by what they inherit from their father”.$
Sex Determination
10 $(a) Name the two types of gametes produced by men. (b) Does a male child inherit X chromosome from his father ? Justify. (c) How many types of gametes are produced by a human female ?$
Sex Determination
13 $How do Mendel’s experiments show that the traits are inherited independently ? Explain.$
Inherited Traits and Mendel's Experiments
Mendel blended his knowledge of Science and mathematics to keep the count of the individuals 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.
14 (a) $What do the F1 progeny of tall plants with round seeds and short plants with wrinkled seeds look like ?$
Inherited Traits and Mendel's Experiments
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.
14 (a) $Why is a pair of sex chromosomes in human beings called a mismatched pair in terms of type and size ?$
Sex Determination
Mendel blended his knowledge of Science and mathematics to keep the count of the individuals 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.
14 (b) $Name the recessive traits in above case.$
Inherited Traits and Mendel's Experiments
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.
14 (b) $Out of male or female, which of them has a perfect pair of sex chromosomes ? In case of a perfect pair, will the gametes produced be of the same kind or of a different kind ?$
Sex Determination
Mendel blended his knowledge of Science and mathematics to keep the count of the individuals 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.
14 (c) $Mention the type of the new combinations of plants obtained in F2 progeny along with their ratio, if F1 progeny was allowed to self pollinate.$
Inherited Traits and Mendel's Experiments
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.
14 (c) (i) $Name two animals whose sex is not genetically determined. Explain the process of their sex determination.$
Sex Determination
Mendel blended his knowledge of Science and mathematics to keep the count of the individuals 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.
14 (c) OR $If 1600 plants were obtained in F2 progeny, write the number of plants having traits : (i) Tall with round seeds (ii) Short with wrinkled seeds Write the conclusion of the above experiment.$
Inherited Traits and Mendel's Experiments
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.
14 (c) (ii) $With the help of a flowchart only, show how sex is genetically determined in human beings.$
Sex Determination
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 can 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.
15 (a) $In what way are the sex chromosomes ‘X’ and ‘Y’ different in size ? Name the mismatched pair of sex chromosome in humans.$
Sex Determination
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 can 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.
15 (b) $Write the number of pair/pairs of sex chromosomes present in human beings. In which one of the parent (male / female) perfect pair / pairs of sex chromosomes are present ?$
Sex Determination
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 can 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.
15 (c) $Citing two examples, justify the statement “Sex of an individual is not always determined genetically”.$
Sex Determination
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 can 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.
15 (c) OR $Draw a flow chart to show that sex is determined genetically in human beings.$
Sex Determination

The 10 board questions to practise first

The questions and kinds of question the board asked most often come first. Each one says how often and when.

Question 1
Asked 4 times: 2023*, 2024*, 2025*, 2026 (* = numbers or wording changed that year) See all 4 times it was asked
14 $\mathrm{F_1}$
Question 2
Asked 4 times: 2022*, 2024*, 2025*, 2026 (* = numbers or wording changed that year) See all 4 times it was asked
13 $\mathrm{F_1}$
Question 3
This type asked 4 times: 2022, 2023, 2025, 2026. This one: 2026 See all 4 of this type
Two statements are given, one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer from the options given below.
8 $Assertion (A) : Bacteria that can withstand heat have better chances of survival in a heat wave. Reason (R) : Accumulation of variations in a species increases the chances of its survival in changing environment.$
Tap an option to check your answer.
Question 4
This type asked 4 times: 2023, 2024, 2025, 2026. This one: 2026 See all 4 of this type
1 $\text{F}_2$
Tap an option to check your answer.
Question 5
This type asked 4 times: 2022, 2024, 2025, 2026. This one: 2026 See all 4 of this type
13 $\mathrm{F_2}$
Question 6 (case study, 4 parts)
This type asked 4 times: 2022, 2023, 2024, 2026. This one: 2026 See all 4 of this type
Mendel took garden pea plants with different characteristics, such as height to study the inheritance pattern of factors (genes). He crossed tall pea plant with short pea plant and obtained all the tall plants in the $\mathrm{F_1}$ generation.
15 (a) $\mathrm{F_1}$
15 (b) $\mathrm{F_2}$
15 (c) (i) $Write one difference between dominant and recessive trait.$
15 (c) (ii) $\mathrm{F_1}$
Question 7
This type asked 4 times: 2022, 2023, 2024, 2025. This one: 2025 See all 4 of this type
8 $\mathrm{F_2}$
Tap an option to check your answer.
Question 8
Asked 3 times: 2024*, 2025*, 2026 (* = numbers or wording changed that year) See all 3 times it was asked
13 $\mathrm{F_2}$
Question 9
Asked 3 times: 2022*, 2024*, 2025 (* = numbers or wording changed that year) See all 3 times it was asked
24 $“Sex of the children is determined by type of sex chromosome which they inherit from their father.” Justify the statement.$
Question 10
This type asked 3 times: 2023, 2024, 2026. This one: 2026 See all 3 of this type
6 $Sex is determined by different factors in various species. However, in human beings, it is determined genetically. Which amongst the following option(s) is/are correct for human beings ? (i) Gamete carrying X chromosome from female parent. (ii) Gamete carrying X chromosome from male parent. (iii) Gamete carrying Y chromosome from male parent.$
Tap an option to check your answer.

Quick revision

5 most-asked questions · 10 key points · 3 formulas · 3 derivations · 2 diagrams

Checked against NCERT Science (CBSE Class 10), Chapter 8.

Most asked in board papers

1
Sex determination: XX and XY, why the father decides the sex, 50 % chance of a boy or girl, animals whose sex is not decided by genes
Asked 5 times: 2026, 2025, 2024, 2023, 2022
2
Monohybrid cross: tall and short or flower colour, $F_1$ and $F_2$ , the 3 : 1 ratio, dominant and recessive traits
Asked 5 times: 2026, 2025, 2024, 2023, 2022 · 1 mark
3
Dihybrid cross: round yellow with wrinkled green, new combinations in $F_2$ , 9 : 3 : 3 : 1, independent inheritance
Asked 5 times: 2026, 2025, 2024, 2023, 2022
4
Genes, proteins and chromosomes: how a gene controls height, how the chromosome number stays the same, DNA
Asked 5 times: 2026, 2025, 2024, 2023, 2022
5
Variation and inherited traits: why offspring are not true copies, earlobes, why variation helps survival
Asked 4 times: 2026, 2025, 2023, 2022 · 1 mark

Must-know points

Variation: Some variation arises even in asexual reproduction, but sexual reproduction gives much more, since DNA from two parents mixes. So offspring are not true copies of their parents. Variations can be inherited and may help some individuals survive.
Heredity and inherited traits: Heredity: the passing of traits from parents to offspring by fixed rules. A child shows all the basic human features but does not look exactly like its parents. Free and attached earlobes are an inherited trait. A skill learnt in life (swimming) is not inherited.
Mendel's contrasting characters: Mendel used garden peas with contrasting visible characters: tall/short plants, round/wrinkled seeds, yellow/green seeds, violet/white flowers, axial/terminal flowers, green/yellow pods, inflated/constricted pods.
Monohybrid cross: Pure tall × pure short: all $F_1$ plants are tall, with no medium height. Self-pollinating $F_1$ gives $F_2$ : 3 tall : 1 short. Both traits were inherited in $F_1$ , but only tallness was expressed. So each trait is controlled by two copies of a factor (now called a gene).
Dominant and recessive: The trait expressed when the two copies differ is dominant (T, tall); the one hidden is recessive (t, short). TT and Tt are tall; only tt is short. A recessive trait can be inherited but not expressed, and shows again when two copies come together.
Dihybrid cross: independent inheritance: Round yellow (RRYY) × wrinkled green (rryy): all $F_1$ are round yellow (RrYy). In $F_2$ : round yellow, round green, wrinkled yellow, wrinkled green in the ratio 9 : 3 : 3 : 1. New combinations appear, so the two traits are inherited independently.
How genes control traits: DNA is the information source for making proteins. A section of DNA giving information for one protein is a gene. Plant height depends on a growth hormone made with the help of an enzyme: an efficient enzyme makes much hormone and a tall plant; an altered, less efficient one makes a short plant.
Chromosomes and gametes: Each gene set is on separate pieces called chromosomes; every cell has two copies of each chromosome, one from each parent. Each germ cell takes one chromosome of each pair, so a gamete has half the number. Fusion restores the full number: equal genetic contribution from both parents and a stable chromosome number.
Sex determination not by genes: In a few reptiles, the temperature at which the fertilised eggs are kept decides whether the young are male or female. Snails can change sex. So sex is not genetically decided in every animal.
Sex determination in human beings: Humans have 23 pairs of chromosomes; one pair is the sex chromosomes. Women have a perfect pair (XX); men a mismatched pair (XY: a normal-sized X and a short Y). Every egg carries X. A sperm with X gives a girl; a sperm with Y gives a boy. So the sex of the child is decided by what it inherits from the father.

Formulas

Monohybrid F2 ratio

\begin{gathered}\text{phenotype } 3 \text{ tall} : 1 \text{ short} \\ \text{genotype } 1\,TT : 2\,Tt : 1\,tt\end{gathered}

Dihybrid F2 ratio

9 \text{ round yellow} : 3 \text{ round green} : 3 \text{ wrinkled yellow} : 1 \text{ wrinkled green}

Chromosomes in humans

\begin{gathered}\text{body cell } 46\ (23 \text{ pairs}) \\ \text{gamete } 23 \\ \text{zygote } 46\end{gathered}

Derivations

Try each one first, then open its outline.

Show Mendel's monohybrid cross: pure tall (TT) with pure short (tt)

Parents TT × tt; gametes T and t.
$F_1$ : all Tt, all tall (tall is dominant).
Self-pollinate $F_1$ : Tt × Tt; gametes T, t from each.
$F_2$ : TT, Tt, Tt, tt: 3 tall : 1 short; genotypes 1 : 2 : 1.

Show Mendel's dihybrid cross: round yellow (RRYY) with wrinkled green (rryy)

Parents RRYY × rryy; gametes RY and ry.
$F_1$ : all RrYy, round and yellow.
$F_1$ gametes: RY, Ry, rY, ry; a 4 × 4 square gives 16 combinations.
$F_2$ : 9 round yellow, 3 round green, 3 wrinkled yellow, 1 wrinkled green. Round green and wrinkled yellow are new combinations.

Show with a flow chart that the father decides the sex of the child

Mother XX gives only X eggs; father XY gives X sperms and Y sperms in equal numbers.
X egg + X sperm gives XX: girl. X egg + Y sperm gives XY: boy.
So the chance of a boy or a girl is 50 % each, and the sperm decides.

Diagrams to practise

Inheritance of traits over two generations (Figure 8.3): label parents TT × tt · F1 all Tt (tall) · F2 TT, Tt, Tt, tt · 3 tall : 1 short
Sex determination in human beings (Figure 8.6): label mother XX, father XY · eggs all X; sperms X or Y · XX: girl · XY: boy

Mistakes that cost marks

Giving the genotype ratio for the phenotype

The

F_2

of a monohybrid cross is 3 : 1 in appearance but 1 : 2 : 1 in genotype. With 1600

F_2

plants of a dihybrid cross: 900, 300, 300, 100.

Self-pollinating the parents instead of F1

Mendel got

F_2

by self-pollinating the $F_1$ plants, not by crossing the parents again.

Saying the mother decides the sex

Every egg carries X, so the mother cannot change the outcome. The sperm (X or Y) decides. Four daughters in a row is chance, not anyone's fault.

Calling a tall plant pure

A tall plant may be TT or Tt. A cross that gives a few short plants shows both parents were Tt; a cross giving 50 % tall and 50 % short is Tt × tt.

Treating acquired traits as inherited

Only traits in the DNA of germ cells are passed on. A skill like swimming or a mark from an injury is not inherited.

8.1 Accumulation of Variation During Reproduction

Questions

8.2 Heredity

Questions

Exercises

What the board has asked from this chapter

2026

2025

2024

2023

2022

The 10 board questions to practise first

Question 1

Question 2

Question 3

Question 4

Question 5

Question 6 (case study, 4 parts)

Question 7

Question 8

Question 9

Question 10

Most asked in board papers

Must-know points

Formulas

Derivations

Diagrams to practise

Mistakes that cost marks