Class 10 Science · Chapter 8

Heredity

285 Class 10 Science questions on Heredity from the NCERT textbook, NCERT Exemplar and CBSE board papers 2017–2026, each with its answer and a step-by-step solution.

NCERT exercises

Exercises (4) Intext Questions (Page 129) (2) Intext Questions (Page 133) (4)

NCERT Exemplar exercises

Multiple Choice Questions (25) Short Answer Questions (11) Long Answer Questions (11)

CBSE Class 10 Science previous year papers, with answers →

Questions · page 10 of 15

Q181 CBSE 2022
A green stemmed tomato plant denoted by (GG) is crossed with a tomato plant with purple stem denoted by (gg).
1. (i)What colour of the stem would you expect in their F1 progeny ?
2. (ii)In what ratio would you find the green and purple coloured stem in plants of F2 progeny ?
3. (iii)What conclusion can be drawn for the above observations ?
Show answer

$F_{1}$ is all green ( $G g$ ); $F_{2}$ shows green : purple $= 3 : 1$ ; green is dominant and purple recessive, inherited unchanged.
(i) All the F1 plants have green stems (genotype $Gg$ ), because green is the dominant trait. (ii) $green : purple = 3 : 1$ in the F2 (genotypes $GG : Gg : gg = 1 : 2 : 1$ ). (iii) Green stem colour ( $G$ ) is dominant and purple ( $g$ ) is recessive. The purple trait, hidden in every F1 plant, reappears in the F2, so the factor for it was inherited unchanged rather than lost or blended: each plant carries two copies of the factor for stem colour, the two copies separate so that each gamete receives only one, and they come together again at fertilisation.

Step-by-step solution →
Q182 CBSE 2022

What are Chromosomes ? Explain how stability of the DNA of the species is ensured in sexually reproducing organisms.

Show answer

Chromosomes carry genes; meiosis halves their number in gametes and fertilisation restores it ( $23 + 23 = 46$ ), keeping the species' DNA stable.
Chromosomes are thread-like structures in the nucleus of a cell, made of DNA (with proteins), that carry the genes, the information for the inheritance of features from parents to offspring. Body cells normally carry their chromosomes in pairs, one member of each pair from each parent (46, or 23 pairs, in human beings). Stability of DNA: the germ cells (gametes) are formed by a special division, meiosis, in which each gamete receives only one chromosome of each pair, that is half the chromosome number and half the DNA of a body cell (23 in human beings). At fertilisation a male gamete and a female gamete fuse, and the zygote receives one set from each parent, which restores the normal number ( $23 + 23 = 46$ ). Because the number is halved in the gametes and restored at fertilisation, each generation has the same number of chromosomes and the same amount of DNA as its parents, so the DNA of the species stays stable.

Step-by-step solution →
Q183 CBSE 2022
In one of his experiments with pea plants, Mendel observed that when tall pea plants (TT) were crossed with dwarf pea plants (tt), in the first generation ( $F_{1}$ ) plants only tall plants appeared.
1. (a)What happens to the traits for dwarfness in this case ?
2. (b)When the $F_{1}$ generation plants were self-pollinated, he observed that in the plants of $F_{2}$ generation both tall plants and dwarf plants were present. Explain briefly why it happened.
3. (c)Mention the ratio obtained in such a cross.
Show answer

Dwarfness is hidden in $F_{1}$ ( $T t$ ) because it is recessive; it reappears in one quarter of $F_{2}$ , giving tall : dwarf $= 3 : 1$ .
(a) The trait for dwarfness is recessive. It is not lost: every $F_{1}$ plant is $T t$ and carries the dwarf factor $t$ , but because tallness ( $T$ ) is dominant, the dwarf trait is not expressed and remains hidden in $F_{1}$ . (b) The $F_{1}$ plants ( $T t$ ) form two kinds of gametes, $T$ and $t$ , in equal numbers, because the two factors separate during gamete formation. On self-pollination the gametes combine at random, giving $T T$ , $T t$ , $T t$ and $tt$ . The $tt$ plants, one in four, have no factor for tallness, so the recessive dwarf trait shows again in $F_{2}$ . (c) The $F_{2}$ plants are in the ratio $3$ tall $: 1$ dwarf (phenotypic ratio); the genotypic ratio is $1 T T : 2 T t : 1 tt$ .

Step-by-step solution →
Q184 CBSE 2022

How do Mendel’s experiments show that the traits are inherited independently ? Explain.

Show answer

Pea plants differing in two traits gave $9 : 3 : 3 : 1$ in $F_{2}$ , with new combinations, because the two factors separate independently.
Mendel crossed pea plants that differed in two traits at once: a pure-breeding plant with round, yellow seeds (RRYY) and one with wrinkled, green seeds (rryy). $F_{1}$ : all the plants had round, yellow seeds (RrYy), because round and yellow are dominant. $F_{2}$ : on self-pollinating the $F_{1}$ plants he obtained four kinds of seeds in the ratio 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green. Round green and wrinkled yellow are new combinations that were present in neither parent. They appear because the factors (genes) for seed shape and for seed colour separate independently of each other when the gametes are formed, each $F_{1}$ plant forming RY, Ry, rY and ry gametes in equal numbers. Had the two traits been inherited together, only round yellow and wrinkled green seeds would have appeared. Hence the traits are inherited independently. (Each trait on its own still follows $3 : 1$ : round : wrinkled $= 12 : 4$ and yellow : green $= 12 : 4$ .)

Step-by-step solution →
Q185 CBSE 2022
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.
1. (a)Why is a pair of sex chromosomes in human beings called a mismatched pair in terms of type and size ?[1]
2. (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 ?[1]
3. (c)(i)Name two animals whose sex is not genetically determined. Explain the process of their sex determination.[2]
4. (c)(ii)OR : With the help of a flowchart only, show how sex is genetically determined in human beings.[2]
Show answer

The male XY pair is mismatched; the female XX gives identical eggs; turtles and snails show non-genetic sex; the sperm decides a child's sex.
(a) In a human male the two sex chromosomes are of different types — one X and one Y — and of different sizes: X is a normal-sized chromosome while Y is a much shorter one. Since the two members of the pair differ in type and in size, the male's pair of sex chromosomes is called a mismatched pair. (b) The female has a perfect pair of sex chromosomes (XX). The gametes (eggs) she produces are all of the same kind: every egg carries an X chromosome (22 + X). (c)(i) Two such animals are turtles and snails. In some reptiles, such as many turtles, the temperature at which the fertilised eggs are kept decides whether the young developing in them will be male or female (in many turtles, eggs kept warmer develop into females). In snails, individuals can change their sex, which shows that their sex is not genetically determined. (Crocodiles, whose sex is also decided by the incubation temperature of the eggs, are another accepted example.) OR (c)(ii) Flowchart (drawn in the figure): Father 44 + XY and Mother 44 + XX → gametes: sperms 22 + X or 22 + Y, in equal numbers, and eggs, all 22 + X → fertilisation: 22 + X sperm + 22 + X egg → zygote 44 + XX → girl; 22 + Y sperm + 22 + X egg → zygote 44 + XY → boy. There is an equal chance of a girl or a boy, and the sperm decides which.

Step-by-step solution →
Q186 CBSE 2022
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.
1. (a)What do the F1 progeny of tall plants with round seeds and short plants with wrinkled seeds look like ?[1]
2. (b)Name the recessive traits in above case.[1]
3. (c)(i)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.[2]
4. (c)(ii)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.[2]
Show answer

$F_{1}$ plants are tall and round; $F_{2}$ shows $9 : 3 : 3 : 1$ with two new combinations; of 1600 plants, 900 are tall round and 100 short wrinkled.
(a) All the $F_{1}$ plants are tall with round seeds ( $T tR r$ ), because tall and round are the dominant traits. (b) The recessive traits are short height (dwarf plants) and wrinkled seeds. (c)(i) On self-pollination of $F_{1}$ , the $F_{2}$ generation has four kinds of plants in the ratio $9$ tall with round seeds $: 3$ tall with wrinkled seeds $: 3$ short with round seeds $: 1$ short with wrinkled seeds. The new combinations, seen in neither parent, are tall plants with wrinkled seeds and short plants with round seeds, in the ratio $3 : 3$ (each $\frac{3}{16}$ of the $F_{2}$ plants). OR (c)(ii) (i) Tall with round seeds: $\frac{9}{16} \times 1600 = 900$ plants. (ii) Short with wrinkled seeds: $\frac{1}{16} \times 1600 = 100$ plants. Conclusion: because new combinations appear, the two characters, plant height and seed shape, are inherited independently of each other, with tall and round dominant.

Step-by-step solution →
Q187 CBSE 2022
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.
1. (a)Mention the dominant traits observed in $F_{1}$ generation.
2. (b)What are the new possible combinations of seeds likely to be observed in $F_{2}$ generation ?
3. (c)(i)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.
4. (c)(ii)OR : What is meant by the terms (I) dominant, and (II) recessive traits ? Explain.
Show answer

Round yellow is dominant; the new combinations are round green and wrinkled yellow; masked recessive factors reappear in $F_{2}$ in the ratio $9 : 3 : 3 : 1$ .
(a) Round seed shape and yellow seed colour are the dominant traits: every $F_{1}$ seed was round and yellow. (b) The new combinations are round and green seeds, and wrinkled and yellow seeds, which neither parent had. (Wrinkled green seeds, absent from $F_{1}$ , also reappear in $F_{2}$ , but that is the combination of one of the parents.) (c)(i) The factors for wrinkled shape and green colour were present in every $F_{1}$ plant ( $R r Y y$ ) but were masked by the dominant factors; they were not lost. When gametes form, the two factors of each pair separate, and the factors for shape and for colour are passed on independently, so some $F_{2}$ seeds receive two recessive factors ( $r r$ or $y y$ ) and show the recessive traits again. $F_{2}$ ratio: round yellow : round green : wrinkled yellow : wrinkled green $= 9 : 3 : 3 : 1$ . OR (c)(ii) (I) A dominant trait is one that is expressed in $F_{1}$ and shows even when only one factor for it is present ( $R R$ or $R r$ gives round seeds). (II) A recessive trait is one that is hidden in $F_{1}$ and shows only when both factors are recessive ( $r r$ gives wrinkled seeds). For example, $R R \times r r$ gives an $F_{1}$ of all round seeds ( $R r$ ); selfing $F_{1}$ gives $3$ round $: 1$ wrinkled in $F_{2}$ , the recessive trait reappearing in one quarter.

Step-by-step solution →
Q188 CBSE 2022
Mendel conducted breeding experiments with garden peas. He crossed tall pea plants (TT) with short pea plants (tt) and obtained only tall pea plants in $F_{1}$ generation.
1. (a)List any two visible contrasting characters taken by Mendel in pea plants apart from height, in his other experiments.
2. (b)State the reason why only tall pea plants were obtained in the $F_{1}$ generation.
3. (c)(i)When $F_{1}$ generation pea plants were self-pollinated a total of 1600 plants were produced. How many of these plants of $F_{2}$ generation would be (I) tall plants, and (II) short plants ? Give the gene (trait) combination of plants produced in $F_{2}$ generation.
4. (c)(ii)OR : If a student performs the same experiment with other angiosperms and gets the similar results, then on the basis of his experiment, can he arrive at a conclusion ? If yes, write the conclusion. If not, give justification for your answer.
Show answer

Seed shape and flower colour; all $F_{1}$ are $T t$ ; $F_{2}$ has $1200$ tall and $400$ short plants; the same rules hold in other angiosperms.
(a) Any two of: round and wrinkled seeds; yellow and green seeds; violet and white flowers; axial and terminal flowers; inflated (full) and constricted pods; green and yellow pods (for example, round/wrinkled seed shape and violet/white flower colour). (b) Every $F_{1}$ plant received $T$ from the tall parent and $t$ from the short parent, so all were $T t$ . Tallness is the dominant trait: a single $T$ makes the plant tall, and the recessive trait (shortness) does not show when $T$ is present. So only tall plants appeared in $F_{1}$ . (c)(i) $T t \times T t$ gives $T T : T t : tt = 1 : 2 : 1$ , that is $3$ tall : $1$ short. Of the $1600$ plants, (I) tall $= \frac{3}{4} \times 1600 = 1200$ and (II) short $= \frac{1}{4} \times 1600 = 400$ (expected numbers). The gene combinations in $F_{2}$ are $T T$ , $T t$ and $tt$ in the ratio $1 : 2 : 1$ , that is about $400$ $T T$ (tall), $800$ $T t$ (tall) and $400$ $tt$ (short). OR (c)(ii) Yes. If the same kind of cross in other angiosperms gives similar results, the student can conclude that in those plants, too, each trait is controlled by a pair of factors (genes), one from each parent; that one trait of a contrasting pair is dominant and shows in $F_{1}$ while the other is recessive and stays hidden; and that the factors separate when gametes are formed, so the recessive trait reappears in $F_{2}$ , in about one plant in four ( $3 : 1$ ). The rules of inheritance found in the pea hold for these plants as well.

Step-by-step solution →
Q189 CBSE 2022
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.
1. (a)In what way are the sex chromosomes ‘X’ and ‘Y’ different in size ? Name the mismatched pair of sex chromosome in humans.
2. (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 ?
3. (c)(i)Citing two examples, justify the statement “Sex of an individual is not always determined genetically”.
4. (c)(ii)OR : Draw a flow chart to show that sex is determined genetically in human beings.
Show answer

XY is the mismatched pair (males) and XX the perfect pair (females); turtles and snails show non-genetic sex; the father's sperm decides sex.
(a) The X chromosome is of normal size, while the Y chromosome is a short one, much smaller than X. The mismatched pair of sex chromosomes in humans is XY, found in males. (b) Human beings have one pair of sex chromosomes (the other 22 of their 23 pairs of chromosomes are autosomes). The perfect pair of sex chromosomes, XX, is present in the female parent (the mother). (c)(i) In some animals, for example many turtles and crocodiles, the temperature at which the fertilised eggs are kept decides whether the young developing in them will be male or female. In some other animals, for example snails, an individual can change its sex during its lifetime. In both cases the sex is decided by an environmental cue, or changes during life, instead of being fixed by the genes inherited, so the sex of an individual is not always determined genetically. OR (c)(ii) Flow chart (see the figure): father ( $44 + XY$ ) forms two kinds of sperm, $22 + X$ and $22 + Y$ , in equal numbers; mother ( $44 + XX$ ) forms eggs that all carry $22 + X$ . Egg ( $22 + X$ ) + sperm ( $22 + X$ ) $\to$ zygote $44 + XX$ $\to$ girl; egg ( $22 + X$ ) + sperm ( $22 + Y$ ) $\to$ zygote $44 + XY$ $\to$ boy. The sex of the child therefore depends on which kind of sperm fertilises the egg, and girls and boys are expected in the ratio $1 : 1$ .

Step-by-step solution →
Q190 CBSE 2022
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.
1. (a)Why is a pair of sex chromosomes in human beings called a mismatched pair in terms of type and size ?[1]
2. (b)If the gametes always have half the number of chromosomes, then how is the original number of chromosomes restored in the organism ?[1]
3. (c)(i)Name two animals whose sex is not genetically determined. Explain the process of their sex determination.[2]
4. (c)(ii)OR : With the help of a flowchart only, show how sex is genetically determined in human beings.[2]
Show answer

(a) In a human male the two sex chromosomes are of different types, one X and one Y, and of different sizes, the X being of normal size and the Y short. Since the two members of the pair match neither in type nor in size, the male’s pair (XY) is called a mismatched pair; a female has a perfect pair, XX. (b) By fertilisation. Each gamete has half the number, $23$ chromosomes; when a male gamete fuses with a female gamete, the zygote gets $23 + 23 = 46$ chromosomes, the original number, and the body cells that develop from it carry $46$ . (c)(i) Turtles and snails (crocodiles and some lizards are other examples). In many turtles the temperature at which the fertilised eggs are kept decides whether the young become males or females. In some snails an individual can change its sex during its life, which shows that its sex is not fixed by its genes. OR (c)(ii) Flowchart (see the figure): Father $(44 + XY) \to$ sperms $(22 + X)$ or $(22 + Y)$ , half of each; Mother $(44 + XX) \to$ eggs, all $(22 + X)$ . Sperm $(22 + X)$ + egg $(22 + X) \to$ zygote $(44 + XX) \to$ girl. Sperm $(22 + Y)$ + egg $(22 + X) \to$ zygote $(44 + XY) \to$ boy. So the type of sperm decides the sex, with an equal chance of a girl or a boy.

Step-by-step solution →
Q191 CBSE 2022
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.
1. (a)What is the statistical probability of getting either a male or a female child ? Justify your answer.[1]
2. (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 ?[1]
3. (c)(i)Name two animals whose sex is not genetically determined. Explain the process of their sex determination.[2]
4. (c)(ii)OR : With the help of a flowchart only, show how sex is genetically determined in human beings.[2]
Show answer

(a) The probability is $\frac{1}{2}$ (50%) for a male child and $\frac{1}{2}$ (50%) for a female child, a ratio of $1 : 1$ . Justification: the mother (XX) produces only one kind of egg, each carrying an X chromosome, while the father (XY) produces two kinds of sperm in equal numbers, half carrying X and half carrying Y. An X-bearing sperm fertilising the egg gives XX, a girl, and a Y-bearing sperm gives XY, a boy; both kinds of sperm are equally likely to fertilise the egg, so each sex has an equal chance. (b) The female has the perfect pair of sex chromosomes (XX). Her gametes are all of the same kind: every egg carries one X chromosome. (c)(i) Turtles and snails. In some animals, such as many turtles, the temperature at which the fertilised eggs are kept decides whether the young developing in them become male or female. In snails, as NCERT describes, individuals can change sex, which shows that their sex is not genetically determined. OR (c)(ii) Father ( $44 + XY$ ) forms sperms $22 + X$ and $22 + Y$ ; mother ( $44 + XX$ ) forms eggs $22 + X$ only. Egg $22 + X$ with sperm $22 + X$ $\to$ zygote $44 + XX$ $\to$ girl; egg $22 + X$ with sperm $22 + Y$ $\to$ zygote $44 + XY$ $\to$ boy. The flowchart is drawn in the figure.

Step-by-step solution →
Q192 CBSE 2022
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.
1. (a)If only one pair of contrasting characters like tall and short plants is taken, plants obtained in F1 generation are not of medium height. Why ?[1]
2. (b)Name the recessive traits in above case.[1]
3. (c)(i)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.[2]
4. (c)(ii)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.[2]
Show answer

(a) Tallness is dominant over shortness. Each F1 plant receives a factor for tallness from the tall parent and one for shortness from the short parent ( $Tt$ ), but only the dominant trait, tallness, is expressed; the two factors do not blend. So all the F1 plants are tall, not of medium height. (b) Shortness (short plant height) is the recessive trait of the tall and short pair; in the round and wrinkled pair of seed shape that part (c) and its alternative use, wrinkled seed is the recessive trait. (c)(i) When the F1 plants (tall with round seeds, $TtRr$ ) self-pollinate, the F2 shows two new combinations not seen in either parent: tall plants with wrinkled seeds and short plants with round seeds, $3$ of every $16$ plants each, that is, in the ratio $3 : 3$ ( $1 : 1$ ) between them. The complete F2 ratio is $9$ tall round : $3$ tall wrinkled : $3$ short round : $1$ short wrinkled. OR (c)(ii) (i) Tall with round seeds: $\frac{9}{16} \times 1600 = 900$ plants; (ii) short with wrinkled seeds: $\frac{1}{16} \times 1600 = 100$ plants. Conclusion: the traits for plant height and for seed shape are inherited independently of each other, which is why new combinations appear in the F2; tall and round are the dominant traits.

Step-by-step solution →
Q193 CBSE 2022
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.
1. (a)What do the F1 progeny of tall plants with round seeds and short plants with wrinkled seeds look like ?[1]
2. (b)What are recessive traits ?[1]
3. (c)(i)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.[2]
4. (c)(ii)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.[2]
Show answer

(a) All the F1 plants are tall with round seeds ( $T tR r$ ), because tall and round are dominant (the parents being pure-breeding, as in Mendel's experiment). (b) A recessive trait is one that does not show in the F1 generation of a cross between pure parents with contrasting traits, being masked by the dominant trait; it shows only when both copies of the gene are recessive, and it reappears in F2. Here short height and wrinkled seeds are recessive. (c)(i) The new combinations (recombinant types, found in neither parent) are tall plants with wrinkled seeds and short plants with round seeds. The F2 ratio is tall round 9 : tall wrinkled 3 : short round 3 : short wrinkled 1, so the two new combinations appear in equal numbers, $3 : 3$ , each $\frac{3}{16}$ of the plants. OR (c)(ii) (i) Tall with round seeds: $1600 \times \frac{9}{16} = 900$ . (ii) Short with wrinkled seeds: $1600 \times \frac{1}{16} = 100$ . Conclusion: height and seed shape are inherited independently of each other (and tall and round are dominant).

Step-by-step solution →
Q194 CBSE 2022
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.
1. (a)In what way are the sex chromosomes ‘X’ and ‘Y’ different in size ? Name the mismatched pair of sex chromosome in humans.
2. (b)Write the number of chromosomes present in human beings apart from sex chromosomes. Which of the parent has a mismatched pair of sex chromosomes ?
3. (c)(i)Citing two examples, justify the statement “Sex of an individual is not always determined genetically”.
4. (c)(ii)OR : Draw a flow chart to show that sex is determined genetically in human beings.
Show answer

(a) The X chromosome is normal-sized, while the Y chromosome is short (smaller than X). The mismatched pair of sex chromosomes in humans is XY. (b) Apart from the two sex chromosomes, a human body cell has 44 chromosomes (22 pairs of autosomes). The father has the mismatched pair (XY); the mother has a perfect pair (XX). (c)(i) Example 1: in some animals, such as many turtles, the temperature at which the fertilised eggs are kept decides whether the young become male or female. Example 2: in some animals an individual changes its sex during its life; the slipper limpet, a sea snail, starts life as a male and later becomes a female. In both, the sex is set by the environment or changes during life rather than being fixed by inherited sex chromosomes, so the sex of an individual is not always determined genetically. OR (c)(ii) Flow chart: Father (44 + XY) × Mother (44 + XX) → gametes: sperms 22 + X or 22 + Y, in equal numbers; eggs all 22 + X → fertilisation: egg 22 + X with sperm 22 + X gives a 44 + XX zygote, a girl; egg 22 + X with sperm 22 + Y gives a 44 + XY zygote, a boy → girls : boys $= 1 : 1$ . The sex of the child is decided by the sperm, that is, by the chromosome inherited from the father (see the flow chart).

Step-by-step solution →
Q195 CBSE 2022
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.
1. (a)What is the statistical probability of getting either a male child or a female child ?
2. (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 ?
3. (c)(i)Citing two examples, justify the statement “Sex of an individual is not always determined genetically”.
4. (c)(ii)OR : Draw a flow chart to show that sex is determined genetically in human beings.
Show answer

(a) The probability of a male child is $\frac{1}{2}$ (50%) and of a female child is $\frac{1}{2}$ (50%): every egg carries X, while half the sperms carry X and half carry Y, so each child is equally likely to be a boy or a girl (and the two chances add up to 1). (b) Human beings have one pair of sex chromosomes. The perfect pair (XX) is present in the female (mother); the male has the mismatched pair XY. (c)(i) (1) In some reptiles, such as many kinds of turtle, the temperature at which the fertilised eggs are kept decides whether the young are male or female. (2) In some animals, such as snails, individuals can change their sex during their lifetime. In neither case is sex fixed by the chromosomes, so it is not always determined genetically. OR (c)(ii) Flow chart. Parents: father ( $44 + XY$ ) $\times$ mother ( $44 + XX$ ). Gametes: the father makes sperms carrying $22 + X$ or $22 + Y$ ; every egg carries $22 + X$ . Fertilisation: an X-carrying sperm and an egg give the zygote $44 + XX$ , a girl; a Y-carrying sperm and an egg give the zygote $44 + XY$ , a boy. So $girl : boy = 1 : 1$ .

Step-by-step solution →
Q196 CBSE 2022
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.
1. (a)Mention the dominant traits observed in $F_{1}$ generation.
2. (b)Write any two contrasting visible characters other than the ones shown above, taken by Mendel for his experiment.
3. (c)(i)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.
4. (c)(ii)OR : What is meant by the terms (I) dominant, and (II) recessive traits ? Explain.
Show answer

(a) Round seed shape and yellow seed colour are the dominant traits: all the $F_{1}$ seeds are round and yellow. (b) Any two other pairs that Mendel used, for example tall and short (dwarf) plants, and violet and white flowers. (Others: axial and terminal flower position, inflated and constricted pods, green and yellow pods.) (c)(i) Every $F_{1}$ plant ( $R r Y y$ ) carries the factors for wrinkled ( $r$ ) and green ( $y$ ) seeds, but they are masked by the dominant $R$ and $Y$ . The factors are not lost or blended: the two copies of each separate when gametes form, and the shape and colour factors are sorted into the gametes independently. On self-pollination some zygotes receive $r$ from both gametes ( $r r$ ) or $y$ from both ( $y y$ ), and these seeds show the recessive traits again. $F_{2}$ ratio: round yellow : round green : wrinkled yellow : wrinkled green $= 9 : 3 : 3 : 1$ . (c)(ii) OR : (I) A dominant trait is one that is expressed even when only one copy of its factor is present, as in the hybrid $F_{1}$ : $R R$ and $R r$ seeds are both round. (II) A recessive trait is one that is masked by the dominant factor and is expressed only when both copies are recessive: only $r r$ seeds are wrinkled. The recessive trait stays hidden in $F_{1}$ and reappears in about one quarter of the $F_{2}$ seeds.

Step-by-step solution →
Q197 CBSE 2020

How many pairs of chromosomes are present in human beings ?

Show answer

Human beings have 23 pairs of chromosomes (46 chromosomes in all).
$23$ pairs ( $46$ chromosomes): $22$ pairs of autosomes and $1$ pair of sex chromosomes (XX in females, XY in males).

Step-by-step solution →
Q198 CBSE 2020
For question numbers 13 and 14, 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 :
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).
1. (a)Both (A) and (R) are true and (R) is the correct explanation of the assertion (A).
2. (b)Both (A) and (R) are true, but (R) is not the correct explanation of the assertion (A).
3. (c)(A) is true, but (R) is false.
4. (d)(A) is false, but (R) is true.
Show answer

(a) Both (A) and (R) are true and (R) is the correct explanation of the assertion (A).

Step-by-step solution →
Q199 CBSE 2020
For question numbers 13 and 14, 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 :
Assertion (A): Wing of an insect and wing of a bird are analogous organs.
Reason (R): The organs which are quite different in fundamental structure and origin but perform same function in different species are called analogous organs.
1. (a)Both (A) and (R) are true and (R) is the correct explanation of the assertion (A).
2. (b)Both (A) and (R) are true, but (R) is not the correct explanation of the assertion (A).
3. (c)(A) is true, but (R) is false.
4. (d)(A) is false, but (R) is true.
Show answer

(a) Both (A) and (R) are true and (R) is the correct explanation of the assertion (A).

Step-by-step solution →
Q200 CBSE 2020

What are homologous structures? Give an example. Is it necessary that homologous structures always have a common ancestor. Justify your answer.

Show answer

Homologous structures share basic structure and origin but differ in function; they always have a common ancestor from which the design was inherited.
Homologous structures are organs that have the same basic structure and origin but are modified to perform different functions. Example: the forelimbs of vertebrates, such as the leg of a lizard, the wing of a bird, the flipper of a whale and the arm of a human, have the same basic design of bones but are used for walking, flying, swimming and grasping. (Any other correct example earns credit.) Yes, it is necessary. The similarity in the basic design of homologous structures shows that their ancestors were common: the design was inherited from a common ancestor and then modified for different functions. Structures that are similar without such common ancestry are analogous, not homologous.

Step-by-step solution →