NCERT Exemplar Solutions Class 10 Science Chapter 8

Why the exchange matters. The phrase "exchange of genetic material" is the examiner's way of asking which mode of reproduction shuffles two parents' DNA into one offspring. In sexual reproduction two events do this work. First, fertilisation pools one full haploid set from each parent into a diploid zygote. Second, just before the gametes form, the paired chromosomes line up and can swap segments during meiosis, so even a single parent's gametes are not all alike.

Reading the wrong options. Vegetative reproduction (a runner, a tuber, a leaf bud) grows a new plant from one parent's body cells; budding (as in Hydra or yeast) buds off a new individual from one parent; both sit inside the larger family of asexual reproduction. None of these involves a second gamete, so the offspring inherit exactly one parent's genome with no exchange. This is precisely why asexually produced organisms show very little variation, while sexually produced ones show a lot.

Spotting the 1:2:1 signal. Examiners pick the 1:2:1 pattern because it points to one specific cross: a heterozygote crossed with an identical heterozygote. The colours red, pink and white tell you this is incomplete dominance, where the pink plants are themselves Rr. So both "parents" carry the same two alleles.

Why selfing, not generic cross-fertilisation. Cross fertilisation only means pollen reaches a different flower; it does not guarantee both parents are the same Rr type. The question states both starting flowers are pink, i.e. both Rr, and the offspring fall in the exact 1:2:1 ratio. A flower pollinating itself is the cleanest description of this set-up, so self pollination is the best answer. "No fertilisation" is ruled out because offspring did appear. Hence option (b).

Reading the F1 generation. The first filial generation (F1) of a pure-tall × pure-short cross is the cleanest test of dominance. Because one parent supplies only T and the other only t, every single F1 plant is the heterozygote Tt. Whatever trait the F1 shows must therefore be the dominant one, since the recessive allele is sitting right there but stays silent.

Eliminating the distractors. The F1 came out all tall, so tall is what wins in the heterozygote: tallness is dominant, which makes option (a) correct and option (c) wrong. If shortness were dominant (option b) the F1 would all be short, which contradicts the data. Option (d) is wrong because the trait clearly tracks the T/t alleles exactly as Mendel showed. This single cross is the basis of Mendel's Law of Dominance.

The gene-to-protein logic. The whole question rests on one idea from the chapter: traits get expressed because a gene tells the cell to make a particular protein. Proteins then do the work of the cell. Enzymes are proteins, so they are coded by genes. Several hormones (insulin, growth hormone) are proteins too, so a gene exists for them. Each of (a), (b) and (c) fits this gene → protein chain and is therefore a true statement.

Where fat breaks the chain. Fats are lipids, not proteins. The body does make and store fat, but it does this using enzymes, and those enzymes are the gene products. There is no direct gene that spells out a fat molecule the way a gene spells out a protein. So the sentence "for every molecule of fat there is a gene" is the incorrect one, which makes (d) the right choice.

Tracking two genes together. This is Mendel's dihybrid set-up. Each parent is pure-breeding (homozygous) for both genes, but for opposite versions: one is round-green, the other wrinkled-yellow. Because each parent can make only one kind of gamete, the F₁ is forced to be uniform: every seed is Rr Yy.

Why round and yellow win. Once you know the F₁ genotype is Rr Yy, dominance settles the look. R beats r, so the seed is round, not wrinkled; Y beats y, so the seed is yellow, not green. The two recessive parental looks (wrinkled and green) disappear in F₁ and reappear only in F₂. So options (b), (c) and (d), which each keep a recessive trait, are wrong, and (a) round and yellow is correct.

What "unpaired" really means. The word "unpaired" here does not mean the chromosome has no partner at all. It means the partner is not an identical match. In 22 of the 23 human pairs (the autosomes) the two chromosomes are alike. The 23rd pair is special: in males it is X together with Y, two chromosomes of very different size and gene content.

Why both X and Y count. Since the male's 23rd pair is made of an X and a Y that do not match, both of these are the odd ones out, not just one. That is why option (c), naming both (iii) Y and (iv) X, is correct, while (b) "(iii) only" is incomplete. Options referring merely to "large" or "small" chromosomes (i and ii) do not specifically identify the sex chromosomes.

The deciding chromosome. The question targets a common misconception, that sex is somehow set by the mother or by chance alone. In humans the egg is uniform: it always carries one X. The variation comes only from the father, whose sperm population is half X-bearing and half Y-bearing. The sex of the zygote is therefore read straight from the sperm's contribution.

Why the Y is the maleness switch. A Y chromosome carries the gene that triggers male development. When a Y-bearing sperm fertilises the egg, the XY zygote develops as a boy; without the Y, the XX zygote develops as a girl. So maleness is determined by the Y chromosome in the zygote, option (b). The X in (a) cannot be the answer because every zygote has at least one X; cytoplasmic effects (c) do not decide human sex; and the sex itself is decided strictly by the chromosome the sperm carries, so (d) is not the precise answer.

Pinning down the genotype. The key fact is that the mother can only give an X. So whatever the father gives becomes the second member of the pair. The question states the father has passed on an X. That fixes the zygote at XX with no ambiguity.

Reading off the sex. An XX individual develops as a female because there is no Y chromosome to trigger male development. So the child is a girl, option (b). Option (a) boy would need a Y from the father, which is not the case here. Option (c) is wrong because the sex chromosomes, including the X here, do decide sex. Option (d) "either" is wrong because the outcome is certain once both chromosomes are known to be X.

Where new combinations come from. The two starting plants were round-green and wrinkled-yellow. The F₁ pooled all four alleles into Rr Yy. When this F₁ is selfed, the R/r gene sorts independently of the Y/y gene during gamete formation, so the F₂ shuffles the traits into all four possible pairings in the classic 9:3:3:1 ratio.

Identifying the recombinants. "New combination" means a trait pairing that neither original parent had. The parents carried round-green and wrinkled-yellow, so those two pairings are old. The remaining two F₂ phenotypes, round-yellow and wrinkled-green, are the recombinants that appear for the first time. These match items (i) and (iv), which is option (b). Independent assortment is the source of these fresh combinations, which is the very reason sexual reproduction generates so much variation.

Why both parents matter. A diploid organism stores its genes in pairs. One member of each pair is inherited from the mother through the egg, the other from the father through the sperm. Because both alleles sit at the same place on the matching chromosomes, both have a say in the trait, whether through dominance, recession or a blended effect.

Ruling out the single-parent options. Saying a trait is from "paternal DNA only" (a) or "maternal DNA only" (b) ignores the fact that the zygote carries a balanced contribution from each parent. Option (d) is plainly wrong because traits are encoded by DNA. Only option (c), both maternal and paternal DNA, fits how inheritance actually works in sexually reproducing organisms.

Locating the two versions. "Two versions of a trait" is the textbook phrase for a pair of alleles. By definition, alleles of one gene occupy the very same position on the chromosome. Since a diploid cell holds two copies of each chromosome (a homologous pair), the maternal allele sits on one copy and the paternal allele on the other copy of that same chromosome.

Eliminating the alternatives. They are not on "two different chromosomes" (b) because that would make them alleles of different genes, not two versions of one trait. They are not restricted to "sex chromosomes" (c) because most traits are coded on autosomes. "Any chromosome" (d) is too loose, since the two alleles are pinned to one specific homologous pair. So option (a), copies of the same chromosome, is the precise answer.

Checking each statement. This is a true/false sorting task. Statement (i) restates the molecular definition of a gene as a defined base sequence in DNA, so it is correct. Statement (iii) captures the idea that each gene has a fixed address on a particular chromosome shared across a species, which is correct and is the reason gene maps are reproducible.

Spotting the false ones. Statement (ii) claims genes do not code for proteins, which contradicts the chapter's central message that genes work by directing protein synthesis. Statement (iv) claims one gene per chromosome, but a human chromosome holds hundreds to thousands of genes lined up along its DNA. Discarding (ii) and (iv) leaves (i) and (iii), so option (b) is correct.

Reading the question precisely. Most students rush to 3:1 because that is the famous monohybrid phenotype ratio (tall : short). But this question does not ask for tall plants; it asks for pure tall plants, meaning the homozygous TT class alone. That single word changes the answer.

Counting from the genotype ratio. The F₂ genotype ratio is 1 TT : 2 Tt : 1 tt. The pure tall class is TT = 1 part. The short class is tt = 1 part. The two heterozygous Tt plants are tall but impure, so they are excluded. Hence pure tall : short equals 1:1, which is option (c). This is a classic "trap" question that rewards careful reading of the word "pure".

Sorting the chromosome set. The human chromosome set is split into two groups. Twenty-two pairs are autosomes, which look the same in both sexes and carry most of the body's genes. The twenty-third pair is the sex-chromosome pair, which decides the individual's sex.

What the zygote receives. When the haploid egg (carrying one X and one of each autosome) fuses with the haploid sperm (carrying its own single set including one X or one Y), the zygote becomes diploid: 23 pairs in all. Among these, exactly one pair is the sex chromosomes, either XX or XY. So the count of sex-chromosome pairs is one, making option (a) correct.

The chromosomal basis of sex. The simplest way to answer is to start from the genotypes: a human female is XX and a male is XY. Because the mother contributes one chromosome from her pair and she has only X chromosomes, every egg is identical in this respect, carrying an X. The father, being XY, produces two kinds of sperm in equal numbers: X-bearing and Y-bearing.

Putting it together at fertilisation. The zygote's sex chromosomes come half from the egg and half from the sperm. The egg's half is always X, so the outcome is read entirely off the sperm. An X-sperm produces an XX zygote (girl); a Y-sperm produces an XY zygote (boy). Since the two sperm types are equally common, boys and girls are expected in roughly equal numbers.

Why the mother's role is fixed. To judge whether the mother "plays a significant role", look at what variation she can supply. Her genotype is XX, so her gametes are uniform in their sex chromosome: each egg carries exactly one X. There is no mechanism by which her eggs could carry a Y. So no matter what other genes she passes on, the sex chromosome she contributes is always X.

Where the real variation lives. The difference between a son and a daughter comes entirely from whether an X-sperm or a Y-sperm wins the race to the egg, and that is set by the father. So the mother's genetic combination does not significantly decide the sex of the newborn. This is an important point for dispelling the harmful and unscientific belief that a woman is responsible for the sex of her child.

Reading off the female's gametes. The answer follows directly from the female genotype. A woman carries two X chromosomes as her 23rd pair. There simply is no Y chromosome in her cells to pass on.

What meiosis does to the pair. When eggs are formed, meiosis separates each homologous pair so that a gamete receives just one chromosome from the pair. Applied to the XX pair, this guarantees that each egg gets exactly one X. Both daughter cells are the same in this respect, so the female is described as the "homogametic" sex, producing only one kind of gamete with respect to the sex chromosome. This is why all human eggs carry an X chromosome.

Where the 1:1 comes from. The even sex ratio is a direct result of how the father's sperm are made. His sex-chromosome pair is XY. When meiosis separates this pair, half the resulting sperm carry the X and half carry the Y, in a clean 1:1 ratio. The mother contributes nothing to the variation, since her eggs are all X.

From sperm ratio to child ratio. Because the two kinds of sperm are equally many and any one of them is equally likely to fertilise the egg, the resulting zygotes are equally likely to be XX or XY. So the statistical probability of a male child equals that of a female child, giving the familiar 50:50. It is worth stressing that this is a probability for each individual birth, not a guarantee about any one family.

Naming the contrasting pairs. A "character" is the feature being studied; its "contrasting traits" are the two visibly opposite versions. For seed colour the two versions are yellow and green; for seed shape they are round and wrinkled. These are two of the seven classic pea characters Mendel followed.

Assigning dominance. Mendel's F₁ crosses settle which trait is dominant. When pure yellow-seeded plants were crossed with pure green-seeded ones, all F₁ seeds were yellow, so yellow is dominant and green recessive. Likewise, crossing pure round with pure wrinkled gave all-round F₁ seeds, so round is dominant and wrinkled recessive. The recessive traits did not vanish; they reappeared in about one quarter of the F₂ plants.

What makes an organism good for genetics. Mendel's success owed a lot to his choice of material. He needed an organism he could grow in large numbers cheaply, that bred fast enough to track several generations, and that showed traits in sharp either/or forms rather than blended shades. The pea fitted on every count: cheap, hardy, and quick, with seven clean pairs of contrasting characters.

The breeding control advantage. Just as important, pea flowers are naturally self-pollinating, so a strain stays pure on its own, giving Mendel reliable pure-breeding parents. Yet the flowers are large enough that he could remove the anthers and apply pollen from a chosen plant, performing precise crosses by hand. This combination of natural purity and easy controlled crossing let him design clean experiments and count clear ratios.

Working through the genetics. Start with what is fixed and what varies. The woman's eggs are all X, so she contributes an X to every child without exception. Each of her daughters is XX, which means the second X in each daughter came from an X-bearing sperm of the father. So every one of her pregnancies happened to involve an X-sperm.

Why this is just chance. The father makes X- and Y-sperm in equal numbers, and which one fertilises a given egg is random. Over a small number of children it is quite possible for all the successful sperm to be X-bearing, just as a coin can land heads several times in a row. So a woman having only daughters is a chance result of repeated X-sperm fertilisation. Her own genotype, being XX, plays no part in selecting the sex.

Drawing the dividing line. The cleanest way to tell the two apart is to ask where the character is stored. Inherited characters live in the genes that sit in the gametes, so they are copied into the next generation each time reproduction happens. Eye colour and pea-seed colour are classic inherited characters because they trace back to specific alleles.

Why acquired characters stop with the individual. Acquired characters develop in response to the environment, use or disuse, or injury, and they change only the body cells. A weightlifter's enlarged muscles or a finger lost in an accident never reach the DNA of the sperms or eggs. So when that individual reproduces, the offspring start from the unaltered genes and do not show the acquired feature. This distinction is the heart of why Lamarck's idea of inheritance of acquired characters does not hold up.

The germ-line rule. The reason acquired characters are not inherited comes down to one rule: only changes in the germ line, the cells that make gametes, can be passed on. A trait gained during life, through exercise, diet, habit or injury, is built in the body's somatic cells. These cells are not the ones that form the next generation.

No path into the gametes. Because the genetic information in the eggs and sperms stays unchanged, there is no route by which the acquired feature can be copied into the offspring's starting DNA. The child therefore begins from the parents' original genes, not from any modification the parent picked up. This is why, for example, the children of a body-builder are not born with extra muscle. The principle directly refutes the inheritance of acquired characters.

Working each cross by its gametes. The fastest, safest way to read each cross is to first list the gametes each parent can make. Parts (a) and (c) involve pure-breeding parents, so each parent makes only one kind of gamete (RY in a, ry in c). Identical gametes combine to give offspring identical to the parents: all round-yellow in (a) and all wrinkled-green in (c).

The heterozygous and test-style crosses. Part (b) is the textbook dihybrid self: each RrYy parent makes four gamete types (RY, Ry, rY, ry), and combining them in a 4×4 Punnett square gives the famous 9:3:3:1 phenotype ratio. Part (d) crosses a pure round-yellow with a pure wrinkled-green; one parent supplies RY and the other ry, so all offspring are RrYy and, by dominance of round and yellow, all look round and yellow even though they secretly carry the recessive alleles.

Reading the cross diagram. The pedigree-style layout gives the parents and the F₁ and leaves the second F₁ partner blank. The phrase "self pollination in F₁" is the clue: a self-cross means the plant is mated with its own genotype. Since the F₁ generation here is entirely RrYy, the partner in the blank must also be RrYy.

Why uniform F₁ makes this certain. Because both original parents were pure-breeding for opposite alleles, every F₁ plant is the same double-heterozygote RrYy. So whichever F₁ plant is used for selfing, its partner is genetically RrYy too. Filling the blank with RrYy sets up the dihybrid self-cross RrYy × RrYy, which in the next question produces the 9:3:3:1 F₂ ratio.

Building the dihybrid result. The cross RrYy × RrYy is the classic dihybrid self. The crucial step is realising each parent makes four equally likely gametes (RY, Ry, rY, ry) because the two genes assort independently. Pairing all four against all four fills a 16-box square, and sorting those 16 genotypes by appearance yields the four phenotype classes.

Reading the 9:3:3:1. Counting the boxes gives 9 plants showing both dominant traits (round, yellow), 3 showing round with the recessive green, 3 showing the recessive wrinkled with yellow, and a single plant showing both recessive traits (wrinkled, green). Two of these four classes (round-green and wrinkled-yellow) are new combinations not present in the original parents. The clean 9:3:3:1 ratio is the hallmark of two independently assorting genes.

The genes-on-chromosomes picture. The mechanism of inheritance is best stated as a short set of rules about genes. Characters are governed by genes; each gene comes in alternative forms (alleles); the alleles sit at fixed positions on chromosomes; and a diploid individual holds two copies of each gene, one from each parent. Dominance explains why one allele can mask the other in a heterozygote.

How the copies move between generations. The dynamic part is what happens at reproduction. When gametes form, the two copies of each gene separate so that a gamete carries just one copy (Mendel's law of segregation). When a sperm fertilises an egg, one maternal and one paternal copy come together, rebuilding the paired condition in the offspring. Different genes assort independently, which reshuffles the combinations each generation.

Tracing the new combinations. The appearance of new character combinations in the F₂ is a direct consequence of how the F₁ makes its gametes. In the double-heterozygote RrYy, each gene pair lines up and separates on its own during meiosis. Because the behaviour of the R/r pair is independent of the Y/y pair, a gamete is just as likely to carry R with Y as R with y.

From four gametes to recombinants. This independence gives four gamete types (RY, Ry, rY, ry) in equal numbers. When these combine freely at fertilisation, the F₂ ends up with trait pairings the parents never had: round-green and wrinkled-yellow alongside the original round-yellow and wrinkled-green. These recombinant types are the new combinations of characters. So the underlying reason is Mendel's law of independent assortment, and it is precisely this reshuffling that makes sexual reproduction such a powerful source of variation.