NCERT Solutions for CBSE Class 10 Biology — 22 solved questions with detailed explanations.
Difficulty: Easy · Topic: Mendel's Experiments
Gregor Mendel used pea plants (Pisum sativum) for his experiments on heredity. He chose pea plants because they have many clearly distinguishable contrasting traits (tall/short, round/wrinkled seeds, etc.), a short generation time, produce many offspring, and can be easily cross-pollinated or self-pollinated.
Difficulty: Easy-Medium · Topic: Monohybrid Cross
When Mendel crossed pure tall (TT) with pure short (tt) plants, all F1 offspring were tall (Tt). The short trait seemed to disappear. This demonstrates that tallness is dominant over shortness. The F1 plants were heterozygous (Tt) — they carried the recessive allele (t) but did not express it because the dominant allele (T) masked it. The recessive trait reappeared in the F2 generation when two recessive alleles came together (tt).
Difficulty: Easy-Medium · Topic: Sex Determination
In humans, the mother always contributes an X chromosome (since she is XX). The father can contribute either an X or a Y chromosome (since he is XY). If the sperm carries X → child is XX (female). If the sperm carries Y → child is XY (male). Therefore, the father's sperm determines the sex of the child. The probability is 50:50 for each pregnancy.
Difficulty: Easy-Medium · Topic: Evolution
Homologous organs have the same basic structure and developmental origin but may serve different functions in different organisms. For example, the forelimbs of a whale, bat, horse, and human all contain the same bones (humerus, radius, ulna, carpals, metacarpals, phalanges) but are used for swimming, flying, running, and grasping respectively. This similarity in structure, despite different functions, is strong evidence that these organisms share a common ancestor — this is called divergent evolution.
Difficulty: Easy-Medium · Topic: Fossils
Fossils are preserved remains, impressions, or traces of organisms that lived in the past. They provide evidence for evolution because: (1) Older (deeper) rock layers contain simpler organisms, while newer (upper) layers contain more complex organisms, showing a progression over time. (2) Some fossils show transitional forms — organisms with features of two different groups (e.g., Archaeopteryx has features of both reptiles and birds). (3) By dating fossils, scientists can reconstruct the evolutionary history of a lineage.
Difficulty: Easy-Medium · Topic: Mendel's Laws
Mendel's Law of Segregation (First Law) states that each organism possesses two alleles for each character, and during gamete (sex cell) formation, these two alleles segregate (separate) so that each gamete receives only one allele. When gametes fuse during fertilisation, the offspring again has two alleles — one from each parent. This explains the reappearance of the recessive trait in the F2 generation.
Difficulty: Easy-Medium · Topic: Evolution
Archaeopteryx is a famous fossil that has features of both reptiles (teeth in the jaw, clawed fingers, a long bony tail) and birds (feathers, a wishbone/furcula, wings). It is considered a transitional form or connecting link between reptiles and birds, providing strong evidence that birds evolved from reptilian ancestors (specifically, theropod dinosaurs).
Difficulty: Easy-Medium · Topic: Heredity
An organism with genotype Tt has two different alleles for the same gene — one dominant (T) and one recessive (t). This condition is called heterozygous. Homozygous means both alleles are the same (TT = homozygous dominant, tt = homozygous recessive). The heterozygous organism (Tt) will express the dominant phenotype (tall) but carries the recessive allele, which can be passed to offspring.
Difficulty: Medium · Topic: Monohybrid Cross
When two heterozygous (Tt) plants are crossed:
Gametes: Each parent produces T and t gametes.
Punnett square:
TT (1) : Tt (2) : tt (1)
Genotypic ratio = 1 TT : 2 Tt : 1 tt
The phenotypic ratio is 3 tall (TT + Tt) : 1 short (tt), but the genotypic ratio is 1:2:1. This distinction between genotypic and phenotypic ratios is crucial for understanding Mendelian genetics.
Difficulty: Medium · Topic: Dihybrid Cross
In a dihybrid cross (e.g., RrYy × RrYy), the F2 generation shows a phenotypic ratio of 9:3:3:1. For example: 9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green. This ratio results from the independent assortment of the two gene pairs. The appearance of new combinations (Round Green and Wrinkled Yellow) that were not present in either parent demonstrates that different traits are inherited independently.
Difficulty: Medium · Topic: Evolution
Wings of a bird (made of bones, muscles, and feathers) and wings of an insect (made of chitin — a thin membrane supported by veins) are analogous organs. They perform the same function (flight) but have completely different structures and origins. Birds and insects did not inherit wings from a common winged ancestor — they evolved wings independently because flight was advantageous in their environments. This is called convergent evolution.
Difficulty: Medium · Topic: Natural Selection
Darwin's theory of natural selection states that individuals with variations best suited to their environment are more likely to survive, reproduce, and pass on their advantageous traits. This is often called 'survival of the fittest' — but 'fittest' does NOT mean strongest or biggest. It means best adapted to the specific environment. A small, camouflaged insect may be 'fitter' than a large, conspicuous one if camouflage helps it avoid predators. The concept depends entirely on the environment.
Difficulty: Medium · Topic: Speciation
When a population is divided by a physical barrier (mountain range, river, ocean), the two groups are geographically isolated. Over many generations, each group independently accumulates genetic changes through mutations, natural selection, and genetic drift. Because each group experiences different environmental pressures, they evolve in different directions. Eventually, they become so genetically different that they can no longer interbreed even if the barrier is removed — they have become separate species. This is allopatric speciation.
Difficulty: Medium · Topic: Evolution
Genetic drift refers to random changes in allele frequencies in a population. In large populations, random fluctuations tend to average out and have little impact. But in small, isolated populations, random events can significantly shift allele frequencies. For example, if a few individuals colonise a new island (founder effect), the new population's gene pool may differ markedly from the original population's, purely by chance. This can accelerate divergence and contribute to speciation.
Difficulty: Medium · Topic: Evolution
Fossil and genetic evidence strongly supports the 'Out of Africa' theory — that all modern humans (Homo sapiens) originated in Africa approximately 200,000–300,000 years ago. From Africa, humans migrated to other continents over tens of thousands of years. DNA analysis shows that African populations have the greatest genetic diversity, consistent with being the oldest human populations.
Difficulty: Medium · Topic: Mendel's Laws
The Law of Independent Assortment states that alleles of different genes are inherited independently of each other during gamete formation. This can only be demonstrated in a dihybrid cross (involving two genes), where the 9:3:3:1 ratio in F2 shows that the two traits segregate independently. A monohybrid cross involves only one gene and thus cannot demonstrate independent assortment. This law holds when genes are on different chromosomes.
Difficulty: Medium · Topic: Evolution
Evolution is defined as the change in the inherited characteristics (allele frequencies) of a biological population over successive generations. Option A is wrong — evolution does not always lead to increased complexity (bacteria are successful despite being simple). Option B is wrong — humans and chimps share a common ancestor but humans did NOT evolve from modern chimps. Option D is wrong — all living organisms evolve, including plants, bacteria, and fungi.
Difficulty: Medium-Hard · Topic: Heredity
Both are true and R correctly explains A. In Mendel's monohybrid cross (TT × tt), all F1 plants were heterozygous (Tt). They showed only the dominant trait (tall) because the dominant allele (T) masks the expression of the recessive allele (t) when both are present in a heterozygous individual. The recessive trait (short) reappeared in the F2 generation when two recessive alleles came together in a homozygous recessive (tt) individual.
Difficulty: Medium-Hard · Topic: Sex Determination
Both are true and R correctly explains A. The mother is XX, so all her eggs carry an X chromosome. The father is XY, so his sperms are of two types — 50% carry X and 50% carry Y. If an X-sperm fertilises the egg → XX (girl). If a Y-sperm fertilises the egg → XY (boy). Since the distinguishing chromosome (Y) comes only from the father, it is the father's contribution that determines the sex of the child.
Difficulty: Medium-Hard · Topic: Heredity
This is a test cross — crossing an organism of unknown genotype with a homozygous recessive (tt). If the tall parent were Tt, the cross Tt × tt would give 50% tall (Tt) and 50% short (tt). But ALL offspring are tall, meaning no tt offspring appeared. This is most likely only if the tall parent is TT, because TT × tt gives 100% Tt (all tall). While Tt × tt could theoretically give all tall offspring by chance (if the sample is small), with a large number of offspring all being tall, TT is the most likely genotype.
Difficulty: Medium-Hard · Topic: Heredity
Colour blindness is an X-linked recessive trait. The cross:
Mother (carrier): XBXb × Father (colour-blind): XbY
Sons receive their X from the mother and Y from the father:
— XBY (normal vision) — 50%
— XbY (colour-blind) — 50%
So half (1/2) of the sons will be colour-blind. For daughters: XBXb (carrier, normal vision) and XbXb (colour-blind) — 50% of daughters will also be colour-blind.
Difficulty: Hard · Topic: Evolution
This is based on the famous peppered moth (Biston betularia) example of natural selection. During industrial pollution, tree trunks became dark with soot, so dark moths were better camouflaged and survived better (industrial melanism). If pollution decreases and tree trunks become lighter again (with lichen), light-coloured moths would be better camouflaged against predators. Natural selection would now favour lighter moths, and their frequency would increase over generations. Evolution can shift direction when selection pressures change — it is not a one-way process.
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