1. Heredity and Inheritance
Heredity is the transmission of traits from parents to offspring. A trait is a characteristic of an organism, such as seed shape or height. Traits are influenced by genes and can be transmitted from one generation to the next.
During reproduction, genetic information is copied and passed to the next generation. In sexual reproduction, genetic material from two parents contributes to the offspring, creating new combinations of traits.
2. Mendel's Experiments
Gregor Johann Mendel studied inheritance using pea plants. He selected contrasting traits and carried out controlled crosses to understand how characteristics are transmitted.
2.1 Why Pea Plants?
- They have several easily distinguishable contrasting traits.
- They can self-pollinate and can also be cross-pollinated manually.
- They have a relatively short generation time.
- Large numbers of offspring can be obtained.
2.2 Dominant and Recessive Traits
When Mendel crossed plants with contrasting forms of a trait, the F1 generation often showed only one form. The trait expressed in the F1 generation was called dominant; the trait not expressed in F1 was called recessive.
When F1 plants were allowed to self-pollinate, the recessive trait reappeared in the F2 generation.
2.3 How Traits Are Controlled
For a simple Mendelian trait, an individual receives one version of a gene from each parent. These versions are called alleles. A dominant allele can determine the phenotype when present with a recessive allele; a recessive phenotype appears when the relevant recessive alleles are present in the required combination.
3. Mendel's Laws and Dihybrid Inheritance
Law of Dominance
When two contrasting alleles occur together in a heterozygote, one may express itself as the dominant trait while the other remains unexpressed in the phenotype.
Law of Segregation
The two alleles of a gene separate during gamete formation, so each gamete receives only one allele of the pair.
Dihybrid Cross
A dihybrid cross follows inheritance of two pairs of contrasting traits at the same time. Mendel's work showed that alleles for different traits can assort independently when the relevant genes are inherited independently.
| Term | Meaning |
|---|---|
| Homozygous | Two identical alleles, e.g. TT or tt. |
| Heterozygous | Two different alleles, e.g. Tt. |
| Genotype | Genetic constitution for a trait. |
| Phenotype | Observable characteristic or appearance. |
4. Sex Determination in Humans
Humans have 23 pairs of chromosomes. One pair is the sex chromosomes; the remaining 22 pairs are autosomes.
| Person | Sex chromosomes | Gametes |
|---|---|---|
| Typical female | XX | Eggs carry X. |
| Typical male | XY | Sperm carry either X or Y. |
Therefore, under the usual XXβXY system, the sperm contributes either an X or Y chromosome. This is why the chromosome combination in the fertilising sperm determines whether the zygote is XX or XY.
5. Variation and Its Importance
Variation means differences among individuals of the same species. Variation can arise from errors or changes during DNA copying and from the reshuffling of genetic material during sexual reproduction.
Why Variation Matters
- It creates differences among individuals.
- Some variations can help organisms survive particular environmental conditions.
- Heritable variations can accumulate over generations.
- Variation provides the raw material on which natural selection can act.
6. Evolution and Natural Selection
Evolution refers to changes in the inherited characteristics of populations over generations. The CBSE 2026β27 material describes evolution in terms of changes in the frequency of inherited traits/genes in a population over generations. ξciteξturn0search21ξ
Natural Selection
When individuals with certain heritable variations survive and reproduce more successfully in a particular environment, those variations can become more common in the population over generations. This is the central idea of natural selection.
Beetle Illustration
Imagine a population of red beetles living on green leaves. If a heritable green variation appears, green beetles may be less visible to predators and may survive and reproduce more successfully. Over generations, green beetles can become more common.
A different variation may become common for another reason. For example, a population bottleneck caused by a random environmental event can leave a small set of survivors. By chance, their inherited characteristics can become more common in the recovering population.
7. Acquired and Inherited Traits
| Inherited trait | Acquired trait |
|---|---|
| Associated with genetic information passed through reproductive cells. | Develops during an individual's lifetime due to environment, use, training, injury or other experiences. |
| Can be transmitted to offspring if the relevant genetic information is inherited. | Generally not transmitted simply because the parent acquired it during life. |
| Example: genetically influenced eye colour. | Example: muscles developed through exercise. |
A change in body cells caused during an individual's lifetime does not automatically alter the DNA of reproductive cells. Therefore, acquired changes are generally not inherited in the Mendelian sense.
8. Speciation
Speciation is the formation of new species. It can occur when populations of the same species become separated and accumulate genetic differences over many generations until they can no longer successfully interbreed to produce fertile offspring.
Factors That Can Contribute
9. Evolution and Classification
Classification groups organisms according to similarities and differences. Similarities in basic body design and molecular characteristics can provide clues about evolutionary relationships.
Homologous Organs
Organs with the same basic structural plan but different functions are called homologous organs. The forelimbs of humans, bats and whales have the same basic skeletal pattern but are adapted for different functions. Homologous structures provide evidence of common ancestry and divergent evolution.
Analogous Organs
Organs with similar functions but different basic structural designs are called analogous organs. The wings of birds and insects perform flight but have different structural origins. Analogous structures illustrate functional similarity that does not necessarily indicate close common ancestry.
| Homologous | Analogous |
|---|---|
| Basic structure similar; function may differ. | Function similar; basic structure may differ. |
| Supports common ancestry. | Often reflects adaptation to similar functions. |
| Example: forelimbs of human and whale. | Example: wings of bird and insect. |
10. Fossils and Evidence of Evolution
Fossils are preserved remains, impressions or traces of organisms from the past. They provide evidence about organisms that lived earlier and help scientists reconstruct evolutionary history.
How Fossils Help
- They provide evidence of extinct organisms.
- Fossils found in deeper rock layers are generally older than those in higher layers, subject to geological context.
- Comparing fossils from different layers can reveal changes in body forms over time.
- Fossil evidence can help identify transitional features in evolutionary histories.
Dating Fossils
The relative age of fossils can be estimated from their position in rock layers. Radiometric methods can provide numerical age estimates by measuring radioactive decay in suitable materials.
11. Evolution by Stages
Complex structures can evolve through a series of small, functional changes rather than appearing fully formed at once. For example, the evolution of the eye can be understood as a sequence in which increasingly complex light-sensitive structures provide useful functions at each stage.
Similarly, the evolution of complex body structures can involve changes in existing structures followed by natural selection acting on heritable variations.
12. Evolution Is Not the Same as Progress
Evolution is not a ladder in which organisms move from lower to higher forms. Different organisms are adapted to different environments. A simpler organism can be highly successful in its environment, and a complex organism is not automatically "more evolved" in a value-based sense.
Evolutionary change reflects survival and reproduction under particular environmental conditions, along with mutation, recombination, genetic drift, migration and other population processes.
13. Human Evolution
Humans are part of the primate group. Human evolution involved a branching history with several hominin species. Modern humans, Homo sapiens, share common ancestry with other primates.
Important evolutionary evidence includes fossils, comparative anatomy, embryological evidence and molecular evidence such as similarities in DNA and proteins.
14. Key Definitions
15. Quick Revision Sheet
16. 30 Practice Questions
- Define heredity and variation.
- What is a gene?
- What is the role of DNA in heredity?
- Why did Mendel select pea plants for his experiments?
- What is a dominant trait?
- What is a recessive trait?
- Differentiate genotype and phenotype.
- Differentiate homozygous and heterozygous conditions.
- Explain the monohybrid cross TT Γ tt.
- State the typical phenotypic and genotypic ratios of a Mendelian monohybrid Fβ cross.
- State the law of segregation.
- What is a dihybrid cross?
- State the classic Fβ phenotypic ratio for a Mendelian dihybrid cross.
- How is sex determined in humans?
- Why does the sperm determine the XX or XY chromosome combination?
- What is variation and why is it important?
- Differentiate inherited and acquired traits.
- What is natural selection?
- Explain the beetle example of natural selection.
- What is evolution?
- What is speciation?
- How can geographical isolation contribute to speciation?
- What is genetic drift?
- What are homologous organs?
- What are analogous organs?
- Give one example each of homologous and analogous organs.
- What are fossils?
- How do fossils provide evidence for evolution?
- Why should evolution not be equated with progress?
- Why is the idea that humans evolved directly from modern monkeys incorrect?
17. CBSE PYQ / PYQ-Based Practice
These questions are designed around recurring CBSE concepts, competency-style applications and the current 2026β27 learning emphasis on Mendelian inheritance and evolutionary change.
- In a cross between pure tall and pure dwarf pea plants, all Fβ plants are tall. Explain why the dwarf trait is not visible in Fβ.
- Show the expected Fβ genotypes and phenotypes for Tt Γ Tt.
- Why can a recessive trait reappear in the Fβ generation even though it was not visible in Fβ?
- Differentiate genotype and phenotype with a suitable example.
- Explain the significance of Mendel's law of segregation.
- A plant with round yellow seeds is crossed with a plant having wrinkled green seeds. Explain how a dihybrid cross can produce different combinations of traits in offspring.
- Why does sexual reproduction generally generate more variation than asexual reproduction?
- Explain human sex determination using XX and XY chromosomes.
- Why is it scientifically incorrect to hold the mother responsible for the sex of a child?
- Distinguish between inherited and acquired traits using suitable examples.
- Explain how a heritable variation can become more common in a population through natural selection.
- A population is drastically reduced by a random environmental event. Explain how the surviving population can have a different frequency of traits from the original population.
- How can geographical isolation contribute to speciation?
- Explain why reproductive isolation is important in the formation of new species.
- Differentiate homologous and analogous organs and state what each tells us about evolution.
- Why are the forelimbs of humans, bats and whales considered homologous?
- Why are wings of birds and insects considered analogous?
- How do fossils help scientists understand evolutionary relationships?
- Explain why evolution should not be described as a ladder of progress from "lower" to "higher" organisms.
- Give scientific evidence supporting the common ancestry of humans and other primates.