Class 10 Science • Complete Chapter Notes
Heredity is the transmission of characteristics from parents to offspring. The study of heredity helps us understand why offspring resemble their parents while also showing variations.
Evolution refers to changes in inherited characteristics of populations over generations. Variations generated during reproduction can become more or less common in populations depending on environmental conditions and natural selection.
Variation means differences in characteristics among individuals of the same species.
Variations can arise because DNA copying is not perfectly accurate and because sexual reproduction combines genetic material from two parents.
Heredity is the transmission of traits from parents to offspring through genetic material.
Traits are influenced by genes, which are segments of DNA that carry information related to inherited characteristics.
A gene is a functional unit of heredity. Genes are located on chromosomes and contain information that contributes to specific traits.
A particular gene can occur in different forms called alleles. Different combinations of alleles contribute to different inherited characteristics.
Gregor Johann Mendel studied inheritance using pea plants. He selected contrasting traits and followed their inheritance across generations.
His experiments helped establish the idea that inherited characteristics are controlled by discrete units, now understood as genes.
Mendel observed that one version of a trait can mask the expression of another version in a heterozygous condition. The expressed version is described as dominant and the masked version as recessive.
A monohybrid cross studies inheritance of one pair of contrasting traits.
The F₁ generation receives one allele from each parent. When two heterozygous F₁ individuals are crossed, the typical Mendelian genotype ratio is 1 : 2 : 1 and the phenotype ratio for complete dominance is 3 : 1.
A dihybrid cross follows inheritance of two pairs of contrasting traits simultaneously.
When the relevant conditions of independent assortment are satisfied, the classical Mendelian F₂ phenotypic ratio is 9 : 3 : 3 : 1.
Organisms with different genotypes can sometimes have the same phenotype when one allele is dominant over another.
During reproduction, offspring receive genetic material from their parents. In sexual reproduction, one set of chromosomes is contributed by each parent through gametes.
The combination of alleles inherited by an offspring determines its genotype for particular traits.
Human females generally have XX sex chromosomes, while human males generally have XY sex chromosomes.
An ovum normally carries an X chromosome. Sperm may carry either an X or a Y chromosome.
Inherited traits are characteristics influenced by genetic information passed from parents to offspring.
Acquired traits develop during an organism's lifetime due to environmental influences, use, behaviour or experience and are generally not passed to offspring through the same inherited genetic mechanism.
Not all variations have the same effect on survival and reproduction. Environmental conditions can favour some inherited variations over others.
When a particular inherited characteristic gives an advantage, organisms possessing it may survive and reproduce more successfully. Over generations, the associated genes can become more common in the population.
Natural selection is a process in which individuals with heritable characteristics that improve survival or reproductive success in a particular environment tend to leave more offspring.
The frequency of inherited characteristics can therefore change over generations.
Imagine a beetle population with a heritable colour variation. If green beetles are less visible to predators on green leaves, they may survive and reproduce more successfully than red beetles.
As generations pass, the proportion of green beetles can increase in the population.
Speciation is the formation of new species from existing populations.
When populations become sufficiently different and can no longer successfully interbreed to produce fertile offspring, they may become separate species.
Geographical isolation occurs when physical barriers such as mountains, rivers or large distances separate populations.
Once separated, populations can accumulate different variations over time. Different environmental pressures and genetic changes can contribute to divergence.
Genetic drift is a change in allele frequency caused by chance events, especially in relatively small populations.
A random event may leave only a small number of survivors. The genetic composition of the next population can therefore differ from the original population simply because of chance.
Fossils are preserved remains, impressions or other traces of ancient organisms found in rocks.
Fossils provide evidence about organisms that lived in the past and can help scientists reconstruct evolutionary history.
In undisturbed layers of sedimentary rocks, fossils found in deeper layers are generally older than fossils in overlying layers.
Classification groups organisms according to similarities and differences. Organisms sharing more characteristics may have a more recent common ancestor, although classification should be interpreted together with genetic and evolutionary evidence.
Similarities in basic body plans, molecular sequences and other characteristics can be used to infer evolutionary relationships.
Homologous structures have a similar basic structural plan and evolutionary origin but may perform different functions.
Analogous structures perform similar functions but have different structural origins.
DNA and protein sequence similarities can provide evidence about evolutionary relationships between organisms.
Generally, greater similarity in important inherited molecular sequences can indicate a closer evolutionary relationship, when considered with other evidence.
Humans and other modern primates share common ancestry. Human evolution involved a long sequence of population changes rather than a simple linear progression from one modern species into another.
Fossil discoveries, comparative anatomy, genetics and other scientific evidence are used together to understand human evolutionary history.
Evolution does not have a predetermined goal of making organisms more complex, more intelligent or “better”.
A trait is advantageous only in relation to a particular environment and set of conditions.
| Concept | Meaning |
|---|---|
| Heredity | Transmission of inherited characteristics from parents to offspring. |
| Variation | Differences among individuals of a population. |
| Natural Selection | Differential survival and reproduction associated with heritable characteristics. |
| Evolution | Change in inherited characteristics of populations over generations. |
| Speciation | Formation of new species. |
These notes follow the Class 10 Heredity and Evolution concepts reflected in current CBSE material. For complete textbook content and current curriculum details, consult the official sources below.
CBSE Science Reading Material 2026-27 → CBSE Class 10 Competency-Based Science Material → NCERT Class 10 Science Textbook → CBSE Academic Curriculum 2026-27 →