Heredity is the passage of traits from one generation to the next. Variation is the differences between individuals. Together they drive evolution. CBSE Class 10 introduces heredity through Mendel’s work and basic sex determination.
Core Concepts
Traits and variation
A trait is any characteristic like eye colour or height. Variations arise from different alleles of the same gene, and from new mutations. Variation is raw material for natural selection.
Sources of variation:
- Genetic recombination during meiosis (crossing over shuffles alleles)
- Independent assortment of chromosomes during gamete formation
- Mutations — changes in DNA sequence (point mutations, insertions, deletions)
- Random fertilisation — any sperm can fertilise any egg
Types of traits:
- Qualitative traits: Discrete categories (blood group A/B/AB/O, flower colour red/white). Controlled by one or few genes.
- Quantitative traits: Continuous variation (height, weight, skin colour). Controlled by many genes (polygenic inheritance) plus environment.
Genes and chromosomes
Genes are segments of DNA that code for proteins or RNA. Chromosomes are long DNA molecules with associated proteins. Humans have 23 pairs — 22 autosomes and 1 sex pair (XX female, XY male).
Key terminology:
- Locus: The specific position of a gene on a chromosome
- Allele: Alternative forms of a gene at the same locus (e.g., T for tall, t for dwarf)
- Homozygous: Same alleles (TT or tt)
- Heterozygous: Different alleles (Tt)
- Dominant: Allele expressed in heterozygous condition
- Recessive: Allele masked in heterozygous condition
- Genotype: Genetic constitution (TT, Tt, or tt)
- Phenotype: Physical expression (tall or dwarf)
Human chromosome facts:
- 46 chromosomes total (23 pairs)
- 22 pairs of autosomes + 1 pair of sex chromosomes
- Females: 44 + XX. Males: 44 + XY
- Total genes: approximately 20,000-25,000
- Total DNA per cell: about 6.4 billion base pairs (3.2 billion per haploid set)
Mendel’s experiments
Mendel crossed pure-breeding tall and dwarf peas. F1 were all tall. F2 showed a 3:1 ratio. This was the first quantitative proof that traits are discrete and heritable, not blended.
Why Mendel chose peas:
- Many clearly contrasting traits (7 pairs)
- Short generation time (one season)
- Large number of offspring per cross
- Both self-pollination (natural) and cross-pollination (artificial) possible
- Pure lines available
The seven traits Mendel studied:
| Trait | Dominant | Recessive |
|---|---|---|
| Stem height | Tall | Dwarf |
| Seed shape | Round | Wrinkled |
| Seed colour | Yellow | Green |
| Pod shape | Inflated | Constricted |
| Pod colour | Green | Yellow |
| Flower colour | Purple | White |
| Flower position | Axial | Terminal |
Mendel’s F2 results for monohybrid cross: Total plants: 787 tall, 277 dwarf. Ratio: 2.84:1 (close to 3:1).
For dihybrid cross (round yellow × wrinkled green): F2: 315 round yellow, 108 round green, 101 wrinkled yellow, 32 wrinkled green. Ratio: 9.8 : 3.4 : 3.2 : 1 (close to 9:3:3:1).
Sex determination in humans
Males make X and Y sperm in equal ratio. The egg is always X. So the father determines the child’s sex. This is a frequent social myth-busting question in CBSE.
Sex determination mechanisms in different organisms:
| Organism | System | Female | Male |
|---|---|---|---|
| Humans, most mammals | XX-XY | XX | XY |
| Birds, butterflies | ZW-ZZ | ZW | ZZ |
| Grasshoppers | XX-XO | XX | XO |
| Honeybees | Haplodiploidy | Diploid (2n) | Haploid (n) |
In humans, the SRY gene on the Y chromosome triggers male development. Without SRY (or if the Y chromosome is absent), the default developmental pathway is female.
Probability of having a boy or girl:
| X (from mother) | X (from mother) | |
|---|---|---|
| X (from father) | XX (girl) | XX (girl) |
| Y (from father) | XY (boy) | XY (boy) |
Wait — this table is wrong. Let me fix: mother always gives X. Father gives X or Y with equal probability. So each child has a 50% chance of being male and 50% female, regardless of previous children.
Acquired vs inherited traits
Acquired traits during lifetime (muscle from exercise, tan from sun) are not passed to offspring. Only changes in gametic DNA are inherited. This disproves Lamarck’s ‘inheritance of acquired characters’.
Lamarck proposed that organisms develop traits based on use/disuse and pass them on. Example: giraffes stretching their necks → longer necks in offspring. Darwin and modern genetics showed this is wrong — only mutations in germ cells (eggs/sperm) can be inherited. Somatic changes die with the individual.
However: Epigenetic changes (DNA methylation, histone modification) can sometimes be inherited for a few generations without changing the DNA sequence. This is an active area of research that blurs the line slightly — but it does not support Lamarckism in the classical sense.
Evolution and natural selection
Variation + differential survival = natural selection. Individuals with traits better suited to the environment survive and reproduce more, passing those traits to the next generation. Over many generations, this changes the population.
Evidence for evolution:
- Fossil record: Shows gradual changes over geological time
- Homologous organs: Same structure, different function (human arm, whale flipper, bat wing)
- Analogous organs: Different structure, same function (bird wing, insect wing)
- Molecular evidence: DNA/protein sequence similarities reflect evolutionary relationships
- Biogeography: Similar species on different continents suggest common ancestry
Worked Examples
Brown is dominant. Both parents could be heterozygous (Bb). F2 can include bb, which is blue. 25% probability in each pregnancy.
Mother contributes X only. Father contributes X (gives daughter) or Y (gives son). The father is the source of the Y chromosome.
During meiosis, independent assortment creates (about 8.4 million) possible gamete combinations from each parent. Crossing over adds even more variation. And random fertilisation means any sperm can meet any egg. The number of possible genetic combinations from two parents is essentially infinite.
Attached earlobes are recessive (e). Free earlobes are dominant (E). Both parents have free earlobes but a child has attached earlobes.
This means both parents must be heterozygous: Ee × Ee → 1 EE : 2 Ee : 1 ee. The child is ee. Probability = 1/4.
Common Mistakes
Writing that blending inheritance is correct. It is not — Mendel proved traits are particulate.
Saying mother determines child’s sex. Father does, via the sex chromosome in sperm.
Confusing gene and allele. A gene is the locus; alleles are the variants at that locus.
Thinking acquired traits can be inherited. A bodybuilder’s muscles are not passed to their children. Only changes in DNA of germ cells (sperm/egg) are inheritable.
Confusing homologous and analogous organs. Homologous organs have the same evolutionary origin but different functions (arm, flipper, wing). Analogous organs have different origins but similar functions (bird wing, insect wing).
Exam Weightage and Revision
CBSE Class 10 boards ask about heredity in every paper — 5-8 marks on Mendel’s experiments, sex determination, and evolution. NEET tests this at a deeper level with genetics problems. This is a foundational topic that builds into Class 12 genetics.
When a question gives a scenario, identify the core mechanism first, then match it to the concepts above. Most wrong answers come from reading the scenario too quickly.
Three facts to lock — 23 pairs, father determines sex, 3:1 is Mendel’s ratio. All basic heredity PYQs build on these.
Practice Questions
Q1. A pea plant with round seeds (Rr) is crossed with a plant with wrinkled seeds (rr). What is the expected ratio?
Rr × rr → Rr (round) and rr (wrinkled) in equal numbers. Phenotypic ratio: 1 round : 1 wrinkled. This is a test cross — the 1:1 ratio confirms the round-seeded parent is heterozygous.
Q2. Why are variations important for species survival?
Variations ensure that some individuals in a population are better adapted to changing environments. If conditions change (new disease, climate shift, new predator), individuals with suitable variations survive and reproduce. Without variation, an entire population could be wiped out by a single environmental change. Variation is the raw material for natural selection.
Q3. In which organisms does the mother determine the sex of offspring?
In organisms with the ZW-ZZ system (birds, butterflies, some fish). The female is ZW (produces Z and W eggs) and the male is ZZ (produces only Z sperm). The egg determines whether the offspring is male (Z egg + Z sperm = ZZ) or female (W egg + Z sperm = ZW).
Q4. What are homologous organs? Give two examples.
Homologous organs have the same embryonic origin and basic structure but may perform different functions. Examples: (1) Human arm and whale flipper — both have humerus, radius, ulna, carpals, but arm is for grasping and flipper is for swimming. (2) Human arm and bat wing — same bones, but wing is for flying. Homologous organs are evidence of common ancestry (divergent evolution).
Q5. How does natural selection differ from artificial selection?
In natural selection, nature determines which traits are advantageous — organisms with better-adapted traits survive and reproduce. In artificial selection, humans choose which traits are desirable and selectively breed organisms with those traits (dog breeding, crop improvement). Both change allele frequencies in populations, but artificial selection is faster and directed towards human goals.
FAQs
Are all mutations harmful? No. Most mutations are neutral (they occur in non-coding DNA or do not change protein function). Some are harmful (sickle cell mutation when homozygous). A few are beneficial (sickle cell trait provides malaria resistance when heterozygous). The effect depends on the environment.
Can two brown-eyed parents have a blue-eyed child? Yes, if both parents are heterozygous (Bb). There is a 25% chance of a bb (blue-eyed) child in each pregnancy. This is a classic application of Mendel’s law of segregation.
Why do some diseases run in families? Genetic diseases are caused by mutations in specific genes. If both parents carry a recessive disease allele (are carriers), they can have affected children (25% chance). This is why diseases like sickle cell anaemia, cystic fibrosis, and thalassemia appear more frequently in certain families.
What is the difference between heredity and inheritance? The terms are often used interchangeably. Technically, heredity refers to the general phenomenon of traits being passed from parents to offspring. Inheritance refers to the specific pattern by which a particular trait is passed (dominant, recessive, X-linked, etc.).
Heredity is where genetics becomes real for students. Connect it to family traits you can see, and the abstractions become obvious.