Heredity & Variations (Basic Genetics)
Heredity & Variations (Basic Genetics)
Heredity and Variations
Heredity is the transmission of characters (traits) from parents to offspring. It explains why children resemble their parents.
Variation means the differences seen among individuals of the same species. It explains why no two individuals are exactly alike (except identical twins).
| Term | Simple meaning | Exam-focus examples |
|---|---|---|
| Trait | Any observable character of an organism. | Height, eye colour, blood group, flower colour, seed shape. |
| Gene | Unit of heredity; a segment of DNA that controls a trait. | Gene for tallness in pea, gene for blood group in humans. |
| Chromosome | Thread-like structure in the nucleus carrying genes. | Humans have 46 chromosomes in body cells, 23 pairs. |
| DNA | Chemical molecule carrying genetic information. | Full form – Deoxyribo Nucleic Acid; double helix structure. |
Types of variations (basic level)
| Type of variation | Key points | Examples |
|---|---|---|
| Inherited variation | Passed from parents to offspring through genes. | Natural hair colour, blood group, ability to roll tongue. |
| Acquired variation | Develops during life due to environment, habits or training; not passed to next generation. | Muscles of an athlete, language learnt, scars on body. |
| Continuous variation | Shows many small differences; no clear groups. | Height, weight, skin colour in a population. |
| Discontinuous variation | Shows clear-cut categories; no middle values. | Blood groups (A, B, AB, O), ability to taste PTC. |
Why heredity and variation are important
- Heredity ensures continuity of species from one generation to next.
- Variation provides raw material for evolution and better adaptation to environment.
- In agriculture, knowledge of heredity helps in developing high-yielding, disease-resistant varieties of crops and animals.
Memory Line: “Genes on chromosomes carry traits; variations among genes create differences.”
Exam Tip: If the question asks whether a change is inherited or acquired, check: “Is it present from birth and passed to next generation?” → inherited. “Developed due to practice or environment only?” → acquired.
Mendel’s Experiments on Inheritance
Gregor Johann Mendel (1822–1884) is called the “Father of Genetics” for his work on garden pea plants.
Why Mendel chose pea plant (Pisum sativum)
- Short life cycle – several generations in few years.
- Many contrasting traits (tall/dwarf, round/wrinkled seeds, etc.).
- Flowers are normally self-pollinated but can be easily cross-pollinated by hand.
- Large number of seeds produced – good for counting ratios.
| Trait studied | Dominant form | Recessive form |
|---|---|---|
| Plant height | Tall | Dwarf |
| Seed shape | Round | Wrinkled |
| Seed colour | Yellow | Green |
| Pod colour | Green | Yellow |
| Flower colour | Violet | White |
Monohybrid cross – simple example
Cross between two plants differing in only one trait (for example, tall × dwarf).
| Generation | Cross / result | Phenotype ratio |
|---|---|---|
| P (parental) | Pure tall (TT) × pure dwarf (tt) | – |
| F₁ (first generation) | All plants tall (Tt) – only dominant trait appears. | All tall |
| F₂ (second generation) | Selfing of F₁ (Tt × Tt) | 3 tall : 1 dwarf |
Key words for exam
| Term | Meaning |
|---|---|
| Dominant trait | Trait that appears in F₁ generation (e.g., tallness). |
| Recessive trait | Hidden in F₁, reappears in F₂ (e.g., dwarfness). |
| Phenotype | Observable appearance (tall, dwarf, round seed). |
| Genotype | Genetic make-up (TT, Tt, tt). |
| Alleles | Different forms of same gene (T and t). |
Mendel’s basic ideas (school level)
- Traits are controlled by pair of factors (genes), one from each parent.
- In a pair, dominant factor expresses in F₁; recessive is hidden.
- During gamete formation, the pair of factors separate so that each gamete gets only one factor.
- In F₂, factors come together in new combinations giving simple ratios (3:1 in monohybrid).
Memory Line: “TT, Tt = tall; tt = dwarf; F₂ ratio 3 tall : 1 dwarf.”
Exam Tip: For any one-trait cross with complete dominance, if parents are pure contrasting types, F₁ will all show the dominant trait and the F₂ phenotype ratio will be 3 : 1. Watch carefully whether question talks about phenotype or genotype.
Sex Determination
Sex determination means deciding whether the offspring will be male or female. In many organisms this is controlled by sex chromosomes.
Sex determination in humans (XY system)
| Cell type | Chromosome set | Notes |
|---|---|---|
| Body cell of male | 44 autosomes + XY (46 chromosomes) | X and Y are sex chromosomes. |
| Body cell of female | 44 autosomes + XX (46 chromosomes) | Two X chromosomes. |
| Sperm | 22 autosomes + X or 22 autosomes + Y (23 total) | Half of sperms carry X, half carry Y. |
| Ovum | 22 autosomes + X (23 total) | All ova carry X only. |
How child’s sex is decided (simple explanation)
| Combination at fertilisation | Resulting child | Deciding gamete |
|---|---|---|
| X (ovum) + X (sperm) → XX | Female child | Sperm carrying X chromosome. |
| X (ovum) + Y (sperm) → XY | Male child | Sperm carrying Y chromosome. |
Thus, in humans the father’s sperm decides the sex of the child because sperm may carry either X or Y, while all eggs carry X only.
Other patterns (only for awareness at school level)
- Birds – opposite pattern: male is ZZ, female is ZW (here mother decides the sex).
- Some reptiles – temperature of the egg during early development can influence sex (environmental sex determination).
Social message linked to sex determination
- Sex of the child is decided naturally by chromosomes, not by behaviour or wishes of parents.
- It is scientifically wrong and socially unfair to blame mother for birth of girl child.
- Equal respect and opportunities should be given to both girls and boys.
Memory Line: “Mother gives X, father gives X or Y – so father’s sperm decides XX (girl) or XY (boy).”
Exam Tip: Typical questions ask: “Who determines the sex of the child?” or give options like XX, XY. Remember: XX = female, XY = male in humans, and the deciding gamete is the sperm.