Class 10 › Science › Chapter 8
Heredity
How variations accumulate, Mendel's rules of inheritance (dominant and recessive traits, independent inheritance), how genes express traits, and sex determination in humans.
1. Accumulation of Variation During Reproduction
- Offspring inherit a common body design plus subtle changes. Each generation inherits the variations of the previous one and adds new ones, so diversity accumulates over generations.
- Asexual reproduction (e.g. bacteria): very minor differences, only from small DNA-copying inaccuracies (a sugarcane field shows little variation). Sexual reproduction: much greater diversity (clearly visible variation in humans).
- Variations do not have equal survival chances; the environment selects variants (heat-resistant bacteria survive a heat wave). This is the basis of evolution.
- Trait that is more common (60% vs 10%) in an asexually reproducing population is likely to have arisen earlier, since it had more time to spread.
2. Heredity and Inherited Traits
Heredity: the reliable inheritance of traits and characteristics from parents to offspring, following definite rules.
- A child has all the basic human features but does not look exactly like its parents.
- Both parents contribute practically equal amounts of genetic material, so each trait has two versions in the child (one maternal, one paternal).
- Example variants: free and attached earlobes.
3. Mendel's Experiments: Inheritance of One Trait
Who
Gregor Johann Mendel (1822–1884)
Grew garden peas; combined science and mathematics and was the first to count individuals showing each trait in every generation.
Traits used
Contrasting characters
Round/wrinkled seeds, tall/short plants, white/violet flowers.
Tall × short pea plants (monohybrid cross)
Parents (P): pure tall (TT) × pure short (tt).
F1: all tall (Tt). No 'medium-height' plants: only one parental trait is seen, not a blend.
F2 (F1 self-pollinated): 3 tall : 1 short (¼ short). Genotype ratio 1 TT : 2 Tt : 1 tt.
Conclusion: both tallness and shortness were inherited in F1, but only tallness was expressed. Each plant carries two copies of a factor (now called genes) for each trait, which may be identical or different.
| Term | Meaning |
|---|---|
| Dominant trait | Expressed even with a single copy (T). Both TT and Tt are tall. |
| Recessive trait | Expressed only when both copies are the recessive form (tt: short). |
| F1 / F2 | First and second filial generations |
- Cross summary (Tt × Tt): gametes T or t from each parent; offspring TT, Tt, Tt, tt.
- Blood group question: a man of group A and a woman of group O having a daughter of group O is not enough to say which trait is dominant; the child receives one copy from each parent, so the data fit more than one possibility.
4. Inheritance of Two Traits (Dihybrid Cross)
Round-green × wrinkled-yellow seeds (RR yy × rr YY)
F1: all round and yellow (Rr Yy). So round and yellow are dominant; wrinkled and green are recessive.
F2 (F1 self-pollinated) gives four types in the ratio 9 : 3 : 3 : 1: round yellow, round green, wrinkled yellow, wrinkled green (Mendel counted 315 : 108 : 101 : 32 out of 556 seeds).
New combinations (round-green, wrinkled-yellow) appear that were not in the parents.
- Tall round × short wrinkled peas: F1 all tall and round (dominant). In F2, some are tall and wrinkled, or short and round.
- Independent inheritance: traits such as seed shape and colour (or height and seed shape) are inherited independently of each other, so new combinations form in F2.
- Exercise logic: tall violet × short white gives all violet, about half short, so the tall parent must be TtWW.
5. How do Traits Get Expressed?
- Gene: a section of DNA providing information for making one protein. Genes control traits through proteins.
- Example (height): plant height depends on the amount of a growth hormone. The gene for an enzyme in its synthesis, if working efficiently, makes more hormone and the plant is tall. An altered gene gives a less efficient enzyme, less hormone and a short plant.
- Each body cell has two copies of each gene, one from each parent. Each germ cell has only one copy (one set).
- Chromosomes: the gene set exists as separate independent pieces (chromosomes), not one long thread, so genes can be inherited independently. Cells have two copies of each chromosome (maternal and paternal); each germ cell takes one of each pair.
- When two germ cells fuse, the normal chromosome number is restored, keeping the DNA of the species stable. This ensures equal contribution of the male and female parent. Asexually reproducing organisms follow similar rules (the copy comes from one parent).
6. Sex Determination
Environmental
Temperature
In a few reptiles, the incubation temperature of fertilised eggs decides male or female.
Sex change
Snails
Individuals can change sex, so sex is not genetically determined.
Genetic
Human beings
Largely determined by the genes (chromosomes) inherited from parents.
Sex chromosomes in humans
Humans have 22 pairs of matching chromosomes plus one pair of sex chromosomes.
Women: XX (perfect pair). Men: XY (a normal-sized X and a short Y).
Every egg carries an X. Sperms carry X or Y in equal numbers.
Father's X sperm + egg → XX girl. Father's Y sperm + egg → XY boy. So half the children are boys and half girls, and the sex is decided by the father's chromosome.
7. Quick Revision
| Cross | F1 | F2 ratio |
|---|---|---|
| Monohybrid (TT × tt) | All tall (Tt) | Phenotype 3 tall : 1 short; genotype 1 TT : 2 Tt : 1 tt |
| Dihybrid (RRyy × rrYY) | All round yellow (RrYy) | 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green |
| Term | Remember |
|---|---|
| Gene | DNA section that codes for one protein |
| Dominant | Expressed with one copy |
| Recessive | Expressed only with two copies |
| Sex of child | Determined by the father (X or Y); mother gives X |
Common mistakes:
• Saying F1 plants are a medium blend; all F1 plants show only the dominant trait.
• Mixing up phenotype ratio (3 : 1) with genotype ratio (1 : 2 : 1).
• Writing 9 : 3 : 3 : 1 for a single-trait cross or 3 : 1 for a two-trait cross.
• Saying the mother determines the child's sex; the father's X or Y decides it.
• Giving gametes two copies of a gene; each gamete has only one copy of each gene (one chromosome of each pair).
• Concluding which trait is dominant from a single parent-child observation.
• Saying F1 plants are a medium blend; all F1 plants show only the dominant trait.
• Mixing up phenotype ratio (3 : 1) with genotype ratio (1 : 2 : 1).
• Writing 9 : 3 : 3 : 1 for a single-trait cross or 3 : 1 for a two-trait cross.
• Saying the mother determines the child's sex; the father's X or Y decides it.
• Giving gametes two copies of a gene; each gamete has only one copy of each gene (one chromosome of each pair).
• Concluding which trait is dominant from a single parent-child observation.