Chapter in a nutshell: Genetics is the study of heredity (passing of traits) and variation. Mendel's experiments on garden pea gave three laws — dominance, segregation and independent assortment. A monohybrid cross gives a 3:1 phenotypic ratio in F₂; a dihybrid cross gives 9:3:3:1. In humans, sex is decided by the XX/XY chromosomes, and the father determines the child's sex.
1. Heredity, Variation & Mendel
- Heredity: transmission of characters from parents to offspring. Variation: differences among individuals of a species.
- Gregor Mendel ("Father of Genetics") worked on the garden pea (Pisum sativum).
- Why pea was ideal: clear contrasting characters (tall/dwarf, round/wrinkled seeds), normally self-pollinating (pure lines) but easy to cross-pollinate, short life cycle, many offspring.
2. Key Terms
| Term | Meaning |
|---|---|
| Gene | unit of heredity (a DNA segment) controlling a trait |
| Allele | alternative form of a gene (e.g. T and t) |
| Dominant | allele expressed in the hybrid (T = tall) |
| Recessive | allele masked in the hybrid (t = dwarf) |
| Genotype | genetic make-up (e.g. Tt) |
| Phenotype | observable trait (e.g. tall) |
| Homozygous | identical alleles (TT or tt) — "pure" |
| Heterozygous | different alleles (Tt) — "hybrid" |
| F₁ / F₂ | first / second filial (offspring) generation |
| Monohybrid / dihybrid | cross involving one / two pairs of traits |
3. Monohybrid Cross (one trait)
Cross a pure tall (TT) with a pure dwarf (tt):- F₁: all Tt → tall (dominance of T).
- F₁ × F₁ (Tt × Tt): Punnett square —
| T | t | |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
- F₂ phenotypes: 3 tall : 1 dwarf (3:1).
4. Mendel's Laws
- Law of Dominance: in a hybrid, only the dominant allele is expressed.
- Law of Segregation (Purity of Gametes): the two alleles of a trait separate during gamete formation, each gamete carrying only one.
- Law of Independent Assortment: alleles of different traits are inherited independently of one another (basis of the dihybrid cross).
5. Dihybrid Cross (two traits)
Round-yellow (RRYY) × wrinkled-green (rryy): F₁ all round-yellow (RrYy). F₁ selfed → F₂ phenotypic ratio $$\textbf{9 : 3 : 3 : 1}$$ (9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green) — demonstrating independent assortment.6. Sex Determination in Humans
- 23rd pair = sex chromosomes: female XX, male XY.
- Eggs always carry X; sperm carry X or Y.
- X-sperm + egg → XX (girl); Y-sperm + egg → XY (boy).
- So the father determines the sex of the child; the chance of a boy or girl is 50 : 50 each time.
7. Sex-Linked Inheritance
Genes on the X chromosome (X-linked recessive) cause traits like haemophilia (failure of blood to clot) and colour blindness. These appear more often in males because a male has only one X — a single recessive allele is enough to show the trait; a female needs both X's affected.8. Worked Genetics Problems (ICSE pattern)
Q1. Cross a pure tall (TT) with a dwarf (tt). Give the F₁ genotype and phenotype. Solution: F₁ = Tt, all tall.Q2. In a Tt × Tt cross, what fraction of offspring are dwarf? Solution: 1/4 (genotype tt).
Q3. A test cross (Tt × tt) — give the offspring ratio. Solution: 1 Tt (tall) : 1 tt (dwarf) → 1:1.
Q4. Phenotype ratio in F₂ of a monohybrid cross? Solution: 3 : 1 (dominant : recessive).
Q5. A colour-blind man marries a normal (non-carrier) woman. Will their sons be colour-blind? Solution: No — sons get the Y from father and a normal X from mother (the daughters become carriers).
9. Key Terms — Quick Glossary
| Term | One-line definition |
|---|---|
| Heredity | transmission of traits parent → offspring. |
| Variation | differences among individuals of a species. |
| Allele | alternative form of a gene. |
| Dominant / recessive | expressed / masked allele in a hybrid. |
| Genotype / phenotype | genetic make-up / visible trait. |
| Homozygous / heterozygous | identical / different alleles. |
| Monohybrid cross | cross for a single trait (3:1 in F₂). |
| Dihybrid cross | cross for two traits (9:3:3:1 in F₂). |
| Sex chromosomes | XX (female), XY (male). |
10. Common Mistakes to Avoid
- Confusing genotype (Tt) with phenotype (tall).
- Writing the F₂ phenotypic ratio as 1:2:1 — that is the genotypic ratio; phenotypic is 3:1.
- Saying the mother determines the child's sex — it is the father (X or Y sperm).
- Mixing up homozygous (pure) and heterozygous (hybrid).
- Forgetting that the dihybrid ratio is 9:3:3:1 (not 3:1).
11. Likely Exam Questions (with crisp answers)
- Define heredity and variation. → Heredity = transmission of traits to offspring; variation = differences among individuals.
- Why did Mendel choose the garden pea? → Clear contrasting traits, self- and cross-pollination possible, short life cycle.
- Differentiate genotype and phenotype. → Genetic make-up vs observable trait.
- State the F₂ phenotypic and genotypic ratios of a monohybrid cross. → 3:1 and 1:2:1.
- State Mendel's law of segregation. → The two alleles separate during gamete formation; each gamete gets one.
- State the law of independent assortment. → Alleles of different traits are inherited independently.
- What is the F₂ ratio of a dihybrid cross? → 9:3:3:1.
- Who determines the sex of a child and why? → The father — he contributes either an X or a Y sperm.
- Why are sex-linked recessive disorders commoner in males? → Males have a single X, so one recessive allele shows the trait.
- Name two sex-linked disorders in humans. → Haemophilia and colour blindness.
- Define a test cross. → Crossing an individual with a homozygous recessive to find its genotype.
- What is an allele? → An alternative form of a gene.
- Define homozygous and heterozygous. → Identical alleles (TT/tt) vs different alleles (Tt).
- What chance does each child have of being a girl? → 50 % (1 in 2).
12. Mendel's Seven Pea Characters (contrasting traits)
| Character | Dominant | Recessive |
|---|---|---|
| Stem height | Tall | Dwarf |
| Seed shape | Round | Wrinkled |
| Seed colour | Yellow | Green |
| Flower colour | Violet | White |
| Pod shape | Inflated | Constricted |
| Pod colour | Green | Yellow |
| Flower position | Axial | Terminal |
13. Dihybrid Punnett Square (gametes RY, Ry, rY, ry)
Crossing RrYy × RrYy, the 16 combinations give phenotypes in the ratio 9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green. The 9:3:3:1 outcome proves that the seed-shape gene and seed-colour gene assort independently.14. Chromosomal Basis & DNA
- Genes are located on chromosomes; alleles of a gene sit at the same locus on homologous chromosomes.
- During meiosis, homologues segregate (Mendel's law of segregation) and different chromosome pairs assort independently (law of independent assortment) — linking genetics to cell division.
- The gene is made of DNA; the base sequence is the genetic code.
15. Mutation (variation source)
A mutation is a sudden, heritable change in the genetic material (DNA/chromosome). Mutations create new alleles and are a raw material for variation and evolution; some cause disorders. Agents (mutagens) include X-rays, UV light and certain chemicals.16. More Worked Problems
Q6. In peas, round (R) is dominant over wrinkled (r). Cross Rr × Rr. Give phenotype ratio. Solution: 3 round : 1 wrinkled. Q7. A heterozygous tall pea (Tt) is crossed with a dwarf (tt). What fraction of offspring are tall? Solution: 1/2 (Tt) → 50 % tall, 50 % dwarf. Q8. A carrier woman (XᴴX) for colour blindness marries a normal man (XY). What fraction of sons are colour-blind? Solution: Half the sons (those receiving Xᴴ) are colour-blind. Q9. Both parents are heterozygous (Tt). State the genotypic ratio of offspring. Solution: 1 TT : 2 Tt : 1 tt.17. More Exam Questions (with crisp answers)
- State Mendel's law of dominance. → In a hybrid, only the dominant allele is expressed.
- What is a Punnett square used for? → To predict the genotypes/phenotypes of offspring from a cross.
- Define mutation. → A sudden heritable change in the genetic material.
- Why is the dihybrid ratio 9:3:3:1 significant? → It demonstrates the law of independent assortment.
- Where on a chromosome is an allele located? → At a fixed position called its locus.
- Give the genotype of a pure tall and a hybrid tall pea. → TT (pure) and Tt (hybrid).
- What is the source of new variations in a population? → Mutations (and recombination during meiosis).
- Why did Mendel's F₁ generation show only the dominant trait? → Because of the law of dominance — the recessive allele is masked.
- Name the human chromosome pair that decides sex. → The 23rd pair (XX/XY).
18. Quick Difference Tables
| Dominant trait | Recessive trait |
|---|---|
| expressed in the hybrid (F₁) | masked in the hybrid |
| needs only one allele to show | needs both alleles (homozygous) to show |
| Homozygous | Heterozygous |
|---|---|
| identical alleles (TT, tt) | different alleles (Tt) |
| breeds true | does not breed true |
19. Applications & Importance of Genetics
- Predicting inheritance of traits and genetic disorders (genetic counselling).
- Plant/animal breeding for better yield and disease resistance.
- Understanding evolution through variation and natural selection.
- Basis of modern biotechnology (e.g. genetic engineering).
20. Final Quick-Revision Q&A
- A trait skips a generation — is it likely dominant or recessive? → Recessive (masked in the carrier generation).
- Why is Tt called heterozygous? → It has two different alleles for the trait.
- Name the dominant and recessive seed-shape traits in pea. → Round (dominant), wrinkled (recessive).
- Define genetic counselling. → Advising prospective parents about the risk of inherited disorders.
- State one reason variation is important. → It helps a species adapt and is the basis of evolution.
- If both parents are Tt, can a child be dwarf? → Yes, with probability 1/4 (tt).
- What is the phenotype of a pea with genotype Tt? → Tall (T is dominant).
- What is meant by "purity of gametes"? → Each gamete carries only one allele of a gene pair (law of segregation).
- Name the plant Mendel used and its scientific name. → Garden pea, Pisum sativum.
- In humans, which sperm produces a female child? → An X-bearing sperm (X + X = XX).
- Define filial generation. → The offspring generations (F₁, F₂) of a genetic cross.
- Why could Mendel's conclusions be drawn confidently? → He studied one trait at a time over many plants and analysed the numerical ratios statistically.