Heredity — Class 10 Biological Science

"Why do you look like your parents — but not EXACTLY like them? The answer is HEREDITY — the passing of traits from one generation to the next."

1. About the Chapter

Heredity explains how traits are INHERITED. This chapter covers Mendel's laws, monohybrid and dihybrid crosses, sex determination, and the theory of evolution by natural selection.

Key Terms

2. Mendel's Experiments

Gregor Mendel (1822–1884), an Austrian monk, bred PEA PLANTS. He chose 7 CONTRASTING characters: Tall/Dwarf. Round/Wrinkled seeds. Yellow/Green seeds. 'Mendel's genius: he used MATHEMATICS (counting ratios). He started with PURE-BREEDING lines (homozygous).'

Mendel's Three Laws

Law 1 — Dominance: In a heterozygote, the DOMINANT allele masks the RECESSIVE. Tt = Tall. 'The recessive allele is PRESENT in the genotype but HIDDEN in the phenotype.'

Law 2 — Segregation: The two alleles for a trait SEPARATE during gamete formation. Each gamete receives ONE allele. 'This is why Tt parent produces T gametes AND t gametes — in EQUAL numbers.'

Law 3 — Independent Assortment: Genes for DIFFERENT traits assort INDEPENDENTLY during gamete formation. 'The gene for height does NOT influence the gene for seed colour. They are inherited INDEPENDENTLY.'

3. Monohybrid Cross (One Character)

Cross: TT (tall, pure) × tt (dwarf, pure). F1: ALL Tt (tall — T is dominant). Self-pollinate F1: Tt × Tt. Gametes: T, t (from each parent). Punnett square: TT, Tt, tT, tt.

Phenotype ratio: 3 Tall : 1 Short (two phenotypes). Genotype ratio: 1 TT : 2 Tt : 1 tt (three genotypes).

'If Mendel had stopped here, he would have discovered dominance and segregation. But he went FURTHER — to dihybrid crosses.'

4. Dihybrid Cross (Two Characters)

Cross: RRYY (round yellow, pure) × rryy (wrinkled green, pure). F1: ALL RrYy (round yellow — both dominant). Self-pollinate F1. Gametes: RY, Ry, rY, ry (4 types from each parent — independent assortment!).

F2 Phenotype ratio — 9 : 3 : 3 : 1

• 9 Round Yellow (R_Y_) — both dominant traits
• 3 Round Green (R_yy) — round dominant, green recessive
• 3 Wrinkled Yellow (rrY_) — wrinkled recessive, yellow dominant
• 1 Wrinkled Green (rryy) — both recessive traits

'This 9:3:3:1 ratio is the SIGNATURE of a dihybrid cross. It PROVES that alleles for DIFFERENT traits are inherited INDEPENDENTLY — Mendel's Law of Independent Assortment.'

5. Sex Determination

Humans: 46 chromosomes (23 pairs). 22 pairs = AUTOSOMES. 1 pair = SEX CHROMOSOMES. Female: XX. Male: XY.

Who determines the sex of the child? Mother ALWAYS contributes X (all her eggs carry X). Father contributes either X (→ XX, girl) or Y (→ XY, boy). 'The FATHER determines the sex of the child. It is the SPERM — carrying X or Y — that decides. The mother has NO role in sex determination.'

6. Evolution by Natural Selection

Darwin's Theory: 1. VARIATION exists in populations. 2. More offspring are PRODUCED than can SURVIVE. 3. STRUGGLE for existence. 4. Individuals with ADVANTAGEOUS variations survive and reproduce MORE. 5. These traits are PASSED ON. Over GENERATIONS → the population EVOLVES.

Evidence for Evolution

7. Common Mistakes

1. 'Mendel's laws apply to all traits' — They apply to traits controlled by SINGLE genes with CLEAR dominant/recessive patterns. Many traits (height, skin colour) are POLYGENIC — controlled by multiple genes.
2. '9:3:3:1 is the monohybrid ratio' — 9:3:3:1 is DIHYBRID. Monohybrid = 3:1 (phenotype).
3. 'Mother determines the child's sex' — The FATHER determines it (sperm carries X or Y). 'This is a deeply important fact for AP students to know — it counters the social prejudice that blames women for the sex of the child.'

8. AP SSC Exam Focus

9. Mendel's Laws — Detailed Explanation with Punnett Squares

Law of Dominance — Step by Step

'When Mendel crossed PURE-BREDING tall plants (TT) with pure-breeding dwarf plants (tt), ALL the F1 offspring were TALL. Why? Because the T (tall) allele is DOMINANT over the t (dwarf) allele.'

Parent generation: TT (tall) × tt (dwarf) Gametes: T, T from one parent; t, t from the other parent. F1 generation: ALL Tt (tall — T masks t). 'Even though the F1 plants LOOK tall, they CARRY the dwarf allele. The dwarf trait is not LOST — it is HIDDEN.'

Law of Segregation — The Punnett Square

'When two F1 plants (Tt × Tt) are crossed, the two alleles for height SEPARATE during gamete formation. Each gamete gets EITHER T or t — NOT both.'

        T         t
   T    TT        Tt
   t    Tt        tt

Genotype ratio: 1 TT : 2 Tt : 1 tt Phenotype ratio: 3 Tall : 1 Short

'The 3:1 ratio is the SIGNATURE of a monohybrid cross. It proves that alleles SEGREGATE during gamete formation.'

Test Cross

'A cross between an organism with a DOMINANT phenotype (but unknown genotype — could be TT or Tt) and a HOMOZYGOUS RECESSIVE (tt).'

If the dominant parent is TT: ALL offspring are Tt (ALL tall). If the dominant parent is Tt: 50% are Tt (tall), 50% are tt (short).

'The test cross REVEALS the hidden genotype. This is how Mendel CONFIRMED his law of segregation.'

10. Dihybrid Cross — Detailed Analysis

'In a dihybrid cross, Mendel studied TWO traits simultaneously: seed shape (Round R / Wrinkled r) and seed colour (Yellow Y / Green y).'

Parental generation: RRYY (Round Yellow) × rryy (Wrinkled Green) F1 generation: ALL RrYy (Round Yellow — both dominant traits expressed)

'When Mendel self-pollinated the F1 (RrYy × RrYy), something remarkable happened. The F1 plants produced FOUR types of gametes: RY, Ry, rY, and ry — in EQUAL proportions. This PROVED that the genes for seed shape and seed colour assort INDEPENDENTLY.'

        RY          Ry          rY          ry
   RY   RRYY        RRYy        RrYY        RrYy
   Ry   RRYy        RRyy        RrYy        Rryy
   rY   RrYY        RrYy        rrYY        rrYy
   ry   RrYy        Rryy        rrYy        rryy

F2 Phenotype Ratio — 9:3:3:1

• 9 Round Yellow (R_Y_)
• 3 Round Green (R_yy)
• 3 Wrinkled Yellow (rrY_)
• 1 Wrinkled Green (rryy)

'The 9:3:3:1 ratio is the DEFINITIVE proof of Mendel's Law of Independent Assortment. If the genes were LINKED (on the same chromosome), this ratio would NOT appear.'

11. Sex Determination in Humans — A Deeper Look

'Human cells have 23 pairs of chromosomes: 22 pairs of AUTOSOMES + 1 pair of SEX CHROMOSOMES. The sex chromosomes determine the sex of the child.'

Female: XX — produces EGGS, each carrying an X chromosome. Male: XY — produces SPERM, half carrying X, half carrying Y.

'At fertilisation: if an X-carrying sperm fertilises the egg → XX → FEMALE. If a Y-carrying sperm fertilises the egg → XY → MALE.'

Why the Father Determines Sex

'The mother ALWAYS contributes X. The father contributes either X (→ girl) or Y (→ boy). The sex of the child is determined by the SPERM — which chromosome the father's sperm carries.'

Probability: 50% chance of a boy, 50% chance of a girl. 'THIS IS WHY blaming women for the sex of a child is scientifically ABSURD. The father's sperm determines the sex.'

Sex Determination in Other Organisms

12. More Worked Examples — Monohybrid Crosses

Example 1: In pea plants, purple flower (P) is dominant over white flower (p). A heterozygous purple-flowered plant (Pp) is crossed with a white-flowered plant (pp). What are the genotypic and phenotypic ratios of the offspring?

Solution: Gametes from Pp: P and p. Gametes from pp: p and p.

        P        p
   p    Pp       pp
   p    Pp       pp

Genotype ratio: 1 Pp : 1 pp. Phenotype ratio: 1 Purple : 1 White (50% each).

'This cross (heterozygous × homozygous recessive) gives a 1:1 ratio — NOT 3:1. The 3:1 ratio only appears when BOTH parents are heterozygous (Tt × Tt).'

Example 2: In humans, free earlobes (E) is dominant over attached earlobes (e). A man with free earlobes (genotype Ee) marries a woman with attached earlobes (ee). What is the probability that their child will have attached earlobes?

Solution: Gametes from man: E and e. Gametes from woman: e and e. Offspring: 50% Ee (free earlobes), 50% ee (attached earlobes). Probability of attached earlobes = 50% or 1/2.

Example 3: A pure-breeding tall pea plant (TT) is crossed with a pure-breeding dwarf plant (tt). The F1 plants are self-pollinated. What proportion of the F2 plants will be TRUE-BREEDING tall?

Solution: F1: All Tt (tall). F2 from Tt × Tt: TT, Tt, tT, tt. TRUE-BREEDING tall = TT = 1 out of 4 = 25%. (Tt and tT are tall but NOT true-breeding — they will produce some dwarf offspring in the next generation.)

13. Self-Test Questions

1. State Mendel's THREE laws of inheritance. Give an example of each.
2. Draw a Punnett square for a monohybrid cross between Tt and Tt. Write the genotypic and phenotypic ratios.
3. What is the phenotypic ratio of a DIHYBRID cross (9:3:3:1)? Explain what each number represents.
4. Who determines the sex of the child — the mother or the father? Explain with a diagram.
5. Differentiate between HOMOLOGOUS STRUCTURES and ANALOGOUS STRUCTURES with examples.
6. Explain Darwin's theory of Natural Selection. Give TWO pieces of evidence for evolution.
7. A heterozygous round-seeded plant (Rr) is crossed with a wrinkled-seeded plant (rr). What is the probability of obtaining round seeds?
8. What is a TEST CROSS? Why is it used?
9. How do FOSSILS provide evidence for evolution? Give an example.
10. Why did Mendel choose PEA PLANTS for his experiments?

Answers to Selected Questions: Q7: Gametes from Rr: R and r. From rr: r and r. Offspring: 50% Rr (round), 50% rr (wrinkled). Probability of round seeds = 50%. Q10: Pea plants have: (1) Distinct, contrasting traits (tall/dwarf, round/wrinkled). (2) Self-pollinating but can be cross-pollinated manually. (3) Short life cycle — many generations in a short time. (4) Large number of offspring — reliable statistical data.