Is it correct to say: Confusing genotype with phenotype?

GENOTYPE = genetic makeup (Tt, TT). PHENOTYPE = observable trait (tall, dwarf). Two different genotypes (TT and Tt) can share the same phenotype.

Is it correct to say: Writing 3:1 as the genotypic ratio of a monohybrid cross?

3:1 is the PHENOTYPIC ratio. Genotypic ratio is 1 TT : 2 Tt : 1 tt (1:2:1).

Is it correct to say: Saying the mother decides the sex of the child?

Mother is always XX — every ovum carries X. FATHER decides the sex because his sperm can be X (girl) or Y (boy).

Is it correct to say: Confusing heredity with evolution?

HEREDITY = transmission of traits from parents to offspring (within a family). EVOLUTION = change in inherited traits across MANY generations of a population.

Is it correct to say: Mixing up dominant and recessive alleles when writing Punnett squares?

Convention: CAPITAL letter for dominant (T), lowercase for recessive (t). A heterozygote (Tt) still shows the dominant phenotype.

Is it correct to say: Writing that acquired characters are inherited?

Acquired traits (body-builder muscles, cut tail of a mouse) are NOT inherited. Only changes in DNA of gametes pass to offspring — Lamarck's idea is disproved.

Heredity

Mendel's laws, monohybrid and dihybrid crosses, sex determination in humans, DNA structure and base pairing — NCERT Class 10 Biology Ch 9 (evolution rationalised out)

📐 9 formulas✏️ 3 examples🎯 6 practice⚖️ 5 marks🏫 CBSE📚 Class 10✓ 2025–26 syllabus

💡

Board Exam Tips

→Monohybrid cross Tt × Tt with Punnett square + 3:1 ratio — guaranteed 3-mark question.
→Dihybrid cross yielding the 9:3:3:1 ratio — 5-mark HOTS question.
→Sex determination in humans (XX vs XY) with a Punnett square — 3-mark question every 2 years.
→Difference between dominant and recessive traits — common 2-mark distinction.
→State Mendel's TWO laws — segregation and independent assortment (with definitions).

📐 Formulas(9)

Monohybrid Cross F₂ Phenotypic Ratio★ Board fav

T t \times T t \Rightarrow F_{2} = 3 Tall : 1 Dwarf

Symbol	Meaning
$T$	Dominant allele (tallness)
$t$	Recessive allele (dwarfness)

Monohybrid Genotypic Ratio

1 T T : 2 T t : 1 tt

Dihybrid Cross F₂ Phenotypic Ratio★ Board fav

R r Y y \times R r Y y \Rightarrow F_{2} = 9 : 3 : 3 : 1

Human Sex Determination★ Board fav

Father: X Y \times Mother: X X \Rightarrow 50% X X (girl) + 50% X Y (boy)

Human Chromosome Number

Somatic: 2 n = 46 (44 autosomes + 2 sex chromosomes)

DNA — Base Pairing Rule

A = T (2 H-bonds), G \equiv C (3 H-bonds)

DNA Nucleotide

Nucleotide = Deoxyribose (C_{5} H_{10} O_{4}) + Phosphate (P O_{4}^{3 -}) + Base (A, T, G, C)

Test Cross

Unknown (T_) \times tt \Rightarrow all tall \Rightarrow T T, else 1 : 1 \Rightarrow T t

Probability Rule (Independent Traits)

P (A \cap B) = P (A) \times P (B)

✏️ Solved Examples

1Solved Exampleeasy4 steps

In pea plants, tall (T) is dominant over dwarf (t). Cross two heterozygous tall plants (Tt × Tt). Show a Punnett square and state the phenotypic and genotypic ratios.

Each parent produces two gamete types: T and t

Gametes: T, t

2Solved Exampleboard5 steps

Mendel crossed a pure round-yellow (RRYY) with a pure wrinkled-green (rryy). Show the F₁ and F₂ generations and derive the 9:3:3:1 ratio.

P generation: RRYY × rryy. Each parent gives only one gamete type.

Gametes: R Y and r y

3Solved ExampleHOTS5 steps

A woman with blood group A marries a man with blood group B. All four blood groups (A, B, AB, O) are seen in their children. Determine the genotypes of BOTH parents and explain.

For blood group O to appear in a child, BOTH parents must contribute an I° allele. Hence each parent carries at least one I°.

⚠️ Traps & Common Mistakes

⚠️Common Mistakes6

1
Confusing genotype with phenotype
✓GENOTYPE = genetic makeup (Tt, TT). PHENOTYPE = observable trait (tall, dwarf). Two different genotypes (TT and Tt) can share the same phenotype.
2
Writing 3:1 as the genotypic ratio of a monohybrid cross
✓3:1 is the PHENOTYPIC ratio. Genotypic ratio is 1 TT : 2 Tt : 1 tt (1:2:1).
3
Saying the mother decides the sex of the child
✓Mother is always XX — every ovum carries X. FATHER decides the sex because his sperm can be X (girl) or Y (boy).
4
Confusing heredity with evolution
✓HEREDITY = transmission of traits from parents to offspring (within a family). EVOLUTION = change in inherited traits across MANY generations of a population.
5
Mixing up dominant and recessive alleles when writing Punnett squares
✓Convention: CAPITAL letter for dominant (T), lowercase for recessive (t). A heterozygote (Tt) still shows the dominant phenotype.
6
Writing that acquired characters are inherited
✓Acquired traits (body-builder muscles, cut tail of a mouse) are NOT inherited. Only changes in DNA of gametes pass to offspring — Lamarck's idea is disproved.

🎯 Practice Yourself

🎯Practice Yourself6 questions

Q1
Define dominant and recessive alleles with one example each.
Q2
State Mendel's two laws in one sentence each.
Q3
A monohybrid cross between two heterozygous purple-flowered plants (Pp) gives 320 F₂ plants. How many are expected to be white (pp)?
Q4
Give TWO differences between mitosis and meiosis relevant to inheritance.
Q5
Write the base-pairing rule for DNA and state which pair has more hydrogen bonds.
Q6
Why did Mendel choose the pea plant (Pisum sativum) for his experiments? Give any two reasons.

📝 Notes

Heredity

Heredity is the transmission of characters (genetic traits) from parents to offspring. Variation is the differences among individuals of the same species. Both are studied together because heredity supplies the traits and variation supplies the raw material for evolution.

Mendel — the father of genetics

Gregor Mendel (1856-1863) crossed pure-breeding pea plants and counted offspring types. Two laws emerged:

Law of Segregation — the two alleles of a gene separate during gamete formation; each gamete carries only one allele.
Law of Independent Assortment — genes for different traits are inherited independently (when located on different chromosomes).

Key terminology

Gene — a unit of heredity; a stretch of DNA coding for a trait.
Allele — alternative form of a gene (T vs t).
Dominant — expressed even in a single copy.
Recessive — only expressed when both alleles are recessive.
Homozygous — same alleles (TT or tt) — 'pure'.
Heterozygous — different alleles (Tt) — 'hybrid'.
Genotype — genetic makeup.
Phenotype — observable trait.

Monohybrid vs dihybrid crosses

| Cross | Parents | F₂ ratio | |---|---|---| | Monohybrid | Tt × Tt | 3 : 1 (phenotype), 1:2:1 (genotype) | | Dihybrid | RrYy × RrYy | 9 : 3 : 3 : 1 (phenotype) | | Test cross | Tt × tt | 1 : 1 (tall : dwarf) |

Sex determination in humans

Humans have 23 pairs of chromosomes; the 23rd pair decides sex.
Female = XX, Male = XY.
Ovum always carries X; sperm carries X OR Y — hence father decides the sex of the child (50:50 chance).

DNA — the genetic material

Double helix (Watson & Crick, 1953).
Nucleotide = deoxyribose sugar + phosphate + nitrogenous base.
Bases: Adenine, Thymine, Guanine, Cytosine.
Pairing: A=T (2 H-bonds), G≡C (3 H-bonds).
Gene = a segment of DNA that codes for a specific protein/trait.

Acquired vs inherited traits

Only changes in the DNA of gametes are inherited.
Acquired traits (muscles from exercise, cut tail) do NOT pass to offspring — Lamarck's idea has been disproved.

Quick sanity checks

Human somatic cell: 2n = 46; gamete: n = 23.
Punnett square rows/cols = number of distinct gamete types (2 for monohybrid, 4 for dihybrid).
9:3:3:1 = product of two independent 3:1 ratios ⇒ Mendel's second law.