Heredity
Mendel's rules, dominant and recessive traits, and how sex is determined in humans.
Based on NCERT Class 10 Science, Chapter 8
Accumulation of variation during reproduction
- Inheritance passes on a common body design plus small changes to the next generation.
- Asexual reproduction gives only tiny variations caused by errors in DNA copying; sexual reproduction creates much more diversity.
- Variations are not equally useful — the environment selects the variants that survive better, and this selection is the basis of evolution.
8.2 Heredity — inherited traits
- Rules of inheritance come from the fact that both mother and father contribute equal amounts of genetic material to the child.
- So each trait can be influenced by both a maternal and a paternal DNA copy.
- Mendel worked with garden pea (Pisum sativum) and chose contrasting visible traits: tall/short plants, round/wrinkled seeds, white/violet flowers.
Parents (2 copies of every gene) → gametes (1 copy) → offspring (2 copies again)Watch it happen — Punnett Square Cross
Controls
Live result
Mendel's monohybrid cross of Tt × Tt gives a 3:1 phenotype and 1:2:1 genotype ratio.
Monohybrid cross — dominant and recessive
- Tall (TT) × short (tt) gives an F1 generation that is all tall (Tt): tallness is dominant, shortness is recessive.
- Selfing the F1 gives an F2 ratio of 3 tall : 1 short, i.e. genotypes 1 TT : 2 Tt : 1 tt.
- A trait can therefore stay hidden for a generation and reappear — proof that the two copies of a gene separate independently into gametes.
F2 phenotype ratio = 3 : 1 F2 genotype ratio = 1 : 2 : 1Watch it happen — Punnett Square Cross
Controls
Live result
Mendel's monohybrid cross of Tt × Tt gives a 3:1 phenotype and 1:2:1 genotype ratio.
Q1.A tall pea plant (TT) is crossed with a short one (tt). Give the F₁ and F₂ genotype and phenotype ratios.
3 marksDihybrid cross — independent inheritance
- Round-yellow (RRYY) × wrinkled-green (rryy) gives all round-yellow in F1.
- F2 shows 9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green, so seed shape and seed colour are inherited independently of each other.
Dihybrid F2 ratio = 9 : 3 : 3 : 1Q1.State the phenotypic ratio of a dihybrid cross in F₂ and what it proves.
3 marksHow do traits get expressed?
- A gene is a section of DNA that carries the information for making a protein (usually an enzyme).
- If one copy of the gene makes enough of the working enzyme, one copy is enough — that version behaves as dominant, and the non-working version is recessive.
- Traits controlled by genes are inherited; changes made only to body tissue during a lifetime (acquired traits) are not inherited.
Watch it happen — Trait Inheritance Probability
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Live result
A recessive trait appears only when both copies of the allele are recessive.
Q1.How do genes control the characteristics or traits of an organism?
2 marks8.2.4 Sex determination in humans
- Humans have 23 pairs of chromosomes; the last pair decides sex. Women are XX, men are XY.
- All eggs carry an X chromosome. Half the sperms carry X and half carry Y.
- If an X-sperm fertilises the egg the child is a girl (XX); if a Y-sperm does, the child is a boy (XY). The father's sperm, not the mother, decides the sex of the child.
XX (mother) × XY (father) → 50% XX girls, 50% XY boysWatch it happen — Sex Determination in Humans
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Live result
The father contributes X or Y, so the father's gamete determines the sex of the child.
Q1.Explain how the sex of a child is determined in human beings.
3 marks