Chapter 8
Heredity
HEREDITY is the passing on of characters (traits) from parents to their offspring. The rules that govern it are the rules of inheritance.
ACCUMULATION OF VARIATION: in ASEXUAL reproduction the offspring are almost identical, because only the small errors of DNA copying create differences — but over many generations these small differences ACCUMULATE. In SEXUAL reproduction, DNA comes from TWO parents, so far more variation is produced in a single generation.
A NICHE is the particular place, with its particular conditions, in which an organism lives. If the niche changes drastically (for example a pond heats up), only those variants able to tolerate the new conditions survive. Variation does NOT guarantee survival for an individual — it gives the SPECIES a chance to persist.
WHY MENDEL CHOSE THE GARDEN PEA (Pisum sativum): it grows quickly; it has several pairs of clearly CONTRASTING characters (tall/short, round/wrinkled seeds, yellow/green seeds); it normally SELF-pollinates but can be CROSS-pollinated by hand, so the experimenter controls the cross. Crucially, Mendel also COUNTED the offspring over thousands of plants — that was his real advance.
MONOHYBRID CROSS = a cross following ONE pair of contrasting characters. Pure TALL (TT) × pure SHORT (tt) → the F₁ generation is ALL TALL (Tt). The short character does not appear at all in the F₁.
F₁ selfed → F₂. The short plants REAPPEAR, in the ratio 3 TALL : 1 SHORT. This proves the short character was never destroyed — it was only HIDDEN in the F₁ because tallness is dominant.
TWO RATIOS FROM THE SAME F₂ SQUARE — students mix these up constantly. PHENOTYPIC RATIO (what you can SEE) = 3 tall : 1 short. GENOTYPIC RATIO (the actual allele combinations) = 1 TT : 2 Tt : 1 tt. Always read the question carefully to see which is being asked.
DOMINANT trait — expressed even when only ONE copy of the allele is present (tallness, T). RECESSIVE trait — expressed only when BOTH copies are present (shortness, t). A recessive trait can therefore be hidden for a generation and reappear later.
ALLELES are the alternative forms of the same gene (T and t). GENOTYPE is the combination of alleles an organism carries (TT, Tt or tt). PHENOTYPE is the characteristic you can actually observe (tall or short). TT and Tt have DIFFERENT genotypes but the SAME phenotype.
HOMOZYGOUS (pure) = both alleles the same (TT or tt). HETEROZYGOUS (hybrid) = the two alleles different (Tt). A pure tall plant is TT; a hybrid tall plant is Tt — both look tall.
DIHYBRID CROSS = a cross following TWO pairs of contrasting characters together. Round-yellow (RRYY) × wrinkled-green (rryy) → F₁ all ROUND and YELLOW (RrYy). So round is dominant over wrinkled, and yellow is dominant over green.
The dihybrid F₂ gives FOUR kinds of seed in the ratio 9 : 3 : 3 : 1 — 9 round-yellow, 3 round-green, 3 wrinkled-yellow, 1 wrinkled-green. The four gametes of an RrYy plant are RY, Ry, rY and ry, giving a 4 × 4 Punnett square of 16 boxes.
WHAT 9:3:3:1 PROVES: round-green and wrinkled-yellow are NEW combinations — neither parent had them. The characters have been reshuffled, so the two pairs of characters are INHERITED INDEPENDENTLY of one another. Whether a seed is round or wrinkled has nothing to do with whether it is yellow or green.
HOW A TRAIT IS EXPRESSED: cells work through PROTEINS, and each gene carries the information for making ONE protein. Plant height depends on a plant HORMONE, which is made by an ENZYME (itself a protein). If the gene works efficiently, plenty of hormone is made and the plant is TALL; if the gene is in an ALTERED form, the enzyme is LESS EFFICIENT, less hormone is made, and the plant is SHORT. A gene does not make the trait directly — it makes a protein, and the protein does the work.
BOTH PARENTS CONTRIBUTE EQUALLY: each character is controlled by a pair of alleles, one inherited from the father and one from the mother, so both parents contribute equally to the traits of the offspring.
SEX IS NOT DETERMINED THE SAME WAY IN ALL ANIMALS (NCERT opens the section with this). In some animals the TEMPERATURE at which the fertilised eggs are kept decides whether the young are male or female. In others, such as SNAILS, an individual can CHANGE its sex — so there, sex is not genetically determined at all. In HUMAN BEINGS, however, sex is LARGELY GENETICALLY determined, by the sex chromosomes. (Direct-recall marks: 'name an animal whose sex is decided by egg temperature' / 'name an animal that can change its sex'.)
SEX DETERMINATION IN HUMANS: human cells contain 23 PAIRS of chromosomes. In 22 pairs the two chromosomes match — these are the AUTOSOMES. The 23rd pair is the SEX CHROMOSOME pair: a woman has XX, a man has XY.
Because the mother is XX, EVERY egg carries an X. Because the father is XY, HALF his sperms carry X and half carry Y. X sperm + X egg → XX = GIRL. Y sperm + X egg → XY = BOY. Therefore the SEX OF THE CHILD IS DETERMINED BY THE FATHER, and the chance of a boy or a girl is equal (50 : 50). The mother can never determine the sex of the child.
A PUNNETT SQUARE is the standard way to work out a cross: write the gametes of one parent along the top and the gametes of the other down the side, then fill each box with the combination. Always show the gametes — marks are given for the working, not only the answer.
Monohybrid cross
TT × tt → F₁ all Tt (tall) → F₂ 3 tall : 1 short
The recessive character disappears in F₁ and reappears in F₂.
The two F₂ ratios
Phenotypic 3 : 1 Genotypic 1 TT : 2 Tt : 1 tt
Phenotype = what you see; genotype = the alleles carried. The commonest slip in this chapter.
Dihybrid cross
RRYY × rryy → F₁ all RrYy → F₂ 9 : 3 : 3 : 1
9 round-yellow, 3 round-green, 3 wrinkled-yellow, 1 wrinkled-green.
Gametes of a dihybrid
RrYy → RY, Ry, rY, ry
Four gamete types, so the Punnett square is 4 × 4 = 16 boxes.
What the dihybrid proves
new combinations appear → characters are inherited INDEPENDENTLY
Round-green and wrinkled-yellow were in neither parent.
How a gene acts
GENE → PROTEIN (enzyme) → hormone → TRAIT
An altered gene makes a less efficient enzyme, so less hormone, so a shorter plant.
Human chromosomes
23 pairs = 22 pairs of autosomes + 1 pair of sex chromosomes
Female XX, male XY.
Sex determination
mother: all eggs X. father: ½ sperms X, ½ Y. X+X = girl, X+Y = boy
The FATHER determines the sex of the child; the chance is 50 : 50.
A cross between a pure tall pea plant (TT) and a pure short one (tt) gives an F₁ generation that is:
easyTry answering on paper first — then reveal the model answer. 📄
Solve in your notebook. Stuck? Take the hint before the solution. ✏️
1. You are given a tall pea plant, but you do not know whether it is TT or Tt. Describe a single cross that would tell you which it is, and state the result you would expect in each case.
medium2. In the dihybrid F₂ of 16 seeds, work out how many would be expected to be (a) round and yellow, (b) wrinkled and green, (c) round in shape regardless of colour. Explain how you obtained (c).
hard3. A woman blames herself for having given birth to three daughters and no son. Using your knowledge of chromosomes, explain carefully why she is not responsible, and state the probability that a fourth child would be a son.
medium4. Two pea plants that both have round seeds are crossed. Among their offspring, some seeds are wrinkled. What does this tell you about (a) which character is dominant and (b) the genotypes of the two parents? Show the cross.
medium5. Explain why two plants with different genotypes can look exactly alike, while two plants that look different must have different genotypes. Use pea height as your example.
hard6. A gene controls the enzyme that makes a growth hormone in pea plants. Predict what would happen to plant height if (a) the plant had two efficient copies of the gene, (b) one efficient and one altered copy, (c) two altered copies — and relate your answer to the terms dominant and recessive.
hard7. A population of beetles living on a green bush is all green. Explain how variation could allow the species to survive if a bird that hunts by sight arrives, and state clearly what variation does NOT guarantee.
medium