Sec 4 Combined Science (Biology) Inheritance notes

From Chapter 4: a gene is a section of DNA coding for one polypeptide. But the same gene can exist in different forms called alleles — think of a recipe card with several printings that differ in one word. The height gene might come as a tall allele (T) or a short allele (t). Every individual carries two alleles of each gene — one on each chromosome of a pair, one from each parent. If both copies say T, the plant is pure tall; if they disagree — T and t — the allele written as the capital wins.

Continuous variation produces a smooth spread of values with every intermediate possible: height, body mass, hand span. Measured across a class, the values pile up in the middle and tail off at the extremes — a bell-shaped spread. Discontinuous variation falls into distinct categories with nothing between: blood group (A, B, AB or O), tongue-rolling ability (roller or non-roller), seed shape (round or wrinkled). You are one category or another — no in-betweens.

The test question to ask yourself: "could I be halfway?" Halfway between 165 cm and 175 cm makes sense (continuous); halfway between blood group A and blood group B does not (discontinuous).

A dominant allele shows its effect even when only one copy is present — it is written as a capital letter (T for tall). A recessive allele shows only when both copies are recessive — lowercase (tt for short). Homozygous means the two alleles match (TT or tt — pure-breeding); heterozygous means they differ (Tt — a hybrid carrying a hidden recessive). The genotype is the pair of alleles an organism carries (Tt); the phenotype is what you actually see (a tall plant). One genotype — Tt — and you cannot tell it apart by eye from TT: both phenotypes are tall. Only tt shows the recessive phenotype.

A monohybrid cross follows one gene through a mating. Draw a genetic diagram: write the parents) genotypes, the gametes each parent can make, then combine the gametes in a grid. Crossing two heterozygous tall plants (Tt × Tt): each parent makes T and t gametes, so the grid fills with TT, Tt, Tt, tt — genotypes in the ratio 1 : 2 : 1, and phenotypes 3 tall : 1 short (75% : 25%). Crossing a heterozygote with a recessive (Tt × tt): gametes T, t against t, t gives Tt, Tt, tt, tt — 1 : 1 (50% : 50%).

Generations get names: the F1 is the first filial generation — the offspring of the original parents; the F2 is the next generation, made by crossing (or self-pollinating) the F1. The classic 3:1 appears when two F1 heterozygotes are crossed — the hidden recessives resurface in a quarter of the F2.

A 3:1 ratio is an expectation — what the averages approach over many offspring. Actual families are small: four children might be three boys and one girl, or all four the same sex, even though the probability of each child is 50:50. Fertilisation is a random event; each new offspring is an independent draw. With small numbers of progeny, chance easily produces observed ratios far from expected — a 3:1 family of four could even show 4:0. Only by pooling hundreds of offspring do the counts settle toward the expectation.

Of the 23 chromosome pairs in a human body cell, one pair decides sex. A female carries XX; a male carries XY. Eggs always carry an X chromosome; sperm carry X or Y in equal numbers. At fertilisation: X sperm + X egg → XX (girl); Y sperm + X egg → XY (boy) — each pregnancy a 50:50 chance. Note the direction of the logic: the father's sperm determines the sex of the child, since the mother always contributes an X.

The grid is just a monohybrid cross in another costume — and it yields the same 1:1 expectation, with the same small-family wobble as any other ratio.

A mutation is a change in a gene or in chromosome number. A gene mutation alters the base sequence, so the polypeptide the gene codes for changes: sickle cell anaemia results from an altered haemoglobin gene — the changed haemoglobin deforms red blood cells into sickle shapes that carry oxygen poorly. A chromosome mutation changes the number of chromosomes: Down syndrome results from an extra copy of one chromosome — 47 instead of 46.

Mutations happen naturally at a low rate, but two factors increase the rate: ionising radiation (X-rays, radioactive materials) and chemical mutagens (certain chemicals in tobacco smoke and industrial settings). Both damage DNA, raising the chance of a changed gene — which is why exposure to either is minimised.