Sec 4 Biology Inheritance notes

A gene is a sequence of DNA coding one polypeptide (chapter 5). An allele is one version of that gene — the gene for seed shape exists as a round allele and a wrinkled allele. Diploid cells hold two alleles per gene, one on each chromosome of the homologous pair. The pair an organism carries is its genotype; what the organism actually looks like — the trait the alleles build — is its phenotype.

If both alleles of the pair are the same, the genotype is homozygous (HH or hh); if different, heterozygous (Hh). A dominant allele (written as a capital, H) shows its effect even in a heterozygote; a recessive allele (lower case, h) shows only when homozygous (hh). So HH and Hh look the same — tall — while only hh shows the recessive trait.

Cross two heterozygous tall plants, Hh × Hh. Each parent makes H and h gametes in equal numbers; the Punnett square gives genotypes 1 HH : 2 Hh : 1 hh — and phenotypes 3 tall : 1 short. That 3 : 1 ratio is the fingerprint of a cross between two heterozygotes with one dominant allele. Cross a heterozygote with a homozygous recessive instead (Hh × hh) and the phenotypes split 1 tall : 1 short — a 1 : 1 ratio that instantly reveals a hidden heterozygote.

A caution the syllabus makes explicit: ratios are expectations, and real families are small. A 3 : 1 expectation over 40 offspring predicts about 30 : 10 — but a real brood of 40 might give 32 : 8 (80% tall). That is not a failed law of genetics; it is sampling variation, the luck of which gametes meet. The larger the progeny, the closer the observed ratio drifts toward the expected.

Some allele pairs are co-dominant: in a heterozygote, both alleles are expressed and neither masks the other. The ABO blood groups run on this plus multiple alleles — three alleles of one gene: IA, IB and IO. IA and IB are co-dominant with each other (an IAIB person is blood group AB, expressing both); IO is recessive to both (group O appears only as IOIO).

The 23rd chromosome pair carries the sex chromosomes: XX in females, XY in males. Every egg carries an X; every sperm carries an X or a Y. At fertilisation, an X sperm gives XX (a girl) and a Y sperm gives XY (a boy) — so the father’s sperm determines the sex of every child, and each pregnancy is an independent 50 : 50 event.

A mutation is a random change in a gene or chromosome, happening spontaneously and occasionally — and it can be passed on if it arises in gametes. A gene mutation alters the base sequence: in sickle cell anaemia, one changed allele codes a slightly different haemoglobin, so red blood cells carry less oxygen and can stiffen into sickle shapes that clog capillaries. A chromosome mutation changes whole chromosomes or their number: in Down syndrome, a person has 47 chromosomes — one extra chromosome 21 — producing a characteristic set of developmental traits.

Mutation rates are not fixed: ionising radiation such as X-rays and chemical mutagens both increase the frequency of mutations by damaging DNA. That is why radiographers shield themselves and why chemical-safety rules exist.

Populations are full of variation. Continuous variation — height, body mass — forms a smooth range of values, shaped by genes plus environment. Discontinuous variation — blood group, seed shape — falls into distinct categories determined by genes alone. Both kinds supply the raw material for selection.

Natural selection runs on three facts: individuals vary; resources are limited, so individuals compete; and the variants best suited to the prevailing conditions survive and reproduce more, passing their alleles on. Environmental selection factors — food supply, predators, disease, climate — do the choosing. Over generations the helpful alleles become more common and the population’s characteristics change gradually: this is evolution, natural selection acting as its mechanism.