CoachThe genotype of an individual is the set of alleles it carries; its phenotype is the set of its observable characteristics. The phenotype can be described at three scales:
- the molecular scale: which proteins are present, in which form, with what activity;
- the cellular scale: the shape, contents and behaviour of the cells;
- the organismal scale: the visible traits, from eye colour to the symptoms of a disease.
Each scale is the consequence of the one below it.
Examples
Example 15.2 (Sickle-cell disease at three scales)
Molecular: one amino acid of the haemoglobin chain (glutamate, the sixth) is replaced by valine; the changed molecules stick to one another into long fibres when oxygen is low. Cellular: the fibres deform the red blood cell into a rigid crescent that blocks small vessels and breaks easily. Organismal: anaemia, crises of pain when vessels block, damage to the spleen and kidneys. One substitution in one codon of one gene, and three descriptions of its consequence.
Example 15.8 (Phenylketonuria, and how a diet changes a phenotype)
About one newborn in ten thousand carries two non-working alleles of the gene for enzyme 1. Phenylalanine from food is not converted; its concentration in the blood rises twentyfold and it poisons the developing brain. Untreated, the child suffers severe intellectual disability. Detected at birth by a drop of blood on a card — every newborn is tested — and fed a diet almost free of phenylalanine, the same child develops normally. The genotype is unchanged; the phenotype, at every scale above the molecular one, is that of a healthy person. The environment, here the diet, decided.