CoachA gene is one determinant made precise: a segment of a chromosome carrying the instructions for one trait’s piece of the build — one gene for the blood-group machinery, others for eye-color pigments, thousands upon thousands along the 46 threads, each at its fixed address. A gene can exist in several versions — alleles: the blood-group gene’s A, B and O versions, the eye-color gene’s brown and blue. Same address, different spelling.
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Example 65.3 (Blood groups, fully mechanized)
One gene, three alleles, two copies per person: A with A or O shows group A; B with B or O shows B; A with B shows AB (both expressed — alleles are not always silencable); O with O shows O. The donation register’s whole pattern — including every “surprise” — is the arithmetic of two-copy deals.