Its seven sexes are rather prosaically named I, II, III, IV, V, VI and VII. An individual of a given sex can mate with individuals of any except its own, so there are 21 possible orientations.
In most animals, what sex you are is straightforward. A human with two X chromosomes is female, while someone with an X and a Y is male. Other species use different systems, but they are all clear cut when it comes to sex determination and mating.
Not so for Tetrahymena. Its sex is controlled by a gene called mat, but it is not as simple as one version of the gene encoding one sex. Instead, each allele of the gene sets out a series of probabilities. For instance, an individual born with the mat2 allele has zero chance of being type I, a 0.15 chance of being type II, a 0.09 chance of being type II, and so on.
There are at least 14 of these alleles, each offering a different set of probabilities. They are divided into two major groups called A and B: A alleles produce every sex except IV and VII, while B alleles produce everything except I.
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