Tetrahymena thermophila is a single cell covered with a coat of hairs called cilia. The cilia wave back and forth, powering it through the water.
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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