#pennapost

Oddly among sparrows, they have two distinct color morphs as adults, unrelated to sex: "white-striped" and "tan-striped," the differences in head pattern color of which can be seen easily. (photos mine)

The white-striped morph is caused by a large inversion on chromosome 2, containing around 1000 genes (a "supergene"). White-striped individuals (of both sexes) are more aggressive, territorial, seek extra-pair copulations more frequently, and are poorer parents. Tan-striped individuals are more attentive parents and less highly territorial.

These behavioral extremes are balanced by pairs almost always forming between individuals of the opposite color morphs - white females choosing tan males, and tan females choosing white males. This behavior is called disassortative mating and the white-throated sparrow shows the highest level of it known in vertebrates, with up to 98% of pairs being between one of each color morph. The result is that homozygous white individuals (having two copies of the inverted chromosome) are extremely rare - they would have to have two white parents, and receive the inversion from both. This maintains the white and tan morphs at about even numbers throughout the population, in functionally the same way the balance between sexes is maintained - the inverted chromosome 2 acting as a sort of neo-sex chromosome, alongside the actual Z sex chromosome of birds.