CORDELIA MOLLOY / SCIENCE PHOTO LIBRARY CORDELIA MOLLOY / SCIENCE PHOTO LIBRARY
Tortoiseshell cat (Felis catus). Tortoiseshell (or calico) colouration occurs in a cat when it simultaneously expresses both the dominant orange gene, XO and its recessive form Xo. These are present at the same location on the X chromosone. For this reason, tortoise shell cats are only male when they carry an extra X chromosone.
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