Downless (Edardl), Ectodysplasin-A receptor, Chr. 10, 29.0 cM, Genome Coordinates: 58350589-58425509
The mouse downless mutation initially arose spontaneously in the late 1950s on the A/H strain. There are currently 5 known allelic mutations. Downless is one of a group of phenotypic mimics that have long been know in which their phenotypes are essentially identical yet they map to different genes. They were historically known for over 40 years as the tabby mimics or the tabby-crinkled-downless syndrome. It is now know that tabby (Chr. X) is the ligand, ectodysplasin-A, downless (Chr. 10) is the receptor, ectodysplasin-A receptor, and crinkled (Chr. 13), is involved with modulation of the kinetics of interaction between the first two.
Downless mutant mice are very similar to tabby and crinkled mice. Mutant mice have thin, shiny hair coats with areas of alopecia, notably behind the ears and on the tail. The tail skin completely lacks the pilosebaceous units which gives it the smooth, hairless appearance at the gross level. The trunk has only awl hairs. Abnormalities of the teeth include missing or malformed incisors and molars. Eyelids may not open completely and corneal opacities may develop. Eyelids lack the large modified sebaceous gland known as the Meibomian gland. This gland produces lipid to coat the aqueous secretions from the lacrimal glands and is a critical part of the fluid barrier of the cornea. Many other glands are missing as well as described for the other phenotypic mimics.
This mouse mutation is a model for some forms of anhydrotic or hypohydrotic ectodermal dysplasia in humans and other species.
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