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Images of Imprinting
Edward R. Winstead

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Newborn children and animals inherit genes from their mothers and fathers. Many of those genes, through a process called imprinting, actually remember which parent they came from. And they behave accordingly. An imprinted gene is expressed one way if it comes from the father and another way if it comes from the mother. About two dozen human imprinted genes have been identified; many are known in animals as well. Sheep for instance.

The callipygous animals (1 and 3) compared to normal animals (2 and 4).

At the first meeting on Sex and Gene Expression, researchers illustrated the effects of imprinting with images of sheep buttocks. Some sheep have heftier haunches due to a mutation in a gene called callipyge, Greek for "beautiful buttocks." Breeders sought the bulkier animals, but the trait proved elusive when they could not determine which animals would produce the effect when mated. Then, in 1996 a team of U.S. and Belgian researchers described the unusual combinations of normal and mutated versions of callipyge that yield the generous chops.

A normal-looking ram admires his bulky progeny. The combination of two copies of callipyge mutation "silenced" the effect in the father.

Lambs develop beautiful buttocks only when the callipyge mutation is inherited from the father, a typical imprinting effect. But lambs that inherit two copies of callipyge mutation, one from each parent, look perfectly normal. This is considered rare: A trait usually emerges when an offspring inherits two mutated copies of the same gene. The mechanisms governing the expression of callipyge are still not known.

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Cockett, N.E. et al. Polar overdominance at the ovine callipyge locus, Science, 273. 236-238 (July 12 1996).

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