Researchers at Harvard Medical School in Boston, Massachusetts, took a gene normally found only in chicken legs and transferred it to the forming wings of chick embryos.

Dr Malcolm Logan: "We have partially transformed a chicken wing into a chicken leg."

The resulting structures lost many of their wing characteristics and gained those of a leg.

Feathers vanished, the beginnings of clawed fingers appeared at the end of limbs, and muscles usually confined to the leg could clearly be seen.

The scientists believe the work may eventually help us to understand how legs and arms develop in humans, raising the possibility - one day - of preventing or correcting deformities.

Important roles

The research team focussed their study on a gene called Pitx1. It is one of three genes thought to play a primary role in determining the identity of upper and lower limbs.

A virus was used to deliver the gene into the budding wings of the developing chick embryos.

Leg-specific muscles are clearly identifiable in the new limbs

The researchers found that it switched on another gene, Tbx4, normally activated only in the leg.

Two other "leg" genes, HoxC10 and HoxC11, believed to play a secondary role in limb development, were also triggered.

But Pitx1 had no effect on the gene Tbx5 that normally controls the development of the wing. This would explain why the new "legs" still retained some of the features usually seen in wings.

Human dimension

The scientists believe the study will help us understand how vertebrate limbs acquire their identity.

All three of the genes thought to play such crucial roles in the development of limbs are also present in mice.

Co-researcher Professor Clifford Tabin explains the purpose behind the work

In humans, mutations in one of the genes are associated with a condition called Holt-Oram syndrome, which produces truncated forearms.

By understanding the developmental processes at the molecular level, the researchers hope science can eventually prevent such problems occurring.

"It is important for us to understand something about normal leg development because in the human we know that there are a large number of congenital defects which lead to incorrect limb formation," lead researcher Dr Malcolm Logan told the BBC.

"So by better understanding these genes which play central roles in normal limb formation, we will have a much better understanding of what's going wrong in human congenital defects.

"And by having a better understanding of what's going wrong, we can then think about possible interventions to correct those defects."

The results of the US study are reported in the journal Science.