A new study has revealed what gives carrots their orange colour.

Researchers looked at the genetic blueprints of more than 600 types of the root vegetable, and found that three specific genes are needed to make carrots orange.

They were surprised to discover that these three required genes, which are also linked to health benefits of the vegetable, all need to be recessive, or turned off.

The findings shed light on the traits important to carrot improvement efforts and could lead to better health benefits from the vegetable.

The study also helps to confirm when carrots were first domesticated, and when they gain popularity in Europe.

Massimo Iorizzo, an associate professor of horticultural science with North Carolina State University, USA, said: “Normally, to make some function, you need genes to be turned on.

“In the case of the orange carrot, the genes that regulate orange carotenoids – the precursor of vitamin A that have been shown to provide health benefits – need to be turned off.”

Carrots, especially orange carrots, contain high quantities of carotenoids, which can help reduce the risk of diseases like eye disease.

Researchers say the orange carrot is the most abundant plant source of pro-vitamin A in the American diet.

The researchers worked with colleagues from the University of Wisconsin-Madison to sequence 630 carrot genomes in a continuing examination of the history and domestication of the vegetable.

The study, published in Nature Plants, also adds further evidence that carrots were domesticated in the 9th or 10th century in western and central Asia.

Dr Iorizzo said: “Purple carrots were common in central Asia along with yellow carrots.

“Both were brought to Europe, but yellow carrots were more popular, likely due to their taste.”

He added that orange carrots made their appearance in western Europe in about the 15th or 16th century, and may have resulted from a white and yellow carrot being crossed.

Dr Iorizzo said: “This study basically reconstructed the chronology of when carrot was domesticated and then orange carrot was selected.”

The colour and sweeter flavour of the orange carrot drove its popularity and farmers selected it for those traits.

Different types of orange carrots were developed in northern Europe in the 16th and 17th centuries, matching the appearance of different shades of orange carrots in paintings from that time.

Orange carrots later grew in popularity as greater understanding of alpha- and beta-carotenes, the precursor of vitamin A in the diet, progressed in the late 19th and early 20th centuries.