MANILA, Philippines—Sheep farmers have been breeding their sheep not just for better wool quality but also color. Dark wool supposedly isn’t worth as much on the market because it can’t be dyed other colors, unlike lighter-colored wool. The term “black sheep,” used to refer to infamous or unsuccessful family members, is believed to have originated from sheep farmers because of this valuation.

Despite the farmers’ best attempts though, every now and then a black sheep appears in the flock. Australian researcher Belinda Norris is said to have estimated that about 15 percent of the white Merino sheep in the country have the recessive version of the gene that leads to black wool.

So it’s unsurprising that Norris and her colleagues at Australia’s CSIRO Queensland Bioscience Precinct have been working on understanding the genetics of sheep coat color. In a study published online this month by the journal Genome Research, the researchers announced that they’d identified the genes responsible for the sheep coat color. The information may eventually lead to a test to help farmers find out ahead of time if their sheep carry the versions of the genes involved making dark coats.

4 scenarios

To understand the odds of finding a black sheep in the flock, consider the following, simpler example involving attached or unattached earlobes. On a global scale, most people will have unattached earlobes, which makes it a dominant trait. To understand the odds of someone having attached or unattached earlobes, consider the following four scenarios:

In the first possibility, both of your parents have unattached earlobes because all four of your grandparents did too. You and all your siblings will therefore have unattached earlobes.

The converse of the above prospect says both of your parents have attached earlobes because all four of your grandparents did too. You and all your siblings will therefore have attached earlobes.

Now let’s assume one of your parents has both copies of the version of the gene for attached earlobes and your mom has both copies of the version of the gene for unattached earlobes. Each of their kids then has a 50/50 chance of having attached or unattached earlobes.

In the final version, if both of your parents each have both versions of the earlobes gene, there’s one chance in four that they’ll have a child with attached earlobes.

Since farmers have been focused on raising sheep with light coats, there are probably more people with unattached earlobes in the world than there are dark-coated sheep. In wild sheep populations though, the proportions of light coats to dark coats haven’t been as skewed as they are in the domestic breeds. Researchers are now finding that coat color preference in wild sheep is changing as well.

Lightening their coats

Earlier this year British and Australian researchers found that the populations of Soay sheep, believed to be the most primitive sheep breed because they came to the Scottish islands 4,000 years ago, well ahead of the Vikings and Romans, were lightening their coats.

Untamed and isolated, about 75 percent of the Soay sheep have dark coats. They are also bigger than the lighter-coated sheep that make up the remaining 25 percent of that sheep population. Usually larger size corresponds to better odds of survival.

The researchers found that over the last 20 years however, the dark sheep were dying out faster. Their theory: sheep with the dark coat also inherit versions of the genes that make them bigger but less fit to survive and pass on their genetic information to the next generation. If this continues—and spreads from the wild populations to the tame sheep—maybe the genetic test for black sheep won’t be necessary after all.

E-mail the author at massie@ massie.com.