I think the report I read…
http://www.straightdope.com/mailbag/mblacksheep.html
…missed the point:
What is the origin of “black sheep” with respect to sheep genetics? I understand the colour symbolism, but about the sheep…
does a black sheep show up from time to time in a family with only white parents? Is this poor sucker an outcast, or does it act differently than the norm?
Silly questions maybe, but I’m a city boy, I don’t know anything about these things.
This is not a very scientific answer, but it could be AN answer. I had an ex-girlfriend whose family was a large sheep farming family (no jokes please). They had about 2500 head of sheep on about 3000 acres. Virtually all of them were “white-face” sheep, meaning they were all white. Now, the white sheep are very “lemming” like in their behavior. If the front one goes forward they all seem to go forward. If the front one stops, they all stop. So, the white-face sheep pretty much stick together, in harmonious family bliss.
They also had about 10 “black-face” sheep. These are all black (wow). Now, they do NOT stick with the flock. They stray and are very much into solitude. The problem, and the fun, of these solipsistic animals take a couple hundred of the white-face with them. So, when it comes time to round them up, you have several different groups of a few hundred with a ornery leader who does NOT want to go with you. Hence, the black-face of the group.
And just for clarification: I hate sheep. They should all die. They are dumber than dumb.
Well, GL, are you interested enough to wade through these sheep genetics websites? 
Short answer: black fleece color is a recessive characteristic, but that’s as deep as I wanna get into it, my brain’s gonna explode.
http://www.sheepusa.org/resource/handbook/chbreed.htm
http://www.hawksmountainranch.com/colorpatterngenetics.html
http://www.agric.wa.gov.au/agency/Pubns/farmnote/1999/f12999.htm
So, a “black sheep”, being recessive, would show up periodically, inexplicably, and inconveniently, but the only inconvenience would be to the shepherd, who would prefer easily dyed white wool. AFAIK, sheep don’t care what color the other sheep are. I’ve heard stories about chickens pecking the “odd bird”, but I’ve never heard of sheep behaving similarly. I would think that if sheep (ewes) did indeed reject or abandon their oddly-colored offspring, that the recessive gene for black coat color would have been removed from the gene pool pretty quickly, as the black lambs wouldn’t have survived to pass on their black coat color gene.
“Missed the point”? The question was about the origin of the phrase, which way pre-dates any knowledge of sheep DNA.
Asking sheep is silly because they Do Not Answer. They have always been like that.
::: sheesh :::
I grew up in a ranching community with a lot of sheep ranchers in the area and wanted to add a tangent. Black sheep are often used as “flock-markers”. You put one black sheep in a herd for every 100 or so white sheep. Then, if you need a quick count to see if a large portion of the herd has wandered off, you simply count the easy-to-spot black sheep. This doesn’t prevent individual sheep from wandering off, but it is a quick way to keep track of a large herd.
C K, Dex mentions an old proverb - “There’s a black sheep in every flock.” He says that black sheep were not as valuable as white ones - without any references. He then goes on to speculate that the reason had to do with dyability of the wool.
He also goes off on an apparent tangent about how “black” has all sorts of negative connotations in other uses. I guess the non-spelled-out-for-the-slow connection is that black carried a stigma of evil and combined with a black sheep being rare, that the odd person out who was the disgrace was the odd one out, like the black sheep is the odd one out, and the black = evil connection helps the metaphor.
Maybe green lantern felt he left out the part he presumed was well known - sheep come in white and black, with black being rare and white being common? I think that’s pretty much green lantern’s question - so are there black sheep and how common are they? The specific genes not being as important as the biology of the situation. Yes, white sheep breed and make black sheep. Surely that was known before the expression was created.
And if you look at green lantern’s continued questions, he’s asking if there’s any connection between the expression and behavior of sheep. Seems like a point Dex could have addressed.
Seems to be some confusion, here, Irish. I never tried to address green lantern’s questions. The Staff Report addressed the question posed by donmac, which didn’t mention sheep genetics, inheritance patterns, recessive genes, quality of wool, nasal cavaties, or grazing patterns.
If someone wants to answer green lantern’s questions, that would be very interesting. But I don’t see how green lantern thought that I missed the point, by failing to answer questions that weren’t asked.
Er, right, yea, what was I saying?