Well, if you bred a Merino to some other non-Merino sheep the offspring would be half Merino, and you would expect that lamb's wool to have half the character of the Merino fleece. Further, since most other sheep have longer, coarser wool than Merinos do, you would expect that half of the character of that lamb's wool would also be longer and coarser. So, what you would get is called a 1/2 blood fleece and you would expect it to have a larger fiber diameter than Merino wool, a somewhat larger crimp, and a staple length of about 3 to 3 1/2 inches.
Then, if you crossed that half blood lamb with another sheep the offspring's wool would be called a 3/8ths blood fleece, and so on. The seven grades of wool, by the blood system are:
Wool Grading by the American Blood Count System
Once upon a time - a long time ago - farmers (shepherds) were out standing in the field trying to compare one flock with another. They could all agree upon what a Merino fleece looked like: it was about 2 1/2 inches long and the fiber diameter was very fine and the crimp (that waviness in the fiber) was very small and close together. They decided that since everybody knew what a Merino fleece looked like, that it would be the standard for comparison and it would be called "Fine" (meaning the fiber diameter, not anything to do with quality).
(The information is taken from information published by Dr. Glen Spurlock and Dr. Vern B. Swanson, CSU Cooperative Extension Bulletin) Following is a table which compares the different sheep breeds and the wool they produce. Note, there are only Wool breeds listed. There is no information on Hair breeds. Also please notice that some breeds have a very large range of fleece grades within the breed. This means two things: there can be a great difference individual to individual, and that genetically there can be more variables in fleece to work with. As in trying to purchase purebred breeding stock: the buyer would have to look very closely at the individual sheep to decide if that animal had the fleece qualities desired.
This is not to say that all sheep today contain Merino blood. This system is merely used as a comparative system. In fact, the Corriedale breed was originally a cross between Merinos and Lincolns (a very lustrous, long, coarse wool breed). The desired result was a sheep which produced a 1/2 blood fleece with at least a 4 inch staple. This wool was especially desirable in the "sweater trade", because yarn spun from it was both soft and strong.
One of the surprises of breeding sheep is what will the wool look like? That is why a lot of breeders stick to a particular breed of sheep - you can expect certain wool qualities. However, there is that occasional individual who seems to get more of one ancestor's qualities than an equal share. That individual whose fleece is a lot finer than it should be or a lot coarser than you hoped for.
Perhaps you have heard of the English (Bradford) Spinning Count System. This originated in the 19th century (along with mechanized spinning equipment). It is the number of hanks of yarn, each 560 yards in length, that it is possible to spin from one pound of clean wool. The finer the wool fiber, the more hanks (greater length, thinner yarn) that can be obtained from one pound.
Or, perhaps you have heard of the latest and greatest: the Micron System. For this you need a microscope and a background slide with micron crosshairs for comparison.
Wool Grading