Big paper II

On June 16 I wrote about my attempt to figure out how ovine spotting works. Researchers have told us that the S-locus is the genetic location that determines spotting and that one copy of the spotting gene causes top-of-head and tip-of-tail spotting, while two copies create much more than that. But how that is determined has remained a mystery. Spotting is always white in sheep, so the darker the sheep, the more obvious the spotting. In comparison to most sheep breeds, the Romeldales carry a lot of spotting — and that’s what drew me in. I kept looking at our new lambs and wondering how the white-spotted ones got that way. Where did all of that come from?

This twin looks exactly like his sire, both without spotting. All of the white you see is symmetrical and due to the pattern that he carries from the sire.

So in mid-June I started tracing certain family lines and the spotting they carry. I used a huge piece of paper and dozens of colored markers in hopes I’d unlock the relationships between the various spotting displays. Instead, I spent a LOT of time drawing out the many spotted lambs and their very convoluted family relationships, only to realize that nothing was resulting from all of that work. I tore up the multicolor-marked paper and placed a couple of strong rubber bands around the markers, hiding them in a bottom drawer. I decided I needed to think about this further and determine another approach. Life got busy, but every once in a while I would find myself lost in thought, trying to make connections between those spotted genetic lines.

This spotted twin carries exactly the same pattern genetics as his brother, but he also inherited white spotting from both parents — even though I believe his sire’s spotting has remained hidden until this generation.

Last week my life calmed down a bit and I decided to once again tackle the spotting inventory I have accumulated for my flock in Excel. I wanted to add a couple of lambs born this year. These lambs were interesting to me because their sire was already in my database and did not carry obvious spotting. Their dam was also in my database and carried only head-and-tail spotting. The two brothers, however, were very, very different! One looked identical to his sire and carried no obvious spotting, but the other was extremely spotted. Both brothers carried the same genetic patterns and coloring, so they should have looked identical, but the spotting made them look very different.

This pair crystallized my frustration with the spotting locus: even the spotted lamb could only carry one copy of spotting at S because his sire was not spotted — yet look at him! Rick came home at just the right time for me to vent my frustration. I needed a way to visually see the interrelationships between different types of spotting, and couldn’t figure out how to do that. I sat him down in the kitchen and began to rant. My brain was too small, I told him; I couldn’t hold all of the necessary information to do the needed comparisons. He suggested I try a familial chart. But I had already done that — so I continued to rant. Every few minutes, he would come up with another suggestion, and every time he did, I felt even more frustrated because I had already tried that too! I started to think that even his “looking in from the outside” wasn’t going to help me — but then he did. He asked whether I had tried a Venn diagram for each type of spotting to see if and how much they overlap. That suddenly launched me in a whole new direction!

My first attempt at the Venn diagram that led me to realize that the spotting on front legs is gated by that on the back legs.

This was it; I knew it. This was what I had been missing all along — and I once again needed my big paper! I first sorted my database by head-and-tail spotting, which is the primary indicator that spotting exists. Of the 326 lambs in the database, 170 were spotted and had head-and-tail spotting. I drew a circle with a 16.5″ radius, proportional in area to 170 lambs. Since every lamb who carries spotting has this type of spotting, every other circle I would draw would lie fully within this original circle. I next looked at front-leg spotting — for no specific reason; I just randomly picked that area. There are 22 lambs with front-leg spotting, which created a proportional circle (with a 5.9″ radius) within the bigger circle. I figured I’d stick with the legs — and that’s when things got interesting. There are 49 lambs with rear-leg spotting, so I drafted a circle with an 8.8″ radius. When I looked at the overlap — the front-leg circle within the rear-leg circle — I discovered that all of the lambs with front-leg spotting had rear-leg spotting. This was my first major breakthrough in months! From this new Venn diagram, I could see that the rear-leg spotting was somehow a type of gating mechanism for front-leg spotting. If there was no spotting on the rear legs, then front-leg spotting could not express. Front-leg spotting needed rear-leg spotting!

As I added circle after circle, I eventually realized that I couldn’t continue my work on the big paper. I needed to be able to shift circles around to create the necessary percentages of overlap with other circles. That’s when we decided to move the whole project onto the computer. Thanks to Rick, I was able to get the circles set up relatively quickly in an easily movable format. In fact, I have a lot to thank Rick for! Without his input, I would still be stressing my brain, trying to remember all of the data I needed instead of laying it out in a very visual and easily understandable series of Venn diagrams. Thanks, Rick!

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