[Jacob-list] Basic Jacob color genotype

[Neal and Louise Grose]

I started this a couple of days ago. I have spent a lot of time the last few days riding around in circles mowing hay, so maybe I have just had too much time to think. Fred, do not worry about stoking the fire on these things, I spend a lot of time in hot water and it's not too bad.

While an undergraduate at "State", I took several graduate level courses in genetics. On one level, everything is Mendelian inheritance. We spent hours looking through microscopes at fruit flies.* It needs to be pointed out though that Friar Mendel was quite lucky (act of God?) to get the three genes that conformed perfectly to simple dominant/recessive inheritance. Most genes have more than two forms, and many have incomplete dominance or trait expression is modified by another gene.

 Most of my genetics education and training is in population genetics of farm animals. If Mendelian inheritance is the statistics of the coin toss, additive population genetics is the statistics of throwing a handful of dice and counting the total number that turns up. You do not know which genes are involved, and you don't care. If we throw 10 die, then we know that most of the time the answer will be around 35. (Remember the question about the "black" Jacob sheep.) On rare occasions the answer will be 10 or 60.

 In population genetics, any one gene is not going to greatly effect the outcome. Remember those fruit flies? At some point they found that fruit fly maggots are fairly unique in that they have huge cells in their saliva glands that have numerous repeats of DNA. This DNA lines up together and can be seen in a microscope and read like a bar code. If I remember correctly there are over 1000 genes that contribute to the growth rate of fruit fly maggots. Can you imagine how many genes contribute to fleece on a sheep? These traits may be loosely termed "dominant" or "recessive", but are not the all-or-nothing effect of a single dominant allele. It is better to speak of these traits as having varying degrees of heritability. The equation can also include chaos. So: Phenotype = genotype + environment + chaos. The most heritable trait is usually 'stature', with heritability in the range of 0.45. In other words, 45% of the trait reflects the genetics of each parent, and 10% is left for environment and chaos. [This explains "miniature" Jacobs and giant Suffolks.] In general, traits such as foot structure, milk production, and maternal traits have lower heritability, 0.10 to 0.30. In these cases genetics account for only about one-half of the observed expression. 

It is worth noting that single genes can be shifted in and out of a population with relative ease. Additive gene traits can not. Single gene traits will be the color traits and the polycerate gene.  Additive traits probably account for 95% of what we can see in an animal. Just to complicate things, inheritance does not have to be only Simple Mendelian or additive. It can be a combination of the two. A good example of this may be spotting. Single genes may govern presence of spots, but type of spotting and a general blueprint for the amount of spotting are additive traits. 

The most difficult traits for me to decipher are those that are between simple Mendelian and additive inheritance. This is the problem with color genetics. With lilac and spotting is that the coin keeps landing on its edge. We know that it's not supposed to, but there it is again. We need to occasionally go back and look at that coin. Different inbred lines of Jacob sheep clearly have different marking patterns** as well as differing degrees of dominance. When we crossbreed, we are looking at the influence of a single copy of a gene. We should not get consistent, clear piebald expression in crossbreeds; but, we do.  

When looking at breed conservation, everything that we do is to minimize outside influence, even while we recognize that some squirrelly things have taken place. There is always going to be a tension between preserving phenotype and preserving genotype. The emphasis to preserve phenotype will exclude genes that are a natural part of the genome.  The effort to preserve as much genotype as possible will drag in things that  should not be there. The question remains: What do we do about all of this. Consider our "hornless" ewe. Should she be retained as breeding stock? No. Should the Breed Standard include these animals? Absolutely not. Is she part of the legitimate Jacob genome? Probably at this time, she is.

Neal Grose
North Carolina

* Like grad students, fruit flies work cheap, and get by on a diet of mostly fermentation products. My project was to figure out the genotype of a trait based on 6 generations or so of phenotype. It was a recessive, sex-linked lethal. The TA said "Oh, ya'll are not supposed to have one that hard".

** I also know that there are several piebald variations in Holstein cattle. Primarily black, large patches, and numerous small black marks to name three obvious ones. The small "spot" type frequently has excessive white on the head (sound familiar?) and when crossed to the primarily black type can produce cows with a black body and white head. 
  ----- Original Message ----- 
  From: Jacobflock at aol.com 
  To: ranchrat at telusplanet.net ; jacob-list at jacobsheep.com 
  Sent: Saturday, May 10, 2003 1:59 AM
  Subject: [Jacob-list] Basic Jacob color genotype


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