ICELANDIC/SHETLAND/FINN COLOR GENETICS

4. DETERMINING GENOTYPE

How can I tell my sheep's genotype?  There are two ways. One is by checking the colors on its pedigree, and this will only give you "odds."  The best way is by progeny testing, which means breeding it several times and seeing what it produces.  By the way, this is why I like to keep several moorits and blacks around. On our farm they don't seem to have the best quality fleeces, but since they are the most recessive in the A locus (Aa/Aa) you can pull out patterns pretty quickly.

Let's start by analyzing a sheep, and we'll analyze Blossom, a white ewe. You can pull out the dominant charts in lesson 2 and follow along. (I've structured the chart below like her pedigree reads)

             Sire-Dailley Ram
       Sire-Silverado, black/white
             Dam-Dailley Ewe
Blossom-white ewe
              Sire- Dailley Ram
        Dam-Hoover, white
             Sire (no color indicated) with blue/grey sire, white dam
         
As I look at her pedigree, she came out of a white ewe and a ram described as "Black/white," whatever that means. It could mean the ram actually had both recessives in the spotting locus so he looked like a Holstein calf, or it could mean he had little spots all over at birth that faded quickly-sometimes just a variation of the grey gene. The trail stops there on her Sire's side. The colors on her dam's side...we know are white, but still pretty hazy.

So, would you take a chance on this ewe? Gee, white out of white and plenty of unknowns... would you pass her up, even at a good price in favor of a fawn, for example? We had to figure out what Blossom was by what she produced. All we knew for sure was that her genotype began like this:  Awt/A? in the A locus, with ? meaning unknown, in the B locus, they were both B?/B? and in the spotting locus...since she's expressing white, even if she had the spotting gene showing it wouldn't show up as white markings since she's already white, so that's S?S?

Blossom: Awt/A?, B?/B?, S?S?

Her first year she was bred to Mandrake, a dark grey ram. We knew his genotype was at least:  Ag/A?, (he was expressing grey, so Aa or solid color could be hiding under it)  BB/B? (he was expressing black face and legs, so he was expressing at least one gene of BB-the other gene could also be BB but could be Bb as well so I wrote it B? for now).  He didn't appear spotted, so I knew he was SS in at least one of the spotting genes, tho could be carrying Ss in the other one, or he could be homozygous with a SS in the other one as well.

Mandrake:  Ag/A?, BB/B?, SS/S?

The first year they were bred, Blossom produced a white ewe lamb, Trixie. Another mystery like her mom, and no further clues to what Mandrake's genotype was. The next year we again bred Blossom to Mandrake, and we hit the jackpot. They produced a medium grey/brown ewe and a black ewe. We were able to start filling in the unknowns. Here's what we knew about the lambs:

      Elaine, med. grey brown ewe:   Ag/Aa, Bb/Bb, S?/S?
      Molly, black ewe: Aa/Aa, BB/B?, S?/S?

Since a gene in each set was donated by each parent, we can now fill in like so:

      Blossom: Awt/A?, B?/B?, S?S?     turns out to be:  Awt/Aa, BB/Bb, S?S?

How do we know? In the A locus, since neither of the lambs are white this time, all she could donate was the Aa/Aa to each of her lambs, and we know Blossom has it because Molly is Aa/Aa.  In the B locus, we now know Blossom is BB/Bb because she gave a BB to Molly who is black, and Bb to Elaine who is brown/factored. S locus is still a mystery.

Mandrake:  Ag/A?, BB/B?, SS/S?   turns out to be:  Ag/Aa, BB/Bb, S?S?

We know Mandrake is Ag/Aa in the A locus because he donated the Ag to Elaine to make her a medium grey, and an Aa to black Molly.  In the B locus, we know he had at least one BB black gene because he's expressing it. Since Elaine is brown factored and that only comes through from Bb/
Bb, she had to get a Bb from Mandrake.

Whew. Now take a pedigree from one of your sheep, and do the same exercise..what did they look like and what all have they  produced?  I now have a whole new respect for old Blossom (Elaine has turned out to have the best fleece on this farm) and Mandrake's white Trixie from the first mating has some exciting potential as well.

The S locus in this exercise isn't that important, as I mentioned before, it's more of a minor locus and doesn't really determine a color.

Page 3




(now hit the "back" button then click on page 4)