Breeding theories (was *Muscat)

[Linda_Merims@ne.3com.com]

There are lots of intriguing theories in breeding which are unproved.
We've recently talked about the mitochondrial DNA possibly giving
the dam more than 50% influence on the makeup of the foal.

There are a great set of unproven theories surrounding "prepotence."

A good sire is a prepotent sire.  That is, a higher percentage of his
offspring than usual look like him (or acquire other internal, less
obvious traits of his).  If you're looking for prepotence in a human,
think of Martin Sheen, whose two actor sons Charly Sheen and
Emilio Estevez resemble their father and each other to a remarkable
degree, even though they are "out of" two different mothers.  An
example of an actor who doesn't seem to be especially prepotent
for physical appearance would be James Carradine, whose three
sons (Keith, Robert, and David) don't look especially like him or
each other.

In classical genetic theory, there are two ways to get a prepotent
sire:  linebreeding/inbreeding to get lots of homozygosity, or
just getting a stallion who has lots of desireable dominants,
hopefully in a homozygous state.

But there are other possible theories to account for prepotentency:

1.  Maybe meiosis is not random.

Meiosis is when the parent sire's genetic material divides in half
to produce the haploid (half the genetic content) sperm cell that
eventually connects with the haploid female egg.  The assumption
all along has been that the division of the genetic material is random.
Whatever the sire possesses in the way of a genetic heritage
is more or less put in a shaker like the Lot-O balls and whatever
ends up in an individual sperm is a randomized subset of the sire's
potential genetic contribution.  Pray for homozygosity and lots of
desireable dominants to improve on that randomization.

But suppose meiosis is *not* random.  What if a particular sire's
genetic material tended to divide with one set of chromosones
tending to clump together in about half the sperm, and the other
set of chromosones tending to clump together in the rest of
the sperm?  You'd get a stud who tended to produce two quite
distinct "body types" with great prepotency.

This bipolar prepotent tendancy has been observed in many famous
sires.  The one I am most familiar with is the historic Morgan stallion
Jubilee King.  It was early noted that he tended to produce two kinds
of sons:  a tall, leggy, light-bodied, almost Saddlebred-y "Daniel Lambert"
image of himself represented by sons such as Red Vermont, and a shorter,
chunkier, rounder, more classically Morgan version of himself in
sons such as Juban that resembled the Billy Bodette horses behind
his maternal Daisy line.

This tendancy for non-random meiosis would itself be an inherited
trait not possessed by all, or even most, individuals.

2.  Maybe the most important thing a foal inherits is the particular
ON switch.

In this unproven theory, every foal inherits a variety of potential
body types from his sire and dam.  In the Jubilee King case above,
the foal could be an elegant Daniel Lambert, or he could be a
chunky Billy Bodette.  The most important thing that the foal inherits
is the ON switch for *one* of these body types.  That there is some
kind of master gene that is going to control which related set of traits
this foal is going to manifest.  The master control gene is going
to switch ON the Lambert set of genes or switch ON the Billy Bodette
set of genes, and that is what the foal is going to look like.

What is attractive about this theory over the first theory is that it
provides a way for body types to "skip" generations in the way that
is so often observed in horse breeding.  Somebody else mentioned
this--the old rule of thumb that if you want to see what a horse is
going to look like, don't look at the parents, look at the grandparents.

I observed this tendancy in one Morgan herd that I followed for
three generations, and visited at different farms around the country.
This set of horses had a Beautiful Front Leg (short cannons,
good bone, beautiful definition, long forearm, beautiful pastern slope,
excellent feet)  and a God Awful Front Leg (calf-kneed, light boned,
tied in under the knee with puffy tendons like they were bowed,
short forearm, short upright pastern, club foot).  It was really amazing.
You never knew which front leg you were going to see.  They were
never blended; it was one or the other.  All in this one breeder's
carefully controlled herd, like Jeckyl and Hyde.

Linda B. Merims
lbm@ici.net
Linda_Merims@ne.3com.com
Massachusetts, USA



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