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In 1997 Marianna Haun from the US wrote
a book about the X Factor. X Factor is the name for
characters only inherited through the X chromosome.
In the US, the information about the X Factor has received
lots of attention among the breeders of racehorses,
as research indicates that some of the characteristics
inherited through the X Factor are very important for
racehorses (the size of the heart a.o.). During the
last eight years, experts in the fields of horse cardiology,
genetics and pathology at the University of Kentucky
has worked together to see if certain important inherited
characteristics are sex related. The results show that
they most likely are.
In addition to inheriting 50% of the gene
pool from both sire and dam, each foal also inherit
one sex chromosome from each of the parents. Each foal
has two sex chromosomes. The colts always have X and
Y and the fillies always have X and X. The sire gives
away one X or one Y to each foal, and what sex chromosome
the foal inherits from the sire decides the sex of the
foal. If the foal inherits an X chromosome, the foal
will be a filly. If the foal inherits a Y chromosome,
it will be a colt. The dam has two X's and will give
one of these to each foal. By this we understand that
characters inherited only through the X chromosomes
are inherited from sire to filly, from dam to colt and
from dam to filly. The sire will never give an X chromosome
to a colt:
Scientific
research in Kentucky the last few years, indicates that
certain mares seems to have very important characteristics
associated with their x chromosomes. This might explain
why some stallions do so much better as broodmare sires:
their daughters inherit the good characters through
the x chromosome from the sire, and these good characters
become important when the daughters give this x to their
offspring.
Knowledge of maternal lines becomes extra
interesting in this matter. One can combine the good
characteristics on the mare's x chromosome with the
same characteristics from the gene pool in the stallion,
if the stallion has an x chromosome from the same maternal
line, and hope for a foal with a better physical possibility
to become a champion (larger heart than normal, etc.).
There are many things that are important in order to
make a champion - more than only the "right" genetic
makeup - but the better gene pool the better chance
the horse has from the start. There can be no doubt
that the mares mentioned in this article inherit certain
genes that makes their descendents more victorious than
others, and these genes are dominant (the genes are
more often expressed in each generation). By finding
a stallion from the same successful maternal line as
the mare, one can make progress as the chance is already
doubled when it comes to getting the important X Factor
in the foal.
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