“THE X FACTOR
points out the brood bitch may be more important
than previously realized.”
–
Another Puzzle piece, Patricia Craige, 1998
Thoroughbred
breeding has made a great impact in canine breeding
because of the revolutionary and exciting new
insights gained by the genetics experts in
thoroughbreds. In 1997, Patricia Craige published a
well-known canine breeding book “Born to Win –
Breed to Succeed”. Mrs Craige can be called a
pioneer with the introduction of the thoroughbred’s
X-Factor theory into the Canine world.
Canines have 39
chromosome pairs. Like most mammals, canines have
one pair of sex chromosomes in each cell.
Each sire carries one Y-chromosome and one
X-chromosome, denoted as XY, while each dam carries
two X-chromosomes, denoted as XX. The sire
receives the X-chromosome from his dam and then
passes on it to his daughters while he inherits the
Y-chromosome from his sire and then passes it on to
his sons. The
X-chromosome is considerably larger than the
Y-chromosome in size and Y-chromosome carries lesser
inheritable materials. Different mammals have
different sized X- and Y-chromosomes. For
instance, the X-chromosome in a horse is 70% larger
than the Y-chromosome. Canine X-chromosome’s size
is 139 Mb
which is the largest chromosome among the 39
chromosome pairs. As the Y-chromosome’s size is just
27 Mb, the canine X-chromosome is over 80% larger
than the Y-chromosome.
Different mammals'
chromosomes carry different inheritable materials.
For instance, the X-chromosome of the horse carries
large heart characteristics. According to different
chromosomes, a region of DNA, e.g., quantitative
trait locus (QTL) can be found that is related to a
particular phenotypic trait. X-chromosomes have been
shown to carry genes in relation to sex,
reproduction, mental functions, muscle and a part of
skeletal traits.
As
mentioned above, the X-chromosome contains the genes
associated with reproduction, mental function,
skeletal muscle and a part of the skeleton
characteristics. The sire carries only one X
chromosome while the dam carries
two. Following careful
breeding plans,
the quality brood
bitches in the breed can usually carry two quality
X-chromosomes and when a brood
bitch carries on two
quality X-chromosomes, she can possibly produce the
same quality male and female offspring.
Hence, selecting a quality dam is very important.
When a breeder
chooses a dam for breeding, the breeder must select
a well-constructed bitch with a good producing
record. When selecting a stud dog for breeding, the
breeder must study the structure and the
reproductive record of the stud dog’s dam. In order
to produce good brood bitches in the breeding
program, the breeder should only use the stud dogs
that are produced by great dams since the X of the
sire will be passed onto his daughters. In this
respect, mediocre bitches must be avoided in the
breeding programs.
Referring
to figure 2.1, Venus must contain one of Vanna’s
X-Chromosomes and she has a 50% chance to carry
Jordan’s X-chromosome and a 50% chance to carry one
of two Marilyn’s X-chromosomes.
|
Parents |
2nd Generation |
3rd Generation |
|
TITUS XY
|
CHIEF XY |
FROSTY |
|
ERLENE |
|
VANNA
XX |
JOE |
|
SISKA |
|
PIXIE XX
OR XX |
JORDAN XY |
SPECS |
|
Maggie |
|
Marilyn XX |
Chief |
|
SPREE |
Figure 2.1. Venus’s pedigree
Just like every
other breeding methodology, the X-factor also has
its limitations. Breeders need to be aware of the
sex linked diseases, which is an abnormal gene
contained in the X chromosome. Normally, sex linked
diseases are linked with X-chromosome. It is
extremely rare for sex linked diseases to be
contained in the Y-chromosome. For example, XL-PRA
disease is linked with the X-chromosome in the
Siberian Husky.
