Acres of Clover • Photo by: Stefan & Barbara Wielebinski

● FEATURE
ARTICLE BY: Michal Prochazka, MD, Founder & Director, Pet DNA Services of Az, Tempe, Arizona (www.petdnaservicesaz.com).
ARTICLE SOURCED BY: LEE ANN HALL
Over the last couple of decades,
advances in genetics technology made
it possible for DNA testing to become
the method of choice for a variety of
applications, ranging from human
medicine and forensics to our hobbies
involving pets and domestic animals.
In this article, I have attempted
to present a simple overview of
the use of DNA tests in horses.
w
WHEN talking about genetics and DNA testing, one cannot
avoid the use of specialized vocabulary, which I will explain
briefly, before discussing the applications of this technology
in our equine friends.
Practically every cell in the body contains DNA, which is the
“blueprint” code of instructions determining the appearance
and function of an organism. The DNA is arranged in rodlike
structures called chromosomes located in the nucleus.
Chromosomal segments carrying discrete codes for
specific traits and functions are called genes, and with the
exception of the sex chromosomes all of the other
chromosomes (and genes) in animals and humans are
present in pairs, with each parent contributing one half of
the material. In general, both copies of a gene are identical,
but some carry discrete differences (mutations, variants),
some of which can result in visible differences – for
example a variation in the coat color. Such alternate variant
forms of the same gene are called alleles, and an individual
is referred to as a homozygote when both alleles in a pair
are the same, whereas an individual carrying two different
alleles of the same gene is called a heterozygote.
Depending on their biological properties, two alleles at any
gene can interact in different ways. An allele is dominant
when in a heterozygote carrier only this allele will show an
effect. Furthermore, its effect is the same, regardless
whether the individual has one or two copies of such allele
(= is either a heterozygote or a homozygote for it).
Conversely, an allele which is masked by its dominant
counterpart is called recessive, and the effect can only be
visible in a homozygote. A special situation is that of
incomplete dominance, when one allele shows a certain
effect in a heterozygote, but the expression is more
pronounced when the carrier is a homozygote (a typical
example in horses is the Cream Dilution).
The combination of alleles at one or more genes is referred
to as genotype, while the visible manifestation of their
effects is called phenotype. Due to interactions between
The Paint Horse Journal 31
April/May 2008