Genetics and Gene-Mapping Some Basics
By: Jo Ann White
What is a Gene?
The gene is the basic unit of heredity. Each gene, acting alone
or with other genes, determines one or more canine characteristics. All
of the genes that constitute the hereditary makeup of an organism are
called the genome. Genes occur in strands of genetic material within
the cell called chromosomes. A dog is composed of a large number of cells
that are genetically identical. The first cell of a particular dog formed
when egg and sperm united; it contains one set of chromosomes from each
parent. The canine genome is made up of 39 pairs of chromosomes (one
set from each parent) that contain approximately 3 billion base pairs
of DNA, or around 100,000 or so genes. Each gene generally occupies a
particular position within a particular chromosome.
What is DNA?
The chromosome is made up of two very long single strands of
a chemical called DNA (deoxyribonucleic acid) that are wrapped around each
other to form a double helix. This DNA never leaves the nucleus of the
cell--it is like a reference library that contains the genes (functional
regions) that determine how the organism will develop. DNA strands are
made up of four basic units linked together in pairs. The entire canine
genome contains about 3 billion base pairs. A small gene may contain 100
of these base pairs; a larger one may contain 10,000 base pairs.
What is a Genetic Marker?
Scattered throughout the chromosome are short repeated groups
of these base pairs known as microsatellites, or markers, that can be used
to track defective genes. Hundreds of these distinctive sequences have
been isolated along the canine genome for use in mapping genes. To find
a marker that is "linked" to a disease, researchers may examine
hundreds of markers from animals with and without the disease before they
find one that is located so close to a disease gene that it is almost always
inherited along with the disease caused by that gene. The closer the marker
is to the disease gene itself, the more accurate the test. Finding such
a marker also narrows down where to look for the disease-causing gene,
which could ultimately lead to a more specific test for the gene itself.
What is a Mutation?
A mutation is a genetic mistake that scrambles the instructions
given by a gene. Mutations may be good, bad, or indifferent. In the case
of renal dysplasia, it is believed the presence of mutations in one or
perhaps two different genes causes the glomeruli of the kidney to stop
developing. What are dominant and recessive genes? A dominant gene will
express itself when the puppy inherits only one copy of the gene (from
sire or dam). A recessive gene will express itself only when a puppy inherits
two copies of the gene (one from the sire and one from the dam). If a disease-causing
gene is recessive, a dog with the gene can be bred to a dog without it
and will not produce the disease, although it will produce carriers of
the gene. If the gene is dominant, both parents must be free of it to avoid
producing affected puppies. Again, more than one defective gene may be
needed to produce a disease.
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