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.