While we will not delve into complicated methods of genetic selection that are of interest mainly to breeders, it would seem useful to provide some basic knowledge so that anyone can understand the rather complicated field of genetics and be able to answer at least this basic question: "How are genetic traits transmitted?" In genetics, a trait is the visible or measurable expression of one or more genes. Coat color, a dog's ability to flush game, its height at the withers, and hip dysplasia are all examples of genetic traits in the broadest sense of the term.
The underlying genetics of traits
A gene is a program module located at a precise position (locus) on a chromosome. Each of a dog's cells contains 39 pairs of chromosomes in its nucleus, except cells that lack a nucleus (e.g., red blood cells) and sex cells (spermatozoa and ova). Sex cells have only one copy of each of the 39 chromosomes.
The complete set of chromosomes contains the genes that make up an individual's genetic inheritance, or genome the entire program that determines the individual's appearance and much of its behavior.
The reason that there are two copies of each chromosome (a pair) is that one copy is inherited from the father and one from the mother. Each "copy" is called an allele. When both copies are the same, and so give the same orders to the cell, the individual is said to be homozygotic for the trait under consideration. For example, the allele "b" (for "brown") codes for a brown coat in the puppy if two copies are present, one copy having been inherited from each parent. The puppy is said to be "homozygotic b/b."
Conversely, if the chromosome inherited from the father carries the "B" gene (for "black"), and the chromosome inherited from the mother carries the "b" gene, the dog is said to be "heterozygotic B/b", and its coat will be as black as its father's. The coat will be black in this case because the "B" gene (with a capital letter) is "dominant" with respect to the "b" gene (with a lower-case letter), which is said to be "recessive." A recessive gene cannot be expressed unless the individual is homozygotic for that gene.
How traits are passed on
When sex cells (gametes) are created, the thirty-nine pairs of chromosomes of the original cell are separated through a complex process known as meiosis. The chromosomes are mixed up, as if being shuffled like a deck of cards, and the new combinations are re-distributed as the 39 single chromosomes found in each gamete. The genetic diversity of the gametes ensures genetic variability within each breed of dog.
At fertilization, a spermatozoon and an ovum join to form an egg (zygote), in which the chromosomes inherited from the two parents are again joined into homologous pairs.
In this way, natural selection operates on two involuntary and uncontrolled levels:
- first, during meiosis, when different genetic information is passed to each gamete; - second, at fertilization, since it is impossible to predict which spermatozoon will fertilize which ovum.
Furthermore, genes can mutate, which modifies the characteristics of whatever they encode. At the moment of conception, each individual has about one chance in ten of carrying a mutated gene.
Is appearance controlled by the genes?
As we have seen, both dominant and recessive traits exist, as in the example of coat color, where black (B) is dominant over brown (b). Given a brown dog, it is easy to deduce its genotype (which genes are present), which can only be b/b, since the recessive trait of brown coat color can be expressed only in a homozygous individual. For recessive genes, the phenotype (the characteristic seen in the animal) is a true reflection of the genotype.
However, the "black coat" phenotype could correspond to either of two different genotypes-either B/b (heterozygous) or BB (homozygous). In the first case, the dog is black but carries a "brown" allele that can be transmitted to its offspring. In the second case, the black homozygous parent can transmit only a black allele to its offspring, which will therefore all be black in the first generation, regardless of the other allele they carry.
A plan for genetic improvement of dogs through selective breeding involves study of breeders'notes and family histories, allowing crosses for improved recombination from time to time by means of inbreeding to fix the characteristics obtained.
Genetic diseases in dogs
Currently, there are no less than 250 genetic diseases (called hereditary or genetic defects) in dogs. It is known that among these, about ninety are caused by a recessive gene, fifteen by a dominant gene, and forty-five by combinations of several genes.
Diseases caused by recessive genes appear only if two copies of the gene are present (one each from the father and the mother). A heterozygous individual will not show signs of the disease, but will be able to transmit it to any offspring. This individual is called a healthy carrier.
Such diseases can be prevented by thorough knowledge of the family history of candidates for breeding, and the ultimate goal is to eliminate them (as with Collie eye anomaly).
When disease are caused by a dominant gene, healthy carriers cannot exist and it is easy to prevent the spread of the disease by excluding the affected animals from the breeding pool. However, some diseases, such as progressive retinal atrophy, can be expressed late in the dog's life, sometimes after it has reproduced, which explains why the disease persists in some breeds.
Other diseases, such as those linked to the "merle" coat (namely deafness) may never be expressed in some resistant individuals, which then transmit the disease to their offspring without the owner realizing it.
Still other diseases are caused by several genes together, as the action of any one of the genes alone is too weak to cause the disease. In this case, it is the synergistic and cumulative effects of several undesirable genes, along with unhealthy elements in the dog's lifestyle (unbalanced diet, too much exercise, etc.), that allow expression of the defect. Hip dysplasia, cryptorchidism and dental abnormalities are examples of this type of disease, which is understandably difficult to eradicate. The earliest possible detection is the most effective remedy.
These examples merely serve to demonstrate the complexity of canine genetics. A thorough knowledge of this science allows breeders to produce quality puppies for the general public.
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