SCENARIO:You have just bred your best bitch to the top producing sire in the country with high expectations for a beautiful litter. Results, however, were a disaster. What went wrong?
Itís difficult to understand why puppies may look different from each other as well as from their parents. Gregor Mendelís work with garden peas uncovered the mystery of this fascinating process and helps explain how genes function in the passing on of traits from one generation to another. The intelligent breeding of dogs requires an understanding of how chromosomes and genes work as well as the maintenance of a system of written records which charts the "genetic" progress of a breeding program. We will attempt to break down the potentially complex topic of genetics into easy-to-understand scenarios. Part 1 discusses the role that chromosomes and genes play in making every dog unique in outward appearance (phenotype) as well as genetic make-up (genotype). The process of "chromosome swapping" during reproduction is a phenomenon that every breeder should understand and helps explain the diverse physical appearance of littermates.
WHAT ARE DOGS MADE OF?
A dog is made up of innumerable cells, each
containing a nucleus. Within the nucleus are tiny thread-like structures called chromosomes.
In each species, the number of chromosomes is always constant and even. The
nuclei of all dogs contain 78 chromosomes; man has 46, the cat, 38. Chromosomes
come in different shapes and sizes, with every cell containing two of each
particular kind. In this regard, instead of referring to 78 chromosomes in dogs,
it is preferable to speak of 39 pairs of chromosomes, one member of each pair
having come originally from the sire and one from the dam. Chromosomes which are
members of the same pair are called homologous chromosomes. Strung out
along the length of each chromosome, like beads on a string, are thousands of genes.
Consisting primarily of DNA, genes are carriers of hereditary information which
will ultimately determine an individualís size, temperament and conformation.
As the chromosomes are paired, so are the genes along their length. Genes
located in the same position (locus) in homologous chromosomes will
influence the same trait or traits in the dog. With one member of the homologous
chromosomes coming from the sire and one from the dam, it follows that one
member of each gene pair also came from each parent.
Dawkins (1976) simplifies the discussion of
how genes are passed from one generation to the next by viewing each nucleus as
a ROOM which contains a BOOKCASE of architectís PLANS on how, in our case, to
build a dog. In our discussion, the "volumes" in the bookcase
(nucleus) will be the chromosomes and the "pages" will
refer to genes, which are the carriers of heredity. Because each nucleus
in the dog contains 39 pairs of chromosomes we could say that filed in the
BOOKCASE of every cell nucleus of our dog are 2 alternative sets of 39 volumes
of plans. Call them Volume 1a, Volume 2a and Volume 1b, Volume 2b down to Volume
39a and Volume 39b ("a" volumes came from the father; "b"
volumes from the dam). The identifying number used for volumes, and later,
pages, is purely arbitrary. From this point on, letís consider each volume
(chromosome) to be of a loose-leaf binder type and its removable pages to be the
genes. In the two sets of volumes in each cell, instructions for building any
part of our Basset Hound would be on corresponding pages. For example, if the
genetic instructions for making our dogís ears were on pages 500 to 700
in Volume 1a, ear instructions would also have to be on pages 500 to
700 in Volume 1b. The ears our dog actually inherited were based
on the combined instructions from both volumes.
In the reproductive process, which is called meiosis (pronounced my-o-sis), a specialized kind of cell division takes place in the testicles and ovaries. A sperm cell from the male (or an egg cell from the female) is made when a cell divides, going from two full sets of 39 chromosomes to a cell with only one full set of 39 chromosomes. Sperm and egg cells are collectively referred to as gametes. Using a sperm cell as an example and returning to our analogy of loose-leaf binders, each sperm would contain only one set of volumes 1 through 39. The most important point to be made here is that during the dividing process Volumes 1a through Volumes 39a and Volumes 1b through Volumes 39b do not stay neatly intact, with the "a" set going to one sperm cell and the "b" set going to another sperm cell. Rather a new and unique 39 volume set is produced for each sperm cell in which single pages, or rather multi-page chunks, are detached from the "a" volumes and swapped with corresponding chunks from the alternative "b" volumes. For example, in producing its unique 39 volume set a sperm cell might make up its "ear instructions" (contained in both volumes on pages 500 to700) by taking pages 500 to 575 from Volume 1a and pages 576 to 700 from Volume 1b. A different sperm cellís "ear instructions" may include pages 500 to 600 from Volume 1a and pages 601 to 700 from Volume 1b. In genetic terms, this process of swapping bits of chromosome (in our analogy, gene "pages") is called crossing-over. Our two sperm cellsí other 38 volumes of building plans for all the other parts of the dog would be made up in a similar, one-of-a-kind way. Each gamete (sperm or egg) always ends up with one of each of the 39 volumes, with no duplications or omissions of a volume number.
Remember, our sireís "a" set of chromosomes came from his father and his "b" set of chromosomes from his mother. Due to a random swapping of genes between the two sets, any one of the "new" chromosomes that ends up in a sperm cell is therefore a patchwork or mosaic of his paternal and maternal genes. This chance arrangement of chromosomes and genes makes every sperm "genetically" unique. The same holds true for eggs produced by the dam.
When a sperm cell, with its newly formed set
of 39 chromosomes, pokes its head into an egg cell, with its new set of 39
chromosomes, a fertilized cell or zygote is formed. A new puppy starts
from this single cell, which now contains 39 chromosomes from the sire and 39
chromosomes from the dam. At this point, each puppyís conformation and
temperament are going to result from a double set of plans or genetic
instructions. The question is: "Which set of instructions will be
followed?" This process, discussed in Part 2, deals with the effects of
dominant and recessive genes.
Traits are not transmitted through the blood of an animal but rather through its genes. The mating of our dogs is in reality a "pairing of two hosts of genes," (Onstott 1962) in which an element of randomness occurs. This chance factor takes place in the reproductive process where crossing-over occurs, resulting in any one chromosome in a sperm (or egg) ending up a one-of-a-kind patchwork or mosaic of maternal and paternal genes originally inherited from that individualís parents. The random swapping of chromosome material in the formation of a gamete negates the myth that 50% of a dogís total heritage comes from its parents and 25% from the 4 grandparents; it also explains why a sire, who received half his genes from his sire and and half from his dam, may pass along a concentration of genes from his parents. In such a case a puppy would be more closely related to its grandparents than to its sire. When we realize that gametes (sperm and eggs) do not have the same genetic content though they come from a single individual, it becomes clear why repeat breedings so often fail, why littermates are not identical and why breeders who use the brother or sister of an excellent dog and believe they are using the same genetic material are in error!
With the union of a sperm and egg, the physical and genetic make-up of a puppy will originate from two sets of genetic instructions. Which instructions are followed depends largely on the dominant and recessive nature of the genes involved. We will review this phenomenon in Part 2.
Dawkins, R. 1976. The Selfish Gene. Oxford University Press, New York.
Onstott, K. 1980. The New Art of Breeding Better Dogs. Howell, New York.
Willis, M.B. 1989. Genetics of the Dog. Howell, New York.
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