The body and organs of a dog are made up of millions of minute cells, each a nucleus containing small threadlike structures called chromosomes. These number seventy-eight, regardless of the breed, as they do in the wolf. Other species have different numbers of chromosomes.
The genes which influence the physical make-up, character, behaviour, health and well-being of the dog are found on the chromosomes. They themselves are long molecules of deoxyribonucleic acid (DNA) linked in two strands rather like the sides of a ladder, with the rungs being chemical bases known as adenine, thymine, cytosine and guanine. A gene is believed to be a portion of the double-stranded DNA, consisting of several hundred base pairs; it is the arrangement of these base pairs which determines the action or potential action of that particular gene. A single chromosome may contain hundreds, even thousands of genes.
Genes have the ability to replicate themselves, effectively unchanged, and the ability to control or influence certain biochemical functions that bring about particular effects in the organism. All the genes in a particular cell do not necessarily function, or they function at a certain stage of life only; the consequences of such actions may be crucial to animal breeders.
It would be more accurate to speak of the seventy-eight chromosomes in the dog as thirty-nine pairs. Since the actual units differ in size and shape, and two of each type can be located in a cell. A pair of chromosomes is called a homologous pair; one member of each pair comes from the sire and one member from the dam; therefore one half in each of the thirty-nine chromosome pairs was inherited from the father and the other half from the mother. One parent may appear to have been more influential than the other, but both will in reality have contributed equally to the genetic make-up of their offspring.
In males, one of the homologous pairs is unequal, being made up of a large chromosome (called X) and a smaller one (called Y). This unequal XY set is the determinant of sex or rather of the male sex, since females carry two paired X chromosomes. All females are thus XX and transmit only an X to all their offspring, while males are XY and can transmit either an X – in which case the offspring is female – or a Y – in which case a male progeny results.
During normal growth, cells multiply by one cell duplicating itself to become two, two becoming four, etc. When one cell becomes two by splitting each of the chromosomes into two identical ones, one set of seventy-eight goes to one end of the cell and the other set moves to the opposite end before splitting away to form the second cell. This process, known as mitosis, goes on constantly during the growth process. During the formation of germ cells (ova and sperm) a different process, known as meiosis, is needed if the chromosome number is to be maintained in the species. At fertilization, sperm or ova are formed with only one member of each homologous pair of thirty-nine chromosomes in each sperm (or ovum), resulting in a zygote (fertilized egg) with the correct seventy- eight chromosomes.

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