Copyright information:  information content copyright owned by Cat World expires 70 years from March 1973 at which time the information minus the research notes may be placed in the public domain.
Research Note:  Here is an article which has been partially converted for online presentation.  It is from the March/April 1974  issue of Cat World which holds the copyright for the text.  This text has not been split into separate files so that it makes for easier printing.  Question:  Is it better to allow for printing or optimize for reading and information chunking.  Note the graphics additions to visually break the text.  Note the bolding of terms and the links to other information -- which add a new original dimension to the work and enhance the information offering.

Genes and Chromosomes

Genes occur in linear order along chromosomes and chromosomes occur in the nuclei of all cells. However, they are not visible except at the time the cell divides. The actual number of chromosomes in each cell is usually constant within a species but it does differ between species. For example, the number in the horse if 64 and in man 46. The domestic cat has 38 chromosomes in each cell. This characteristic number is called the diploid number of chromosomes and is symbolized in literature on genetics as 2n. Thus the statement Felis Catus 2n = 38 means simply that the number of chromosomes in each cell of the domestic cat is 38.

There are two types of cell however that do not possess the diploid number of chromosomes and these are the ovum and the spermatozoon (collectively known as the gametes). The gametes possess one half of the diploid number of chromosomes and this is known as the haploid number and symbolized 1n. Therefore the ovum and the spermatozoon of the domestic cat carry the haploid number of 19 chromosomes, each of the 19 having an opposite number in the chromosomes of the ovum or spermatozoon it pairs with. Thus after fertilization the resultant embryo has 19 pairs of chromosomes restoring the diploid number to 38. The two members of each pair are described as homolgous.

Both the haploid ovum and the haploid spermatozoon gametes carry a complete set of genes and the fertilized ovum then carries two sets of chromosomes on which are carried two sets of genes, one set from the dam and one from the sire and the two genes then act on each other in determining the expression of the character they control. The way in which genes act on each other varies tremendously and depends on a number of things as will be described later.

Genes which control any one particular character occur at the same position or location on the chromosome carrying them; the word used to describe this position is the locus and the genes which are so placed in the same position (at the same locus) are called alleles, allemorphs or allels. In literature on cat genetics the word allele seems most common and will be used henceforth. These allelic genes can show dominance or recessivity and sometimes both genes can interact to produce an intermediate expression while in some species genes may even dominate on different parts of the body.

Alleles therefore always occur at the same locus and if one gene is described as allelic to another then the meaning is simply that they both control the same character and are both located at the same place on the chromosome while at the same time differing in their contribution to the character concerned.

Therefore if allelic genes always occur at the same locus then non allelic genes will occur at other loci. The number of genes known to occur at any particular locus can vary--in some cases, i.e. the Brown locus, it may be only two. In other cases there may be several, i.e. the Albino locus where a cat may inherit chinchilla, burmese dilution, siamese dilution, or possibly albino. In this context the word chinchilla {NB this is now referred to as the Inhibitor gene and has subsequently been determined to occupy a different locus]  describes the amount of dilution and its use does not infer that the cat with chinchilla in its genotype will necessarily be known in the fancy as Chinchilla. Similarly the use of the words burmese and siamese describes the coat patterns caused by those genes and does not infer that the cats owning these genes in their genotype will necessarily be those of the breeds known as Burmese or Siamese. A Silver Tabby British Shorthair has chinchilla in its genotype, a Burmese patterned Cornish Rex has burmese in its genotype while a Colorpoint Persian has siamese in its genotype. These are but three examples but serve to prove that the words in this context are only descriptive of the dilution effect. Except in very unusual circumstances any particular cat can only possess two genes at each locus.