College of Cat Genetics: Part II
by Patricia Turner
Study Unit 2
The first of these articles explained that characteristics are transmitted from one generation to another by a system known as heredity and the study of heredity and the nature of life itself has occupied scientists for centuries. It is now accepted fact that most living processes are controlled through complex protein substances known as enzymes. However, proteins, although fundamental to life, are not the key to life--this having been proved to be the materials known as nucleic acids (known as DNA and RNA) responsible for the mechanism of heredity by the control through the protein enzymes of most of the activities of life and living.
The science of heredity is known as genetics, the study of which, in cats, involves the study of the way that kittens may differ from their parents. The variations may be in coat colour, tail length, ear placement, or eye colour, etc.--all these characteristics are heritable.
It is known that a kitten may own thousands of distinct characteristics which together make up its individuality; that each of these characteristics is controlled by genes and that genes are distributed to the kitten from either sire or dam in a manner that is controlled to a great extent by chance.
The exact nature of the gene will be discussed in a later article.
It can be seen that the idea held by animal breeders of the past--namely that characteristics of the parents were blended in their progeny, like the mixing of milk and water or blood and blood was a misconception. This misconception persists to a certain extent in the use of expressions such as "Champions blood", "bloodline", "bloodstock", or the "diluting out" of particular characteristics. As Gregor Mendel showed by his experiments over a hundred years ago, there is no such thing as blended inheritance or dilution of characteristics, and although a particular characteristic may disappear in one generation the kitten may carry it as a hidden recessive so that it may very well reappear in a later generation.
Nor are characteristics inherited in the blood--in fact, the dam's blood is broken down into its elements before ever reaching the kitten embryo and therefore not even the dam's blood passes on directly to the kitten it carries. Right from the beginning, kittens make their own blood and this may even differ in type from that of their dam.
The characteristics of a cat or kitten are controlled by genes which are linear segments of the chromosomes carried in the cell nucleus. The cell itself can be regarded as a sort of factory, the product of the factory varying with the type of factory it is (or type of cell) and the actual link between generation being made by the gametes (the male and female sex cells) which produced by reduction cell division fuse at fertilization and give rise to a zygote from which the new kitten is produced by repeated normal cell division.
The basic functional unit of the cat is thus the cell and any one cat is made up of millions of them. Most cells contain the mechanism for their own reproduction and can use nourishment to provide energy and to make their own constituent parts. They also contain a source of information determining how they function and reproduce. Cells differ greatly in their size and function but the cells of any one type are usually similar in all species.
A liver cell in a lion may be similar in size to a liver cell in a domestic cat although the overall liver size is different. Therefore it can be seen that although a lion may be larger overall than a domestic cat it may be equal in the size of its cells. Like large buildings built of small bricks, lions are large cats built of small cells. The cat's blood cells include a group of small round cells, consisting mostly of nucleus and very little cytoplasm, which do not normally divide--their function is the protection of the cat against infection.
In contrast, the nerve cells in the cat's brain have very large areas of cytoplasm (the area outside the nucleus) and their function is to transmit nerve impulses. There are many, many kinds of specialized cells and yet they all come from a single fertilized ovum and it is clear that the set of instructions inherited from the sire and the dam must give all the information necessary for their production, differentiation and function. Ultimately genes control the manufacture of enzymes which, in their turn, decide that functions go on in a cell and thus in the tissues, organs and the cat as a whole.
The way in which genes affect cells depends not only on the genes themselves but also on the physiology of the cells which, in turn, is related to their function in the cat's body.
That function, as has been explained, is dependent on the instructions received from the genetic inheritance from sire and dam.
Differentiation of Cells
It is known that no one cell type necessarily uses its full potential and the examination of various kinds of cell from any single species shows that certain proteins and enzymes may be synthesized in some sorts of cells and yet not in others. Clearly some sorts of cells have the ability to carry out metabolic reactions that others cannot undertake.
As has been explained there is a great variety of cell types in the cat's body and they are all derived from the single fertilized ovum. The process by which different cell types are formed is known as differentiation, and the varying of function in the different cell types is thought to be accounted for by the differential expression of the instructions contained. This could be brought about by the transcription of different parts of the coded instructions in different cells, or alternatively it could be that there is control over some of the translation of the coded instructions so that only some of them are translated in all the cells.
This transcription of the coded instructions of the genes in DNA by messenger RNA and their translation by other types of RNA to form protein will be discussed later. It has been suggested by some authorities that a particular protein known as histone may be concerned in this control while others feel that hormones may be the important factor.
The nature and function of the whole cat is largely governed by the nature and arrangement of its ingredients which are manufactured according to instructions contained in the genes made up of segments of DNA in the nuclei of its cells. Its final nature is decided by both heredity and environment and although some characteristics such as eye colour may be determined genetically those such as size may be the result of a combination of factors. So it can be said that the cat's coded instructions govern the nature of both the chemical ingredients from which it is made and the nature of the chemical changes in its body during its life.
If the structure of cells is studied it can be seen that they have an outer membrane acting as a barrier between their contents and their surroundings but this membrane is, of course, permeable to small molecules. Inside the cell membrane is the cytoplasm, or the cell sap, which is made up of the various complex structures controlling life processes. All cells need energy to live and they have the ability to use, conserve and process this energy in their cytoplasm. Some cells are able to harness energy and store it as chemical energy while other cells draw on the store to transform it in order to re-create themselves, fulfill their function and to stabilize their environment. To do this they have to make use of catalysts that make possible the chemical reactions and the catalysts are called enzymes, usually being protein molecules built up of hundreds of thousands of amino acid components arranged in precise sequence. The sequences of the amino acids are determined by the nucleic acids (DNA and RNA) carrying the inherited coded instructions.
The cytoplasm also encloses small bodies known as ribosomes composed of nucleic acid (rRNA) bound to protein and which are joined together in groups by strands of nucleic acid (mRNA). Ribosomes have an important role in the synthesis of protein by controlling the sequence of amino acids making up the protein molecules.
Cells almost invariably contain a nucleus which can be described as the brain of the cell. Although the removal of the nucleius does not "kill" a cell immediately, it does cause the activities gradually to cease so that it eventually stops growing and dies. The nucleated cell, however, grows in size and, at a predetermined moment divides into two identical daughter cells. The division is preceded by a division of the nucleus in the parent cell in which the chromosomes present concentrate together, reform, duplicate and re-allocate themselves at different ends of the nucleus so that when it actually divides there are the same number and type of chromosomes in each new nucleus.
The division of the nucleus into daughter nuclei is followed by the division of the whole of the cell into daughter cells. This process is the mitosis or normal cell division.
The main consituents of the nucleus are large molecules (known as macromolecules) of the nucleic acids which have the ability to copy themselves--one variety of nucleic acid (DNA) carrying the entire heredity of the cell in a sort of code. This code includes all the specifications for every component of the cell, controls the differentiation of cells into the specialized groups which make up the organs and tissues and gives the exact details of when they should be constructed, differentiated and assembled to re-create similar cells.
The cell is this the unit of life and can be seen to be a highly efficient unit for the manufacture and construction of active chemical molecules, the instructions being coded in a chemical form within molecules of nucleic acid contained in its nucleus.
The messages from the nucleic acids in the nucleus are transmitted by more nucleic acids to the ribosome assembly units in the cytoplasm so that protein molecules can be constructed. While the cell is growing and building, the master instructions are being duplicated in the nucleus so that when growth is complete the nucleus and the cytoplasm divide with on set of instructions being enclosed in each daughter cell.
The first scientific observations on heredity were presented by Gregor Mendel in 1865. He carried out experiments with "pure breeding lines" of garden peas and examined the inheritance of characteristics from generation to generation. From the results of those experiments he was able to lay the foundations of genetics in what are now known as Mendel's Laws or Principles.
The essence of his discovery was was that the inherited characteristics of an individual are distinct and independent, each characteristic being transmitted as a separate unit from parent to offspring, so that although an individual possesses thousands of separate characteristics each of these are controlled by units of heredity (now known as genes) and distributed from the male and female parents.
Cats are bred in countries where various languages are in general use. While those truly involved in genetic research represent only a very small number in any one nation , it may be difficult for a researcher living in one country and using one language to understand the opinions and hypotheses of a colleague in another country. Yet science has no barriers and the exchange of knowledge and ideas is fruitful. Similarly genetic researchers using the same native tongue may well find difficulty in communication because of differences in in symbol or terminology. Just as in the cat fancy the same breed of cat may be known under different names in different countries so, in the field of genetic research, a strain or mutant might be known under different names in different parts of the country or different countries of the world.
As the problem in genetic nomenclature grew, steps were taken to standardize genetic nomenclature and symbolism and to provide rules which workers in all countries would follow. Proposals for nomenclature symbols and rules were drawn up and over the years have been revised when necessary.
The names of the genes (i.e. piebald white spotting) are written with a lower case initial letter (i.e. NOT Piebald White Spotting) regardless of whether the mutant is dominant or recessive except at the beginning of a sentence or in any place where a capital letter would normally be used (i.e. Oregon Rex, Manx, Abyssinian tabby).
The symbols used for genes are typically abbreviations of the accepted name and for convenience the initial letters of names and symbols should be the same (e.g. l for longhair).
Recessive mutations are indicated by the use of a small initial letter (i.e. lowercase) as the symbol of the mutant gene (e.g. a for non-agouti, b for brown). Dominant mutations are indicated by the use of a capital letter as the symbol of the mutant gene (e.g. W for white, Pd for polydactylia)
The symbol used to describe the locus is the symbol of the first named mutant but any superscript indicative of a specific allele is omitted. Normal or wild type can be designated by:
Multiple alleles determining visible or other clearly characterized distinctions are represented by the locus symbol with an added superscript (i.e. cs for Siamese allele, cb for Burmese allele).
The Committee for the standardization of genetic nomenclature in the domestic cat was listed in the first Study Unit (Cat World, March/April, 1973, p. 25) and it presented its report in 1968. Mr. Roy Robinson, a well-known geneticist noted for his interest in cats, was a member of this committee and describes its work and recommendations.