Cells, Chromosomes, and Genes

From a 35-pound Main Coon to a 5-pound Devon Rex; from the small folded caps of a Scottish Fold to the great, delicate ears of a Balinese; from the 4-inch coat of a Chinchilla Persian to the fuzzy down of a Sphinx; from the deep Ebony of a Bombay to the translucent white of a Turkish Angora; from the solid color of a Havana Brown to the rich tabbiness of a Norwegian Forest Cat: the variety and beauty to be found in the domestic cat is beyond measure. When these characteristics are coupled with the genetically-patterned and environmentally- tailored personalities of the individuals, it can be seen that each animal is as unique as it is possible to be. There truly is a cat for everyone.

Wide as the range of cats is, it pales when compared with the varie- ties of Other Pet. Why should the dog exhibit such a wide spectrum of body types, looking like completely different creatures in some cases, while cats always look like cats (as horses always look like horses)? The secrets behind the wide variations in possible cats, and why cats, unlike dogs, resist gross changes and always look like cats, can be found in its genetic makeup.

In order to understand what happens genetically when two cats do their thing, it is necessary to understand a few basic things about genetics in general. To study genetics, is to study evolution in miniature, for it is through the mechanism of genetics that evolution makes itself felt. In chapter 1, we showed how the gross evolution of the cat came about, and how this gross mechanism was applied to the European Wildcat to evolve the African Wildcat, the immediate forerunner of our cats. We will examine this mechanism itself to better understand how the first domestic cat has become the dozens of breeds available today, and how cat breeders use this mechanism to create new breeds or improve existing ones.

Cats, like people, are multi-cellular creatures: that is, their bodies are composed of cells, lots and lots of cells. Unlike primitive multicellular creatures, cat bodies are not mere colonies of cells, but rather societies of cells, with each type of cell doing a specific task. To one specific type of cells, the germ cells (ova in females and sperm in males), fall the task of passing the genetic code to the next generation. The method the Great Engineer has developed to carry this out is one of the most awesome, most elegant, and most beautiful processes in nature.

The cells of a cat, with few special exceptions, are eukaryotic, that is, they have a membrane surrounding them (acting as a sort of skin), are composed of cytoplasm (cell stuff) containing specialized orga- nelles (the parts that do the cell's task), and have an inner membrane surrounding a nucleus. It is this nucleus that contains all the genetic materials.

Within the nucleus of a cell are found the chromosomes, long irregular threads of genetic material. These chromosomes are arranged in pairs: 19 pairs in a cat, 23 pairs in a human. It is these 38 chromosomes that contain the "blueprint" for the individual cat.

When inspected under a microscope, the chromosomes reveal irregular light and dark bands: hundreds of thousands, perhaps millions per chromosome. These light and dark bands are the genes, the actual genetic codes. Each gene controls a single feature or group of features in the makeup of the individual. Many genes interact: a single feature may be controlled by one, two, or a dozen genes. This makes the mapping of the genes difficult, and only a few major genes have been mapped out for the cat.

The chromosome is itself composed primarily of the macromolecule DNA, (deoxyribonucleic acid): one single molecule running the entire length of the chromosome. DNA is a double helix, like two springs wound within each other. Each helix is composed of a long chain of alternating phosphate and deoxyribose units, connected helix to helix by ladder-like rungs of four differing purine and pyridamine compounds.

It is not the number of differing compounds that provide the secret of DNA's success, but rather the number of rungs in the ladder (uncounted millions) and the order of the amino acids that make up the rungs. The four different amino acids are arranged in groups of three, forming a 64-letter alphabet. This alphabet is used to compose words of varying length, each of which is a gene (one particular letter is always used to indicate the start of a gene). Each gene controls the development of a specific characteristic of the lifeform. There is an all-but-infinite number of possible genes. As a result, the DNA of a lifeform contains its blueprint, no two alike, and the variety and numbers of possible lifeforms has even today barely begun.

Mitosis and Mendel

When a cell has absorbed enough of the various amino acids and other compounds necessary, it makes another cell by dividing. This process is called mitosis, and is fundamental to life.

Not too long ago, it was thought that the chromosomes were generated immediately prior to mitosis, and dissolved away afterwards. This turned out not to be true. The extremely tiny chromosomes, normally invisible in an optical microscope, shorten and thicken during mitosis, becoming visible temporarily.

The rather complex process of mitosis can perhaps be explained simply as a step-by-step process:

Mitosis begins when the cell senses sufficient growth and nutrients to support two cells.

The invisible chromosomes duplicate themselves through the wonder of DNA replication. Various enzymes are used as keys to unlock and unwind the double helix into two single helices. Each of these helices then uses other enzymes to lock the proper parts (the amino acids and other stuff) together to build a new second helix, complete with all transverse rungs, so that the results will be exact replicas of the original double helix. This winding and unwinding of the DNA can take place at speeds up to 1800 rpm! The two daughter chromosomes remain joined at a single point, called the centromere.

The cromosomes then wind themselves up, shortening and thickening, making them visible under the microscope, and attach themselves to the nuclear membrane.

The nuclear membrane then dissolves into a fibrous spindle, with at least one fiber passing through each centromere (there are many more fibers than centromeres).

The fibers then stretch and pull the centromeres apart, pulling the chromosomes to opposite sides of the cell.

The spindles dissolve into two new nuclear membranes, one around each group of chromosomes.

The chromosomes unwind back into invisibility, the cell divides, and mitosis is complete. Genetically, each daughter cell is an exact duplicate of the parent cell.

Since the genetic coding is carried in the rungs of the DNA and only consists of four different materials arranged in groups of three to form words of varying length written with a 64-letter alphabet, the instructions for a "cat" may be considered to consist of two sets of 19 "books," each millions of words long, one set from each of the cat's parents. The numbers of possible instructions are more than astronomical: there are far more possible instructions in one single chromosome than there are atoms in the known universe!

A single gene is a group of instructions of some indeterminate length. Somewhere among all the other codes is a set of instructions composing the "white" gene, and what that set says will determine if the cat is white or non-white.

Since a cat receives two sets of instructions, one from each parent, what happens when one parent says "make the fur white" and the other says "make the fur non-white"? Will they effect a compromise and make the fur pastel? No, they will not. Each and every single gene has at least two levels of expression (many have more), called alleles, which will determine the overall effect. In the case given, the "make the fur white" allele, "W", is dominant, while the "make the fur non- white" allele, "w", is recessive. As a result, the fur may be white or non-white, not pastel (we're only speaking of the "white" gene here, a gray cat is caused by an entirely different gene).

In order to understand how this works, lets run through a couple of simple examples using the white gene. A cat has two and only two white genes. Since each white gene, for the purposes of our examples, consists of one of two alleles, "W" or "w", a cat may have one of four possible karyotypes (genetic codes) for white: "WW", "Ww", "wW", "ww". Since "W" is dominant to "w", the codes "WW", "Ww", and "wW" produce white cats, while the code "ww" produces a non-white cat.

	W	w
W	WW	Ww
w	wW	ww

The double-dominant "WW" white cat has only white alleles in its white genes. It is classed as homozygous (same-celled) for white, and will produce only white offspring, regardless of the karyotype of its mate.

The single-dominant "Ww" or "wW" white cat has one of each allele in its white genes. It is classed as heterozygous (different-celled) for white, and may or may not produce white offspring, depending upon the karyotype of its mate.

The recessive "ww" non-white cat has only non-white alleles in its white genes. It is classed as homozygous for non-white, and may or may not produce white offspring, depending upon the karyotype of its mate.

Assuming these cats mate, there are sixteen different possible karyotype combinations. Since each cat in these sixteen combinations will pass on to their offspring one and only one allele, there are four possible genetic combinations from each mating. There are sixty-four possible combinations of offspring.

	WW W W	Ww W w	wW w W	ww w w
WW W W	WW WW WW WW	WW Ww WW Ww	Ww WW Ww WW	Ww Ww Ww Ww
Ww W w	WW WW wW wW	WW Ww wW ww	Ww WW ww wW	Ww Ww ww ww
wW w W	wW wW WW WW	wW ww WW Ww	ww wW Ww WW	ww ww Ww Ww
ww w w	wW wW wW wW	wW ww wW ww	ww wW ww wW	ww ww ww ww

Inspecting these possible offspring, several patterns emerge. Of the 64 possible offspring, 16, or exactly one-quarter, have any given pattern. This means that one quarter of all possible matings will be homozygous for white, "WW", two quarters will be heterozygous for white, "Ww" or "wW" (which are really the same thing), and one quarter will be homozygous for non-white, "ww". Since homozygous white and heterozygous white will both produce white cats, three-quarters of all possible combinations will produce white cats, and only one-quarter will produce non-white cats. This 3:1 ratio is known as the Mendelian ratio, after Gregor Johann Mendel, the father of the science of genetics.

Further inspection leads us to several conclusions. If a homozygous white cat mates, all offspring will be white. If two homozygous white cats mate, all offspring will be homozygous white. If a homozygous white cat mates with a heterozygous white cat, there will be both homozygous white and heterozygous white offspring in a 1:1 ratio. If a homozygous white cat mates with a homozygous non-white cat, all offspring will be heterozygous white. Thus, a homozygous white cat can only produce white offspring, regardless of the karyotype of its mate, and is said to be true breeding for white.

If two heterozygous white cats mate, there will be homozygous white, heterozygous white, and homozygous non-white offspring in a ratio of 1:2:1. The ratio of white to non-white offspring is the Mendelian ration of 3:1. If a heterozygous white cat mates with a homozygous non-white cat, there will be both heterozygous white and homozygous non-white offspring in a 1:1 ratio.

If two homozygous non-white cats mate, all offspring will be homozygous non-white. Homozygous non-white cats are therefore true-breeding for non-white when co-bred.

Geneticists differentiate between what a cat is genetically versus what it looks like by defining its genotype versus its phenotype. A homozygous white cat has a white genotype and a white phenotype. Likewise, a homozygous non-white cat has a non-white genotype and a non-white phenotype. A heterozygous white cat, on the other hand, has both a white genotype and a non-white genotype, but only a white phenotype.

Naturally, in a given litter of four kittens the chances of having a true Mendelian ratio are slim (slightly better than 1:11), so several generations of pure white kittens could be bred, still carrying a recessive non-white allele. In all good faith you then breed your several-generations-all-white-but-heterozygous female to a similar several-generation-all-white-but heterozygous male and voila! A black kitten! The non-white genotype has finally shown itself.

This Mendelian patterning is the basic rule of genetics. Since the rule is so simple, why is it so hard to predict things genetically? The reason is that we are dealing with more than one gene from each parent. The number of possible offspring combinations is two to the power of the number of genes: one gene from each parent is two genes, two squared is four possibilities; two from each parent is four, two to the fourth is sixteen; three from each is six, two to the sixth is 64;... There are literally hundreds of millions of genes for one cat, yet a mere hundred from each parent produces a 61-digit number for the possible offspring combinations!

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