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Meiosis Since each cell contains the entire chromosome set, 19 pairs, how is it possible for a parent to pass on only the genes from one chromosome of a pair, and not both. This is accomplished via the gametes: the germ cells, ova for females and sperm for males. Within the gonads (ovaries or testes), these special cells go through a division process known as meiosis.
Unlike the normal process of mitosis, where the chromosomes are faithfully replicated into duplicates of themselves, in meiosis the resultant gametes have only half the number of chromosomes, one from each original pair. This involves a double division.
As in mitosis, meiosis begins when the cell senses sufficient growth and nutrients to support division. The invisible chromosomes are duplicated through DNA replication. As usual, the two daughter chromosomes remain joined at the centromere. The chromosomes wind themselves up, shortening and thickening, becoming visible under the microscope. Each new chromosome twin aligns itself with its homologous counterpart: the twin chromosome from its opposite number in the original chromosome pair. The two twin chromosomes intertwine into a tetrad and exchange genes in a not clearly understood process that randomizes the genes between the twins. The tetrad attaches itself to the nuclear membrane. The nuclear membrane dissolves into a spindle, with at least one fiber passing through both centromeres of each tetrad. The fibers stretch and pull the tetrads apart, pulling the chromosomes twins to opposite sides of the cell. Once the chromosome twins are at the poles of the spindle, the spindle dissolves and reforms as two separate parallel spindles at right angles to the original spindle, with at least one fiber through each centromere. At this time there are effectively two mitoses taking place. The parallel spindles pull the centromeres apart, forming four separate groups of chromosomes, each of which consists of one-half the normal number. The spindles dissolve and four new nuclear membranes form, one around each group of chromosomes. The chromosomes unwind back into invisibility, the cell divides into four gametes, each having 19 chromosomes, and meiosis is complete.
At the moment of conception, a single sperm penetrates a single ovum, the ovum absorbs the sperm, merging the sperm's nucleus with its own and pairing the two sets of chromosomes. The ovum has now become a zygote, which begins dividing through the normal mitosis process, and a kitten is on its way.
Male, Female, and Maybe The 19 pairs of chromosomes in a cat carry the numbers 1 through 18, plus "X" and "Y". The "X" and "Y" chromosomes are very special, for they determine the sex of the kitten. A female cat has two "X" chromosomes, "XX", while a male cat has one "X" and one "Y" chromosome, "XY", so if we follow the Mendelian pattern for sex determination we find that the female parent can provide only an "X" chromosome to her offspring, while the male parent can provide either an "X" chromosome or a "Y" chromosome. The resulting kittens are either "XX" or "XY", as determined by the father. The same rule also applies to people (Sorry guys, if you and the wife have seven girls, it's your fault, not hers!).
Since the sex chromosomes follow the same rules as the other chromosomes, why bother mentioning them separately? Because they don't exactly follow the same rules: the "X" chromosome is longer than the "Y" chromosome, and it is this extra length that carries the codes for the female. When there is only one set of these extra codes, they act as recessives, allowing the male characteristic to dominate. When there are two sets, they act as dominants, and suppress the male characteristics. Thus, female and male kittens.
We could end the argument here if it weren't for two complications. First, the extra-length of the "X" chromosome carries some genes that are for other than sex characteristics (such as the gene for orange fur): such characteristics are said to be sex-linked, and operate differently in males and females.
A further complication comes with incomplete separation of the "X" gene twin at the centromere. An "X-X" gene twin has its centromere exactly where "Y"'s would become "X"'s. If an "X" were to fracture at the centromere during the process of separation, it would become an effective "Y". This is rare but by no means unheard of, and produces a "false" "Y" (shown as "y" to differentiate it from a female "XX" parent.
Another variation is incomplete separation, where only a "false centromere" is separated from the gene twin, with or without a part of the twin, causing one gamete to have 18 chromosomes (neither an "X" or a "y" while the other has 20 (either two "X"'s, an "Xy", or two "y"'s, depending on the point and angle of fracture).
These variations on the sex chromosomes mean that a female, being "XX" in nature, can produce ova with the following: "XX", "Xy", "yy", "X", "y", or "O" (no sex chromosome). A male, being "XY", can produce sperm with "XY", "Yy", "X", "Y", "y", or "O". A zygote, taking one gamete from each parent, may then be any of the following 36 possibilities:
XX Xy yy X y O
XY XXXY XXYy XYyy XXY XYy XYO Yy XXYy XYyy Yyyy XYy Yyy YyO X XXX XXy Xyy XX Xy XO Y XXY XYy Yyy XY Yy YO y XXy Xyy yyy Xy yy yO O XXO XYO yyO XO yO OO Since at least one "X" is required (can't build a puzzle without all the pieces), we may immediately ignore "Yyyy", "Yyy", "yyy", "YyO", "yyO", "Yy", "yy", "YO", "yO", and "OO".
In a like manner, "XXXY", "XXYy", and "XYyy" have too many pieces and are unstable, usually dying at conception, in the womb, or soon after birth (and invariably before puberty) from gross birth defects due to over-emphasis of various sex-linked traits.
Turner females, "XO", show all normal female characteristics save that they have difficulty reproducing due to the absence of a paired sex chromosome, which inhibits normal meiosis.
Kleinfelter superfemales, "XXX", tend to exhibit an unusually strong maternal instinct, often refusing to wean or surrender their young. This leads to psychological damage in the young, usually resulting in antisocial behavior.
Kleinfelter supermales, "XYy" or "Xyy", tend to exhibit a generally antisocial behavior, often leading to unnecessary fighting to the point of inhibiting mating. As an interesting aside, among us humans approximately 5 per cent of convicted male felons are supermales. Hermaphrodites, "XXy" and "XXY", have male bodies but tend to exhibit various female characteristics, often adopting orphan kittens or other young. One such cat adopted a litter of mice, which it lovingly raised while gleefully hunting their relatives. Hermaphrodites are invariably sterile, sometime having both sets of sexual organs with neither fully developed. This is the most common of the aberrant sexual makeups.
Pseudoparthenogenetic females, "XXO", or males, "XYO", are identical to normal cats in every way save that their sex and sex-linked characteristics come only from one parent.
Gene-reversal males, "Xy", suffer partial gene reversal, receiving a normal "X" from one parent and a "y" from the other parent's "X". This is the rarest of the aberrant sexual makeups.
Pseudoparthenogenetic and gene-reversal animals often suffer from birth defects and other signs of the aberrant genetic construct.
Normal females, "XX", and males, "XY", are by definition the norm and vastly outnumber all other type combined. Chances are less than 1:10000 that any given cat has a genetically aberrant sexual makeup, the most common of which is hermaphroditism, about 1:11000.
Mutations Going back to genes in general, those genes that are found in the African Wildcat, felis lybica, the immediate ancestor of our cats, are termed "wild." These genes may be considered to be the basic stock of all cats.
Since all cats do not look like African Wildcats (brown tabbies), it is obvious that some changes have taken place in the genetic codes. These changes occur all the time, and are called mutations. Unlike the distortions shown in cheap post-apocalypse or ecological-disaster movies, mutations rarely occur at the gross level, but rather at the level of the genetic codes themselves.
Mutations occur when, in the course of mitosis or meiosis, there is an imperfect replication or joining of the components of the DNA macro- molecule. Such imperfections can occur as a result of a chemical imbalance within the body which affects replication. Most commonly these days such an imbalance is caused by the introduction of some foreign agent into the body (such as nicotine or, for an extreme example, thalidomide) which acts as a catalyst and affects the keying action of the enzymes during replication. Such agents are called mutagens.
The greatest of all mutagens is radiation. It is believed that the vast majority of spontaneous mutations, such as extra toes, long hair, albinism, etc., that keep reoccurring in an otherwise clean gene pool are caused by solar radiation, cosmic rays, the Earth's own background radiation, and most probably, by radioactive isotopes of the atoms making up DNA itself, most significantly carbon-14. (One of the dangers of nuclear war, other than the obvious, is that the increase in background radiation and atmospheric carbon-14 may increase the numbers of spontaneous mutations to the point where the germ cells lose viability, and whole species, even genera, would go the way of the dinosaur.)
Mutations are the very essence of evolution (or of a breeding program, which is merely evolution guided by man). It is through mutation that the survival of the fittest takes place.
To illustrate this, let's assume a species of striped cat living on the plains. He undergoes a mutation creating a spotted coat (the stripes get broken up). For our plains friend, the spots don't blend as well as stripes with the long shadows and colors of the grasses, his prey can see and avoid him better, and he soon evolves out. This was a detrimental mutation (most are).
Now let's assume the same species of striped cat living in woodlands. He undergoes the same mutation creating a spotted coat. In his case, the spots blend better with the dapple of light and shadow playing through the trees, his prey can't see or avoid him as well, and spots are soon the "in" thing. This was a beneficial mutation. From the same parent stock we soon have two differing sub-species, one striped, living on the plains, and one spotted, living in the woods.
In a domestic situation, a litter is born to two normal cats, wherein one of the kittens is hairless. Thinking the hairlessness is different enough to be a desired feature, especially for those with allergies, the kitten is very carefully bred to other cats, back and forth over several generations, until the hairlessness breeds true. Thus the Sphinx, a hairless domestic cat and the ultimate in hypo-allergenic cats, was developed.
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