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GUPPY GENETICS

PART 1- INTRODUCTORY TERMS

By Jack Rosengarten

 To acquaint you with heredity and perhaps to further stimulate your interest, I’ve decided to write a series to try to explain genetics as it relates to guppies. Since the books I have read did not mention guppies, or even fish, I had to use my own judgment in selecting characteristics, which might apply to guppies. Therefore. Any comments about guppies are entirely my own opinion or those of others that I respect. The literature deals almost entirely with the fruit fly and humans, citing other animals to illustrate particular characteristics so that I may likewise be forced to go to these same examples.

• Part one deals mainly with the basic definitions which I hope will not be too repetitious fro most of you, but they will assure that the following articles may be understood. Some of the definitions will require extensive examples so they will be left to future articles.

Now for some very basic definitions:

• GENE: To the breeder this is the smallest unit of inheritance although the geneticist now subdivides this to attempt to explain why genes are different and how they function. We will adhere strictly to what is useful to the breeder.
• CHROMOSOMES: All genes are located on a thread like bodies called chromosomes. These are normally found in pairs. The nucleus of every cell contains a set of chromosomes. The fruit fly has eighteen chromosomes; while humans and guppies have 46 chromosomes (are we related?). It is estimated that humans have as many as 300,00 genes, so guppies probably have a comparable number. If that seems like a lot, remember that every physical characteristic is determined by at least one gene.
• ALLELES: Genes which occupy a specific location on the chromosome usually control a specific trait. Variations of this gene are called alleles and they can cause corresponding variations in the trait. Since the chromosomes come in pairs, the genes like wise come in pairs and whether they are both the same or different is really the backbone of heredity.
• POLYGENES: frequently more than one pair of genes influences a characteristic. These groups of genes are known as polygenes or multiple genes. Obviously breeding gets more complicated when polygenes are involved.
• GENOTYPE: This is the description of the genetic makeup of an organism usually described symbolically with letters.
• PHENOTYPE: this is the appearance of the organism caused by the genetic makeup. Individuals with different genotypes may still have th same phenotype, or appear to be the same.
• HOMOZYGOUS and HETEROZYGOUS: As mentioned earlier, genes usually come in pairs. If both genes of the pair are the same, the organism is said to be homozygous. If both genes are different, the organism is known as heterozygous.
• DOMINANT or RECESSIVE: The relative importance of each allele is classified as dominant or recessive to each other allele. Possession of one dominant allele is sufficient to establish the dominant phenotype. The heterozygous organism will look identical to the oranism that is homozygous for the dominant gene. Both identical recessive genes are needed to express the recessive genotype, unless of course, the odd gene is a third allele that is even more recessive. There can also be an intermediate expression where the heterozygous organism is a different phenotype than either of the homozygous genotypes ( in other words, three different appearances result from various combinations of two different genes). Geneticists use capital letters to denote dominate genes and small letters to symbolize recessive genes; i.e. genotypes for brown eyes could therefore be written as BB, Bb or bb where B is a dominant gene for brown eyes as letters with various superscripts. 

Now that you know the basics, lets progress into how these traits are passed on to the offspring.

• MEIOSOS: This is the process by which cells with a normal number of chromosomes divide to form the sex cells (eggs or sperms) necessary for fertilization. This division separates each chromosome pair so that each sex cell has only half the normal number of chromosomes. When they join during fertilization, the number of chromosomes will again be correct. It is a pure game of chance as to which of each chromosome pair is in each egg or sperm, but all the genes on each chromosome will move as a unit (with some exceptions).
• SEX DETERMINATION: As mentioned earlier, chromosomes occur in pairs. Excluding abnormal cells, these pairs are usually matched in size and approximate sex. In humans, fruit flies and guppies, the male has a pair of chromosomes differing greatly in size. The smaller of the pair is designated as the Y-chromosome and the larger is designated as the X- chromosome. The female, in contrast, has a pair of X- chromosomes. These chromosomes are inherited the same as all the others so that an individual with an XY chromosome pair is male and one with XX chromosome is a female.
• SEX-LINKED GENES: Genes located exclusively on the X- chromosome are called sex-linked genes since their inheritance is related to sex determination. In the hobby this is usually referred to as X-linked and I’ll stick with that usage. A good sample of the characteristic is some of the half-black strains of guppy.
• HOLANDRIC GENES: This term applies to the genes located exclusively on the Y- chromosome or Y-linked. Few genes appear to be located on the chromosome so that this condition is relatively rare. Examples of this in guppies are also certain half-black strains, snakeskins and also the tangential-eye-line.
• AUTOSOMAL GENES: This covers all the genes located on the other chromosomes. Their pattern of transmission is therefore independent of sex determination.
• INCOMPLETELY SEX-LINKED GENES: Genes in this category have alleles on both the X- and Y- chromosomes so that they behave like autosomal genes but their pattern of transmission shows their relation to sex determination. I don’t know of any guppies that fit this pattern but certainly the half-black strains mentioned above are candidates if they are indeed alleles. I think some of the sword tails guppies are also possible candidates but I’m now convinced that the double swords that I have are caused by a dominant autosomal gene.   It should be obvious that outcrosses of this type of gene with other strains will cause some confusing results.
• SEX-LIMITED GENES: These are genes, which maybe present in either sex but are expressed in only one sex. Certainly this must apply to the color and other secondary sexual characteristics of the male guppy. Female guppies treated with male hormones will color like the males and start to acquire male characteristics proving that the females have the genes to make that possible. Hormone treated females can even develop a gonopodium (male anal fin) although the will never be fertile males. In the fruit fry, only the genes for male fertility are located on the Y-chromosome and this appears to be the case with guppies.
• SEX-INFLUENCED GENES: The class of genes, which are dominant in one sex, can be recessive in the other sex. The best example I can think of concerns the X-linked hemophiliac while a woman is an unaffected “carrier of the gene. IN the contrast, a woman with two genes for hemophilia is herself a “hemophiliac”.
• LINKED GENES: The term covers genes, which govern different characteristics but are located on the same chromosome so that they are inherited together. Of course this is a great nuisance to a breeder who is trying to separate an undesirable trait from a desirable trait. I would guess that the small dorsal associated with snakeskin’s are an example of linked genes. The next two terms, however, offer some hope fro the frustrated breeder. It should be pointed put thst if linked genes govern the same trait the breeder will be obvious of the fact and assume that there is only one gene involved.
• CROSSOVER: An entirely unpredictable phenomenon which occurs is that of crossover wherein linked genes are indeed separated. Somewhere in the formation of the gametes (a general term for eggs and sperm) a pair of chromosomes breaks and exchanges halves. If the above example is true, someday a breeder maybe lucky enough to have a large dorsal gene on one chromosome and a snake pattern on its companion when a crossover occurs. Since snakeskin is a Y-linked gene (although some claim there are also X-linked snakeskin’s) this would be a most unusual crossover and could result in sterile males if too much of the Y- chromosome is lost. Hopefully, if the fish turns up it will not be culled fro some other reason before the crossover is noted.
• MUTATION: In the strictest sense, this is the occurrence of a gene, which was not inherited. It maybe a gene that was altered with chemicals, radiation, heat or by accident. Whatever the reason, a new trait may show up and if desirable could lead to a whole new strain of guppies. The Breeder, of course, will probably call anything that wasn’t expected a mutation, even though it may only be a recessive trait that has finally surfaced.