All living things have genes, which is better known as DNA. The genes of a living thing are created when an egg and a sperm unite. All living things have what's known as a chromosome. These are structures inside the living thing's body that genes are located on. Each paired chromosome has two chromosomes, each chromosome is inherited from a parent. A gene for a part of a living thing is located on a certain locus (location) on the chromosome. The location is the same for all of the same specie.
Guppies have 23 different kinds of paired chromosomes, which equals 46 chromosomes in total. Of the 46 chromosomes, 22 of them are "blueprint" for the area of the guppy as another 22, which is why they are called paired chromosomes. The last 2 chromosomes are called sex chromosomes, which will be explained later. The two chromosomes of a paired chromosome in pure strains are nearly identical, this is called homozygous. If they aren't identical, it is call heterozygous. When an egg from a female and a sperm from a male unites, the 23 different paired chromosomes will split into two chromosomes. One chromosome from each the 23 paired chromosomes from both parents will unite into 23 new paired chromosomes, which will act as a blueprint for the new fries. Therefore, the fry inherits half of it's genes from either parent. Since the chromosomes from a paired chromosome are different, and the chromosome that is passed on is random, this will result in difference in the fries. The chromosomes of sex chromosomes can be an "X" or an "Y". Males of any living thing have one "X" and one "Y", while females have two "X"s. When the sex chromosomes split, an "X" is inherited from the female and either an "X" or an "Y" is inherited from a male. If both chromosomes that are inherited are "X"s, then the fry is a female. One "X" and one "Y" would result in a male.
During fertilization, a crossover occurs. This is when a part of a chromosome swaps positions with the same part on its paired chromosome. For example, the only way to get a tuxedo red by breeding a homozygous tuxedo blue with a homozygous full red is with a crossover. What happens is when the two guppies mate, the tuxedo gene from the tuxedo blue gets crossovered to the full red gene, replacing the wild-type (non-tuxedo) genes. So now the guppy has a chromosome for full blue and a chromosome for tuxedo red. However, this does not affect the phenotype (physical appearence) of the current generation, since the guppy still has the same genes on the same locus. However, if the guppy breeds, the next generation will receive either full blue or tuxedo red genes.
If the fry has two different genes on the same locus of a chromosome (heterozygous), three outcomes may occur.
1) The most common one is that one of the genes dominates, or covers up, the other gene. Some gene are dominant over others, which are recessive. If you breed a guppy with a dominant gene with one with a recessive gene, all the off-springs will show the dominant genes. But the guppies still have the recessive genes, it's just that it's covered by the dominant gene. A recessive gene will only show if both parents have the recessive genes.If you breed a gray guppies (dominant) with the genotype GG with an albino guppy (recessive) with the genotype aa, all of the off-springs will be gray with recessive albino genes. The off-springs' genotype will be Ga because GG x aa = Ga. However, if you breed two of the off-springs, the result would be Ga x Ga = GG, Ga, Ga, aa. Basically, 25% will be fully gray, 25% will be fully albino, and 50% will be gray with recessive albino genes. And then you can mate two of the albinos together and all of the off-springs will be albino.
2) Another possibility is that the dominant gene partially covers up the recessive gene, this is called partial domination. This occurrence is extremely rare.
3) The final possibility is just as rare as the previous one. What happens is that the results of the two genes mix. For example, if a gene for red and a gene for blue are on the locus of the chromosome, the guppy may show purple on its body.