Genetics are a very complicated matter: Dominant and recessive genes, phenotypes, genotypes, big A little A, etc. It can be an extremely over whelming subject. One in which we are not qualified to give an exhaustive study on, but we can offer up a basic run down on a simplified version of genetics from what we know, and from our experience with breeding Chihuahuas.
To keep the matter as simple as possible, we will discuss the genetics of the long-coat and the smooth-coat variations, and the genes and gene-combinations that produce each variety. Color combinations in the Chihuahua breed are nearly infinite, so this article will discuss only the coat lengths to keep the gene variations limited to two types. For an exhaustive study, please see our references at the bottom of the page.
How can two smooth-coat Chihuahuas produce a long-coat offspring?
Each parent Chihuahua carries two sets of genes, either two dominant genes, two recessive genes, or a combination of both: SMOOTH/SMOOTH, long/long, or SMOOTH/long. In the case of the Chihuahua, the SMOOTH-coat is considered the dominant trait while the long-coat is considered the recessive one. Being the dominant gene does not mean that it is any more likely to be passed on to their offspring, only that if a Chihuahua has both genes the SMOOTH-coat gene will be expressed and over shadow the long-coat gene. The gene that gets passed on from parent dog to pup is simply a matter of chance (50/50). So if you have two SMOOTH-coat Chihuahuas that both carry a recessive long-coat gene, there are four possible combinations of genes that the pup can inherit:
In the example, 3 out of the 4 pups will have SMOOTH-coats while only 1 will have a long-coat. This is because the SMOOTH gene dominates the long gene. This does not mean that each combination will happen in a liter. You may have a liter where all of the pups only inherited SMOOTH-coat gene. Keep in mind that there are also many SMOOTH-coat Chihuahuas that do not carry a recessive long-coat gene, and only have SMOOTH-coat genes, in which case you can not breed them and get a long-coated pup, even if you breed them to a long-hair:
There are only two combinations of genes, because they inherit one gene from each parent, and in both cases the SMOOTH gene would dominate the long. There is no way of telling whether a smooth-coat Chihuahua carries a recessive long-coat gene or not, except through trial breeding or a DNA test. Trial breeding can take some time to find out if your smooth-coat Chihuahua has any recessive genes. Because the genes are inherited at a 50/50 chance, and the smooth-coat gene is dominant, it is likely to have many liters of pups without getting a long-hair even though the combination is possible.
Long-haired Chihuahuas, on the other hand, when bred together will always produce a long-haired pup. This is because there is no SMOOTH gene in a long-coat Chihuahua. If there were, it would dominate the long-coat gene and the dog would be a SMOOTH-coat.