Technical explanations taken from the book "Current veterinary therapy: small animal clinic" (1984, subsequent editions 1996, Italian ed. Piccin Editore) by Robert Warren Kirk.
"First of all, remember that from homozygous subjects recessive by character (cream, blue, cream blue; which are recessive due to the dilution of the color) coupled with recessives they will give only homozygous recessive subjects, while subjects heterozygous for the aforementioned character, therefore, phenotypically black, red or brindle, if coupled together they can also give recessive homozygotes therefore blue, cream blue or cream in the Mendelian proportion of 1: 4.
Let’s remember that from the point of view of a genetic terminology the word Nero is used to indicate several colors, for example Tabby, Chocolate, Brindle etc. which have in common the fact of not being red or orange.
The very rare occurrence of the birth of black or blue subjects is explained in this way: these subjects are genetically brindle, but, with a very few red or cream fur, they may have also only small spots on the fingertips or on the palate. There is also the very rare possibility that the explanation should be sought in facts of mutation or altered chromosomal aberration (alteration linked to hetero chromosomes) colors such as. male caille x brindle or caille, will produce brindle male children, white and brindle and equal females, continuing the brindle x brindle mates after a few generations, the subjects tend to lighten or darken too much, so on they will have more and more brindle subjects with little or nonexistent stripe effect and some dilutions as blue / cream. However, these subjects are not albinos, nor with particular diseases, both on the skin and on other species.
The matter is when the dilution of the gene is exacerbated, that is, by continuing the repeated couplings between diluted, this method, which preserves the skin and / or hair color, leads to further dilutions, and may cause the albino type phenomenon, therefore subjects with lack of pigmentation; green or blue eyes with red concentric pupillary.
This redness of the pupil is noted with exposure of the subject to sun exposure or to greater light. Albino subjects may have an important photophobia and skin problems.
The same process may occur by introducing cream dogs without black masks into the brindle lines, weakened by excessive coupling between them, in this case we already start with a coupling that denotes a lack of pigmentation at the root. The same triggering factor of skin pathologies, it could be a retrocession in the pairings between subjects, which, over the years had been used to form a breed. For example, it is known that French bulldogs were initially paired with small terriers with large ears held up to correct their plump or pink ears. It is also known that in the bulldog genealogical family there are also pugs ...
Well, the Boston Terrier, for example, has created puppies with more pointed ears, dogs with slimmer build and longer legs, as well as liver-colored puppies and yellow eyes. The coupling with pugs has given thicker hair, with more undercoat, an increase in the already existing skin pathologies in both breeds.
What determines the color of the coat is, or rather, 2 Eumelanin and Feomelanin pigments, these pigments differ in term of chemical structure. The main color is defined by the presence in the hair of the pigment "Eu" i(it s the solid color: black with its recessives); the "Feo" determines the solid red and its diluted one. Disregarding the pigment may create errors. The genes that determine and control the coat's pigment are located on the sex chromosome "X"; the "Y" chromosome doesn’t have it! Starting from the point that the males have chromosome "X" and "Y" their coat is determined by a single pigment .If it is Eumelanin ,we will have males "Eu" and if it is Feomelanin we will have males "Feo", the females unlike two chromosomes "X" and do not have "Y", if both X chromosomes have 2 genes for the Eu there will be females of only one color Eu if both Feo chromosomes the females will be only Feo if the genes are 1 "Feo" el other "Eu" will have Eu- Feo females who will bring both colors in the litters, usually the males take the color of the coat from the mother and inherit the pigments from both parents. The results show the validity of the rules. The recessives appear only after several and excessive mating of the same color, or if, the 2 parents are both carriers of the same recessive gene. Ex: male brindle, son of brindle x cream. Fawn female, daughter of cream x fawn.
If both parents have the same single recessive gene the statistical result of the broods will be the following: 25% no recessive gene; 50% recessive gene; 25% 2 recessive genes. The small brindle will be partly carriers of the recessive gene and partly not, however, it is still impossible to distinguish them by eye if not by following successive mating with subjects carrying the same gene.
If one of the parents is the carrier of the autosomal recessive gene that does not manifest itself in reproduction, we can expect that 50% of the puppies will be a carrier of this gene (autosomal is a gene whose locus is not found in the sex chromosomes "X" "Y". All genes that control recessive colors are autosomal.). Brindle male, without gene x blue, for fawn female with cream gene, will give solid tawny colors, brindle, but all females having fawn will be able to carry in one way or another the dilution gene, both in cream and in blue , therefore the brindle will have fawn as the fawn and here the chances will be very high if this female meets a male with the same gene as the dilute. If one of the parents is a carrier for twice of the same autosomal gene, all the puppies will have at least one of these genes.
Another example: blue male with tawny female we will only get tawny males and brindle females with cream and non-tawny spots, both will transmit the dilution gene by continuing x redundancy of pairs we will get "Blue fau" puppies That is red with blue mask, (hardly distinguishable from the classic ), with light eyes and blue / cream with light eyes for the onset of the albinotype phenomenon ".
