Читать книгу Horse Economics - Vera Kurskaya - Страница 31

The Cream Dilution mutation is located in the MATP (membrane-associated transporter protein) gene on the twenty-first chromosome. This gene is responsible for the synthesis of a protein, which participates in the process of pigment formation and melanocyte differentiation. This protein is also found in large quantities in melanoma tumor cells.

The mutation of MATP is designated “C^cr,” while the “Wild” allele that does not influence the color of the animal is designated by “C” or “Cr.” The “C^cr” allele is incompletely dominant, and in the heterozygous state it causes dilution of pheomelanin-pigmented regions without an effect on eumelanin colored areas. However, observations show that the regions pigmented with eumelanin also show the action of this gene, although it is not always clearly noticeable. An example can be the brownish coloring of the lower part of the legs in some buckskins, or the brownish shades sometimes encountered in smoky black horses. In some cases the “C^cr” gene in the heterozygous state lightens up the iris color in the eye, which then becomes hazel or amber. A horse homozygous for “C^cr” shows strong pigment dilution not only in the hair, but also in the skin and the iris of the eye (Photo 25). The colors caused by Cream Dilution are summarized in Table 3 (see p. 22).

A DNA test for “C^cr” is available, and it is especially useful to determine the smoky black color, which as mentioned earlier, in the majority of cases cannot be distinguished visually from black. Based on the results of this test, the breeder will know if a particular horse can produce certain diluted colors caused by this mutation, and plan appropriate breeding. Tests of Agouti and Extension further help to determine the precise genotypes in double cream dilutes.

Table 3.

Base Color	Ccr/C	Ccr/Ccr
Bay	Buckskin	Perlino
Black	Smoky Black	Smoky Cream
Chestnut	Palomino	Cremello
Seal Brown	Smoky Brown	Perlino