The gene variant that produces the cream dilution phenotype has recently been reported (Mariat, Taourit and Guerin, (2003) "A mutation in the MATP gene causes the cream dilution colour in the horse". Genet. Sel. Evol., 35: 119-135).
Although this published evidence is based on a study of 145 horses, field studies have not progressed to the point where there are sound statistics about the proportion of false negative or false positive results.
Therefore, while the AEGRC is prepared to provide a DNA testing service for this gene it wishes it to be clearly understood at the outset that the test cannot be regarded as field validated, and will accept no responsibility for the subsequent use that Registries and or breeders wish to make of the test.
The cream dilution gene is believed to be different from the two other dilution genes, Dun and silver dapple or taffy colour dilution genes (Bowling (1996) in "Horse Genetics" (CAB International, Wallingford, Oxon UK), pp. 25-33; Gower (1999) in "Horse Coat Colour Explained. A breeders perspective" (Kangaroo Press, East Roseville, NSW, Australia, pp. 28-53).
The cream dilution phenotype
Generally, in a heterozygous (single copy) state the cream dilution gene produces the palomino colour in a sorrel or chestnut (red) background, and the buckskin colour in a bay background. But, because it shows incomplete dominance, the cream dilution gene in a heterozygous state may have a minor or no effect in some black horses (Bowling, 1996). In such horses the dilution effects may be subtle - yellow-brown eyes and only a dulling of the black coat colour.
Bowling (1996) also reports that there may be recessive alleles of the cream dilution gene in some breeds of horses that may account for very pale palominos sometimes called "isabellas", and for very pale buckskins (pink skinned amber horses) occasionally known as "yellow duns". Alternatively it is noted that these may be due to other genes that modify pigment intensity. In the case of yellow duns this may be due to a historical confusion in terminology that has since been clarified - yellow duns are now known to be the result of the unrelated Dun dilution gene (see Gower 1999).
Generally, in a homozygous (double copy) state the cream dilution gene produces the cremello coat colour on a red background and a perlino colour on a bay background (Bowling (1996) in "Horse Genetics" (CAB International, Wallingford, Oxon UK), pp. 25-33).
Gower (1999) notes that the nearly complete absence of pigment in the coats of the cremello and perlino defines them as "pseudo-albinos". As such they generally have blue rather than pink eyes, a feature often resulting in them being referred to a "blue-eyed cream" in some countries.
While it is the generally accepted practice to regard cremellos as homozygous-dilute reds and perlinos as homozygous-dilute bays or blacks, Bowling (1996) notes that these descriptions do not fit all cases. Bowling (1996) cautions that colour of some perlinos can be indistinguishable from their cremello counterparts, and notes that "more cases need to be studied to understand the genotypic and phenotypic differences between them".
Making sense of your CrD DNA screening test data
It should now be clear from all of the above that the test results fit into
one of the following categories:
1. Those in which the genotype is "CrD/N" (where "CrD" signifies cream dilution and "N" normal).
This genotype will be reported in:
(a) palomino horses with the chestnut coat colour. (This is an expected result.)
(b) buckskin horses with a bay coat colour. (This an expected result.)
(c) black or dilute black horses with bay/black coat colour genes. (While this is an expected result, in some cases the dilution effects may be minor).
(d) those grey horses that were either palomino or buckskin as foals. (It is to be expected that some grey horses with chestnut or bay/black coat colour genes can also carry a single copy of the CrD gene.)
When horses with the "CrD/N" genotype are mated together, only up to 25% of their progeny will be "CrD/CrD".
2. Those in which the genotype is "N/N".
It is very unlikely that these will have cream dilution effects on their coat colour.
Horses with this genotype are not expected to produce any progeny with a cream dilute phenotype.
3. Those in which the genotype is "CrD/CrD"
This genotype is expected to be reported in:
(a) cremello horses (with chestnut coat colour genes). (This is an expected result). However, if such horses also carry one or two copies of the (dominant) grey gene then the grey colour will later mask the cremello phenotype (cream body coat, pink skin blue eyes). Grey cremellos are expected to have a white body coat, pink skin and blue eyes.
(b) perlino horses (with black or bay coat colour genes). (This is an expected result). If such horses carry one or two copies of the (dominant) grey gene, then the grey colour will later mask the perlino phenotype, resulting in a white body coat, pink skin and blue eyes.
(c) those grey horses that were either cremellos or perlinos as foals.
When horses with this genotype are mated all of their offspring will have diluted coat colours.
Cream dilution testing in combination with other screening tests:
Is it really a buckskin?
A buckskin is expected to be heterozygous for CrD (ie, positive but with a single copy, or a CrD/N genotype) and also to be either homozygous negative for Chestnut Coat Colour (N/N) or to be heterozygous for Chestnut Coat Colour (ie, be C/N). (Bay, brown or black horses have the CCC genotypes of C/N or N/N).
Dual testing for CrD and CCC is suggested for these cases.
Is it really a palomino?
A palomino would be expected to test homozygous positive (with two copies of genes) for both CrD (CrD/CrD) and CCC (C/C). (A chestnut coloured horse has the CCC genotype, C/C).
Thus dual testing for CrD and CCC is suggested in these cases also.
For any further advice about the interpretation of the test data, please call the AEGRC.