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DP stands for dense pheomelanin and is a concentration of the red pigment in hair cells that causes them to appear darker than normal. When homozygous (though we can't test for it) and in conjunction with homozygous sooty, it gives "black" chestnuts (and "black" palominos and buckskins). Heterozygous DP and homozygous sooty gives liver chestnuts. The gene test now includes "liver chestnut" as the result when appropriate, but the other effects of DP do not show up.

Here is a search on my horses that carry my DP tattoo. The ones that didn't test as liver chestnuts are suspected of carrying heterozygous DP either because a parent was homozygous for it or because their coloring is darker than the amount of Ssty they have would lead me to expect.
http://www.huntandjump.com/adv_search.php?status=any&ageg=&agel=&agee=&erag=&eral=&erae=&f_basic=none&f_adv=none&tattoo=sca+dp+carrier&f_limit=25&stallion=1&mare=1&gelding=1&spayedmare=1&name=&name_lm=like&color=&color_lm=like&bty=1&lined=lined&foundation=1&sire=&dam=&barn=&owner=137592&breeder=&e1=1&e2=1&a1=1&a2=1&f1=1&f2=1&p1=1&p2=1&s1=1&s2=1&s3=1&c1=1&c2=1&c3=1&d1=1&d2=1&g1=1&g2=1&ch1=1&ch2=1&z1=1&z2=1&sa1=1&sa2=1&kit1=1&kit2=1&kit3=1&kit4=1&kit5=1&kit6=1&kit7=1&kit8=1&kit9=1&sp1=1&sp2=1&rb1=1&rb2=1&o1=1&o2=1&l1=1&l2=1&v1=1&v2=1&brindle1=1&brindle2=1&patn1=1&patn2=1&patn3=1&patn4=1&patn5=1&patn6=1&wf1=1&wf2=1&wf3=1&wf4=1&wf5=1&wf6=1

Ammit has also updated the color test to reflect some of the most recent conclusions about bays and browns. Agouti now has four possible variations: A (dominant agouti that gives bays when combined with at least heterozygous black [E]), a (recessive agouti that does not affect black pigment), A+ (wild bay that lightens the black stockings, sometimes gives a dorsal stripe), and At (brown). In the order from most to least dominant they are: A+, A, At, and a.
Wild bay is dominant in all combinations. At only produces browns when combined with a. AtA color tests bay. AtAt produces a lighter brown brown horse than Ata.

Wild bay in various combinations:
http://www.huntandjump.com/adv_search.php?status=any&ageg=&agel=&agee=&erag=&eral=&erae=&f_basic=none&f_adv=none&tattoo=sca+wild+bay+carrier&f_limit=25&stallion=1&mare=1&gelding=1&spayedmare=1&name=&name_lm=like&color=&color_lm=like&bty=1&lined=lined&foundation=1&sire=&dam=&barn=&owner=137592&breeder=&e1=1&e2=1&a1=1&a2=1&f1=1&f2=1&p1=1&p2=1&s1=1&s2=1&s3=1&c1=1&c2=1&c3=1&d1=1&d2=1&g1=1&g2=1&ch1=1&ch2=1&z1=1&z2=1&sa1=1&sa2=1&kit1=1&kit2=1&kit3=1&kit4=1&kit5=1&kit6=1&kit7=1&kit8=1&kit9=1&sp1=1&sp2=1&rb1=1&rb2=1&o1=1&o2=1&l1=1&l2=1&v1=1&v2=1&brindle1=1&brindle2=1&patn1=1&patn2=1&patn3=1&patn4=1&patn5=1&patn6=1&wf1=1&wf2=1&wf3=1&wf4=1&wf5=1&wf6=1

At in various combinations:
http://www.huntandjump.com/adv_search.php?status=any&ageg=&agel=&agee=&erag=&eral=&erae=&f_basic=none&f_adv=none&tattoo=sca+brown+carrier&f_limit=25&stallion=1&mare=1&gelding=1&spayedmare=1&name=&name_lm=like&color=&color_lm=like&bty=1&lined=lined&foundation=1&sire=&dam=&barn=&owner=137592&breeder=&e1=1&e2=1&a1=1&a2=1&f1=1&f2=1&p1=1&p2=1&s1=1&s2=1&s3=1&c1=1&c2=1&c3=1&d1=1&d2=1&g1=1&g2=1&ch1=1&ch2=1&z1=1&z2=1&sa1=1&sa2=1&kit1=1&kit2=1&kit3=1&kit4=1&kit5=1&kit6=1&kit7=1&kit8=1&kit9=1&sp1=1&sp2=1&rb1=1&rb2=1&o1=1&o2=1&l1=1&l2=1&v1=1&v2=1&brindle1=1&brindle2=1&patn1=1&patn2=1&patn3=1&patn4=1&patn5=1&patn6=1&wf1=1&wf2=1&wf3=1&wf4=1&wf5=1&wf6=1