Affected gene: PMEL
Inheritance: autosomal dominant
OMIA link: 000211-9615

Mutation: Insertion of a SINE at the boundary of intron 10 and exon 11, flanked by a 15-bp duplication of the target site on chromosome 10.

Breeds: Australian Shepherd, Border collie, Leopard catahoula, Dachshund, Hungarian Mudi, Shetland

Description:
The merle-colored coat has a random pattern of spots with areas of darker or paler color and pale eyes. The merle color is transmitted dominantly, meaning that only one copy is needed to present this color.
A mutation in the PMEL gene can cause health problems in merle dogs, particularly "double merle" and "cryptic/ghost merle". 
See this site for more details: Labgenvet
Betagen determines the mutation responsible, i.e. an insertion in the PMEL gene, but the length of this insertion associated with the different colors or spotting patterns is not determined.

References:
Clark LA, Wahl JM, Rees CA, Murphy KE. Retrotransposon insertion in SILV is responsible for merle patterning of the domestic dog. Proc Natl Acad Sci USA. 2006 Jan 31; 103(5):1376-81. [PubMed: 16407134]
https://labgenvet.ca/en/dog-genetic-profile-merle/?fbclid=IwAR0SlGJXmEDiIV2QMoyaKLg-wjjfSsWKJZLfK0U_uTlqzx-RnnAoN3170lk