Why do Brindlequins look patchy?

Brindlequins look patchy primarily because the Merle gene affects the dog’s underlying coat pattern.

For a dog to be a Brindlequin, it must inherit the brindle mutation (often represented as K 

br

 ), which creates a striped pattern over a fawn (sable) base coat. However, a Brindlequin also possesses both the Merle and Harlequin modifier genes.

When these genes combine, the Merle gene breaks up the continuous brindle striping, causing the brindle areas to look torn and patchy. Meanwhile, the Harlequin modifier acts on the remaining diluted background color caused by the merle gene, removing the pigment entirely to turn the base coat pure white. The final visual result is a dog with broken, patchy brindle spots scattered across a solid white background.