What made you want to look up xenial? Please tell us where you read or heard it (including the quote, if possible).
Black dog is an oft-used phrase to mean depression. Here's a link that attributes the phrase to Winston Churchill, but I suspect he didn't invent it. I like the way they described it though: Black Dog was Churchill's name for his depression, and as is true with all metaphors, it speaks volumes.
    Coat color and pattern is a polygenic trait, meaning it is controlled by more than one gene or gene complex. A dog with the gene for black color expression may als â¦ o have the gene for suppression of black and so have a red or yellow coat. polygenic trait is a trait in which multiple sets of alleles are used to determine the trait, whereas in a single gene trait aka. a Mendelian trait, only one pair of alleles â¦ is used.