> Yellow tends to stop certain patterns from being expressed e.g. merle, so it could be the same with agouti - anyone know?
Yes, Yellow (ee) does stop Merle from being expressed but in Labradors (and also in this case now I think about it!) it's the Dominant Black gene (K) at work.
Even Yellow Labradors have the K gene, but the presence of
ee prevents all black pigment from being produced. So a yellow Lab is
KK eeWhen crossed to a black and tan GSD -
kk a(t) a(t) EE the result is
Kk -a(t) Ee. the dominant black overrides everything else and as the GSD didn't carry
e (some would - "white" GSDs for instance) then they would be all black.
Not all breeds have the Dominant Black gene but the majority do. It tends to be herding breeds like GSDs and BCs that have recessive black
aa although oddly enough Belgian Shepherds are mainly KK, with rare occurrence of
aa.
The cases where tan-point Labs have occurred is when the parents each only had one K gene -
Kk and the absence of the K gene allowed the tan points (previously totally hidden by the prevalence of the K gene) to show through in one or two pups. I have seen a pure-bred tan-pointed Weimaraner and it's the same genetic quirk.