Since the solid black gene is a double recessive gene, a pup must
inherit the pair of the solid black recessive genes from both parents
(one from each parent) in order manifest the solid black coat. Two
colored GSDs (non solid black) can produce some solid black GSDs pups in
their litter if one of the gene pair of both parents is a recessive
solid black gene (i.e. they are carriers) but if both parents are solid
black GSDs then all the pups in their litter will be solid black German
shepherds.
Therefore except the German shepherd breeder knows the
pedigree of the male (sire) and female (dam) dogs used in breeding, he
or she may be surprise that some of the pups are solid black even though
neither of the parents is a solid black German shepherd. In this case both parents
are carriers of the recessive black gents.
Except in cases where the black pigment formation is acted upon in
some ways e.g. blocked, diluted or masked (as in the case of the white
German shepherd), GSD pups are born black and will stay black through
adulthood and sometimes with white spot on the chest or toes.
The final color of a GSD pup can only be ascertained when the outer coat has fully developed.
Black German Shepherd: Essential facts, pictures and videos
http://www.aboutdogfacts.com/black-german-shepherd/
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