Chocolate is a sex-linked recessive colour. The colour is located on the Z chromosome. Sex chromosomes in poultry work differently to humans. Male chickens have two ZZ chromosomes; hens have ZW. The chocolate gene is found on the Z chromosome, so when a hen inherits one copy any black in her plumage is changed to chocolate. The gene is recessive so cockerels need two chocolate genes, one on each of their Z chromosomes. If they only have one their colouring remains, black (anywhere on the body expected to have black plumage). The cockerel will pass his Z genes on to his daughters. Even though he didn’t look chocolate if he has even one Z chromosome which carries the Chocolate gene he can pass it on to his daughters. Half his daughters will get the Z with the chocolate gene and be chocolate. A cockerel which is chocolate coloured and has two copies of Chocolate, one on each Z so all his female offspring will be chocolate.