1. Dogs were probably the first domestic animals. They are descended from the wolf. They date back at least to the Paleolithic period, and two breeds were known in early Egypt. From earliest times dogs have been bred for special purposes, such as sporting, working, and pets.

genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).

all matter is made up of atoms

in any element all atoms are the same

elements in a compound are always in a constant ratio: apex