Calico cats are always female and the are also refferred to as tortoiseshell. The answer has to do with genetics. Every cat has 38 pairs of chromosomes; half of the pairs are from the mother, the other half is from the father. Within every chromosome there are thousands of different genes.
Every female cat receives one X chromosome from her mother and one X chromosome from her father, while a male receives one X chromosome from his mother and one Y chromosome from his father. Within the X chromosome is a gene for coat color.
In calicos and tortoiseshells, one X has the black gene; the other X has the orange gene. White coat color is associated with a completely separate gene.
At conception, the kitten is a one-celled organism, which divides until there are millions of cells that make up the final kitten. Each time a cell divides, it passes on its genetic material.