Yorkies, like all other domesticated dogs have two oestrus cycles per year ( whereas the gray wolf from whom they were derived has only one). The first season or heat begins at around 6 months of age, while she is still immature ,like a young 11 year old girl having their first period. Your bitch should not be bred from until around 2 years of age BUT better still please consider neutering her so that she will have no seasons at all . Otherwise having puppies just adds to the misery of packed rescue centres because of the overproduction of unwanted litters. Also very many pedigree dogs carry bad recessive genes which can be specific to each breed and so you need to be reasonably sure that if you do mate her that the sire will not be carrying the same recessive mutation as the dam. This can be tricky as obviously all members of the same breed have a high genetic homozygosity ( they are very un iform or clonish)