Here's why:
* Genetics: A female dog (bitch) has two X chromosomes (XX), while a male dog (dog) has one X and one Y chromosome (XY). During mating, the female contributes an X chromosome, and the male contributes either an X or a Y chromosome.
* Chance: The sex of a puppy is determined by which chromosome (X or Y) the male dog contributes. This means there's a 50/50 chance for each puppy to be male or female.
* Large Litters: While it's possible to have a litter with a slight imbalance (slightly more males or females), it becomes increasingly unlikely to have all puppies of the same sex as the litter size increases.
Exceptions:
It's important to note that there are rare genetic anomalies that can influence sex determination in dogs, but these are extremely uncommon.
In summary: It's impossible for a female dog to have a litter consisting entirely of male or female puppies due to the basic principles of canine genetics.