Answer:
There can be a lot of varying factors when it comes to muscle building from family genetics. Genetically my family has always had a low body fat percentage (around 6-8%) so I almost naturally have a six pack, but some families have a high natural body fat percentage, that doesn't mean they cant have a six pack it just means they will have to work a little bit harder to get it. Girls can have a harder time because they don't have high testosterone like men do and that can be a factor. Diet also plays an important roll, If you eating a lot of calories though out the day you better burn them off. Cadio is one of the most important things to achieving a six pack, it keeps your fat percentage down and makes your leg and core muscles more toned (which is what six pack really is). Also it is important to keep a work out routine and stick to it. Over all it can be challenging but all it really requires is persistence.