Genetic mutation and crossbreeding are the more common causes of variation.

The cells of the muscular cows do increase in number, but not quite in size. They simply produce more, and make it more compact, which is how muscles work.

The muscles will often show immediately, or directly at birth. If they don't, they will either be moderately more muscled, or simply average.

Yes, often times muscular cows are the way they are because of genetics and heredity.

Traits can change based on the other members of the family tree not belonging to the original parents which is what changes the traits, although some can be preserved with a stroke of luck and a lot of crossbreeding.

Depending on what genes the parents have and what were passed on, genetics can change a lot about the development of an organism.

Traits vary from person to person because different parents cause different genes to mix, which is why everyone looks different, because we don't all come from the same person.

Sometimes these muscles do come from steroids, but other times, the muscles can come from genetic mutations.

Plants do have genes, but they don't show instantly, like they do with humans or other species.

Depending on the genes of these over muscular cows, their hormones will produce more and more of these muscular cells and tissues than normal, which is why they're so tightly packed.

Yes, half of the genes from each parent could cause patterns of the breed to mix. Be it how they act or look, there is often a mix of the breeds from the parents.

Purebred are born with muscles, and crossbred cows will develop them within a month.