Male Offspring

Dunno whether it’s true or not as stated.
Just take it as a joke. Don't get offended.

TRUE FACT:

Male sperm (Y) swim faster and die sooner than female sperm (X), because female sperm contain heavier genetic material, which slows them down but allows them to live longer.

THEREFORE:
Males with longer penises tend to produce more male offspring because they deposit the sperm closer to the egg and Y sperm cells win the "sprint." Males with shorter penises tend to have more female offspring because sperm is deposited further from the egg, Y sperm die off thus allowing X sperm to win the "marathon."

CONCLUSION:
Look at your family. This tells you whether or not your father had a big one.