Spermatogenesis
As a result of meiosis in man’s testicles four gametes appear. The production of sperm starts with the pubescence and continues until the old age. Many DNA replications increase the probability of a mutation in a single gene for older fathers.
    

Oogenesis
The formation of egg cells (ova, singular ovum) is already advanced in the foetus. The new born girl’s primary egg cells are halted in the first prophase. After pubescence these cells finish meiosis one by one. After first division one cell is larger, the second one is so-called polar body. In the second division again one cell is larger and only one egg cell originates in the meiosis (and 3 polar bodies). The long halt period increases the risk of abnormally chromosome segregation for older mothers.