Complex sentence: Only during meiosis, in which gametes are formed, do these chromosomes cross over, exchanging bits of DNA to form unique genes not found in either parent, though this objection would also apply to the genome of an adult.
Split into simpler sentences: Only during meiosis, in which gametes are formed, do these chromosomes cross over, exchanging bits of DNA to form unique genes not found in either parent. Although this would also apply to the genome of an adult.