Gametes are containing only one set of dissimilar chromosomes, or half the genetic material necessary to form a complete hapliod. Gametes are formed in the process of meiosis.
When Spider man's sperm made contact with Supergirl's egg, the process of meiosis starts to create their offspring. Supergirl and Spider Man each have 2 chromosomes that meet in the egg once the sperm finds the egg. After that, they cross over craeting the law of independent assorment. In other words, the 2 chromosomes create their own punnett square and craete posibilities for the newly coming child. They call that random fertilization when there is a sum of 70 trillion different combinations of chromosomes in one potential offspring.
Supergirl's original DNA strand: CTCCCTGGATTATCGCCAATC and CCTGAATTATATGGGCATACGCACGAGGGATAT
Supergirl's DNA mutated: GAGGGACCUAAUAGCGGUUAG and GGACUUAAUAUACCCGUAUGCGUGCUCCCUAUA
Supergirl's DNA translated: glu, gly, pro, ile, ser, gly, stop and gly, leu, asn, ile, pro, val, cys, val, leu, pro, ile
Spider Man's original DNA strand: CCTCTGCTTTATGGGCAACACGGATGA and CCTCTGCTTTATGGGCAACACGGATGA
Spider Man's DNA mutated: GGAGACGAAAUACCCGUUGUGCCUACU and GGAGACGAAAUACCCGUUGUGCCUACU
Spider Man's DNA translated: gly, asp, glu, ile, pro, val, val, pro, thr and gly, asp, glu, ile, pro, val, val, pro, thr