How can parentage test be accurately done even though crossing over occurs? As crossing over occurs when gametes are secreted, so when a zygote is formed after fertilization, the base pairs of DNA in the child should be different from that of the parents. So when gel electrophoresis occurs, the position of DNA fragments of the child shouldn't be the same as half of their father and mother. But why is this statement wrong? sorry for bad English.

Crossing over is an event that occurs during meiosis, which produces the gametes. This crossing over occurs between 2 homologous chromosomes, which are both from the parent. Hence, even though crossing over occurs, the recombinant chromosome that is passed down to the child will contain an allele that matches with the allele on one of the parent's chromosomes. Within the allele, the base pair sequence would not change.