When [ Select ] occurs, the egg and the sperm unite to form a [ Select ] . This single cell normally contains [ Select ] unpaired chromosomes from the egg and [ Select ] unpaired chromosomes from the sperm. These chromosomes combine to form one set of 23 [ Select ] chromosomes. Each pair combines one chromosome from [ Select ] . Emily and Robin are twins. They were originally a single zygote that split in two. Therefore, Emily and Robin are [ Select ] , or [ Select ] twins. Emily and Robin have [ Select ] of their genes in common. David and Allison are also twins, but they developed from separate eggs and sperm. Therefore, David and Allison are [ Select ] , or [ Select ] , twins. The genetic similarity between David and Allison the same as the genetic similarity between [ Select ] .