A family has a child affected with trisomy 18. A trait related to a gene on chromosome 18 has been studied in the family. The parents have normal phenotypes for this trait. It has been determined that the mother produces only normal protein for this trait, and the father produces equal amounts of its normal and abnormal forms. The child with trisomy 18 has twice as much abnormal as normal form of this protein. The gene for this protein is very close to heterochromatic DNA of the centromere, thus reducing its crossing-over frequency. Which parent contributed the extra chromosome to the child? Explain. In which division did the error most likely occur? Explain.