Show Me The Science Month Day 23 Installment 23

Thanks to your parents, you have two copies of each chromosome, which means that you have maternal and paternal copies of every gene. In most cases, having two copies of a gene is no problem, but in some cases, two is too much, and your cells have to shut one copy down. How does a cell do it?
Shutting down one copy of a gene (or an entire section of a chromosome) is called genomic imprinting. (This is not the same thing as the newly hatched duckling that latches on to the first thing it sees, obviously). Genomic imprinting is a critical process used by placental and marsupial mammals to control the dosage of many genes, but how did this process evolve?
The answer, in part, has been discovered by an analysis of the platypus genome. Genomic imprinting appears to have
evolved from a defense mechanism used by cells to knock down parasitic DNA.