So the forum lately has been hijacked for theistic evolutionist bashing, so I'm posting a thread to get things back on track. A few years ago I found myself wondering--why does a molar pregnancy not involve an embryo? For those who don't know about this, a molar pregnancy (hydatidiform mole) occurs when an egg somehow ends up without its maternal DNA. When it is fertilized it contains only half of the DNA needed. This paternal DNA then copies itself, so the zygote formed contains the 46 chromosomes we need for development. Assuming the sex chromosome contributed by the father is X, the zygote should grow into a baby girl. However, then development goes drastically wrong. Instead of dividing and growing to form the embryo and the placenta needed to feed it, it divides and starts forming beads of placenta-like tissue. This tissue may become invasive and cause a type of cancer called a choriocarcinoma. The usual treatment once molar pregnancy is diagnosed is curettage to remove the abnormal tissue and sometimes even hysterectomy. This is a complete hydatidiform mole, there is also a partial hydatidiform mole caused by two sperm fertilizing one egg to produce a triploid (69n) zygote, but it makes immediate sense that that would not be viable. No source that I read talked about the reason for this at all, so I turned to journals. I ran across an article involving experiments with mice where they made zygotes containing either all paternal DNA or all maternal DNA. When all maternal DNA was used, the embryo began development normally, but the placenta did not form and the embryos eventually died. When all paternal DNA was used, the case was similar to that of a complete mole--placenta-like tissue proliferated, but the embryo did not form. It ends out that the reason for this difference is a process called genome imprinting. Imprinting in mammals involves patterns of DNA methylation. Not much is known about imprinting, and it serves different roles at different times of development. When gametes are formed their DNA is methylated in a different fashion depending on whether they are eggs or sperm. When they combine, the methylation pattern of each DNA duplex determines whether its genes are used in the growth of the placenta or the embryo. During embryo growth, the genome imprinting of the maternal and paternal DNA is erased and then the embryo re-methylates the DNA in a manner that permits proper transcription control for that point in development. Here is a page that talks more about genome imprinting and its significance.