In high school biology class we learned this. These are the combinations you can have if sex chromosomes and live: XX(female), XY(male), X (also XO, female with Turner Syndrome), XXX or XXXX or XXXXX (female with Triple-X Syndrome), XXY or XXXY(male with Klinefelter Syndrome), XYY (male with XYY syndrome). The combination where the baby will die soon after conception is just a single Y chromosome and no X. This is because the X chromosome contains some key instructions on creating you properly and keeping you alive. These conditions all come from when a gamete or sperm/egg was formed when a cell decided itself unevenly. Leaving more chromosomes in one of the resulting gametes. And less in the other.
Hi I am a biologist at Duke university and for anyone who said that having 2 holes is a evolutionary change they are completely wrong. What happens was when you were in the uterus and you were still developing some kind of defect happened and the first hole or non-working hole pretty much just died. There for it had to develop another one which is now the functional one, but that is very uncommon to see because usually the dead one gets closer before the child reaches grade school. Now on some special occasions one hole stops doing only one function incorrect so then it has to produce another hole and sometimes your body pretty much gets lazy and makes that one only do one function.
That is the furthest detail I can go in to without losing most of you guys