I was going to post this as a comment in reply to the comment by darkwinter on my last post, however I feel I have enough material to make a whole new post from it.
Interestingly genetics advancements mean a parent can now choose the gender of a foetus. This is of particular interest to people like myself with X-Linked Conditions. To understand this you have to appreciate that males have an X chromosome and a Y chromosome whereas females are XX.
In my case I have a faulty gene in my X chromosome. This fault manifests itself as Retinitis Pigmentosa(RP). If I have a son I will be passing my Y chromosome which does not carry the faulty gene. My son will not contract this condition. However if I have a daughter I will be passing on my only X chromosome, which is faulty. This means my daughter will carry the defect. However as she has two X chromosomes and a certain degree of error checking occurs my daughter will not contract RP she will merely carry it.
If my daughter has a child there is a 50% chance she will pass the defective gene on. As she will only contribute one of her two X chromosomes to the creation of the foetus. If she has a son and passes on the faulty X chromosome he will contract the condition.
As my ability to convey this information may be lacking I have drawn a terrible little diagram with the following key -
Underlined X chromosomes carry the gene for RP.
1. The faulty chromosome has to be carried for the creation of a daughter. As a female requires an X from each parent. The daughter is a carrier
2. A son obtains his X chromosome from his mother. The fathers Y chromosome is unaffected. Assuming the mother is not a carrier of the condition the son will be unaffected.
3. There is a 50% chance a mother will pass the condition to the daughter as the mother only contributes one of the two X chromosomes to her daughter. In this case the chromosome is carried.
4. Again there is a 50% chance the son will inherit the faulty X chromosome. In this case he inherits the chromosome without the fault and is thus unaffected.
5 and 6. As the parent did not inherit the condition the faulty chromosome will not be passed down this line.
People often consider me weird for my fascination with my condition. However it is incredible. Despite the 50% odds every time a female with the gene procreates this condition is still passed on potentially thousands of years after the first mutation occurred. The fact we can now not only understand how this inheritance occurs but exactly which gene is effected is something I find terribly exciting. In my case the gene fault is the deletion of a single base pair. Two tiny nucleotides were removed from this gene generations ago. When you consider how many nucleotides are in the human genome its hard not to find this breathtakingly impressive.