What Is Turner Syndrome?

Down Syndrome and Turner Syndome?

• It is known that about 80% of Down Syndrome are due maternal meiotic nondisjunction, while 80% of Turner Syndrome is due paternal meiotic nondisjunction. What are the possible causes? Why is it that the "X" contributed by one specific parent gives rise to a specific Syndrome and not the other? Also, Down Syndrome is Trisomy 21, which has nothing to do with the sex chromosomes. Any help on this question is appreciated!

• Answer:

  I've never heard that 80% figure for either DS or TS, so I don't know what your source is, but assuming it's accurate: TS - perhaps this has something to do with the fact that the male produces so many more sperm than the female does eggs, so the odds of any one egg missing its X chromosome are less than the odds that in any given semen sample, there will be one or more sperm that are missing a sex chromosome? BTW, this maternal/paternal dichotomy ignores the fact that some women w/ TS received an X chromosome from both parents but some of their cells lost one of them in a nondisjunction accident during one of the early cell divisions after fertilization. DS - Perhaps this is related to the fact that DS is correlated with maternal age...the older the woman, the poorer the quality of her eggs, perhaps including chromosomal abnormalities like an extra 21st chromosome? Whereas there doesn't seem to be any similar age-related decline in quality of sperm to my knowledge. Just guessing.