pivoxa15 said:
Still, eggs produced within a period of time can be different to eggs produced at a later time. Same as sperms. So non identical twins can still be more similar than to siblings born years apart? Say 10 years would be a noticable difference.
This raises the question: Is it true that the sperm and egg 'quality' decline as one ages? Although someone define quality as I don't know how to. If so when is the peak times for male's sperm and female's egg?
You're exactly right about egg and sperm quality being related to age of the parents. My guess is that the ideal time
physically is probably between age 17 and 34. There's the mother's ability to carry the baby safely, of course, and most damage to genes occur as a mother approaches mid 40s. Father's sperm health also declines, but more gradually--that's why we still sometimes hear about 80-year-old fathers. I did recently read about certain types of defects actually being more common with very young mothers, but now I can't remember the details.
Of course, the differences in children that are related to age of parents isn't really a matter of benign forms of similarity or differences like hair color, build, and so on. Those difference are caused strictly by which of the mother's genes happened to end up in the egg that was used and which of the father's genes happened to end up in the sperm that was used. Those things are not related to age of the parent. The age of the parents affects only the probability of birth defects and gestational health of mother and baby.
For eggs, they are all created when the the mother is in HER mother's womb. For sperm, they are created within months of being used (older ones just die). The mother's eggs degrade over time and the father's specialty cells that produce sperm also degrade over time. Also various toxins collect in everyone's body over the years (like xenoestrogens) which damage the reproductive efficiency and reliability, cause birth defects and bad fetal health, and also cause many other common life-threatening problems in the adults, or in a baby when it grows up. So the only difference in later-born children really has strictly to do with the probability of birth defects rather than how similar they look to siblings near their age.
On a side note, this gives me a chance to propound that the xenoestrogen problem is really bad and people need to take notice. The ill effects of xenoestrogens don't show up for years, sometimes decades--so we are only now noticing it. Xenoestrogens don't biodegrade much at all, and they collect in fatty tissues (so they build up in the food chain). Studies over the last couple decades are now proving that they are the cause of a myriad of life-threatening problems. They are the cause of the 50% drop in men's sperm counts of the last few decades, damage to sexual development in children, and they are the cause of most breast, prostate, and ovarian cancers that occur! Xenoestrogens come from herbicides, insecticides, cosmetics, plastics food containers, certain common solvents and emulsifiers, synthetic clothing (through the skin and when the lint is inhaled) and other places.
That would be much more rare? How rarer compared to non identical twins?
Looks like the probability of having identical twins is about 1 in 280, fraternal twins 1 in 125.
-- either that or they like the idea of maternity leave too much after the first has arrived 