Location: Outside U.S.
Date: Spring 2011
Does each individual sperm have different DNA so that its
"luck" as to which sperm reaches an egg and can create a unique
person. Or are all the sperm from one man the same so there is no
chance of variables, at least as far as the sperm is concerned.
One of the great adaptations for survival is the genetic diversity which occurs in sexual reproduction. Indeed, both in the production of sperm, the male sexual cell, and egg, the female sexual cell, there are a series of cell divisions coupled with chances for "shuffling" of genetic material. While mathematically there is probably some very very very very small chance of 2 sperm having identical genetics, the more likely situation is that each of the sperm has genetic variation, which, when coupled with the particular egg's genetic variation, will produce a unique individual.
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If you have learned about the process of meiosis, you know that sperm give half the chromosomes to the new offspring. In the process of meiosis, there are 3 different processes that create genetic diversity in the new sperm. One is crossing over in prophase, where parts of adjacent chromosomes switch places, so each now has a bit of the other. The other is when they line up in the middle of the cell to be split apart (metaphase), there is no set order that they line up, so each half gets a unique combination of traits. The same thing is happening as eggs are being made. The third way is that it is completely random which sperm gets to the egg first, and which egg matures and gets ovulated is also random. Humans have 46 chromosomes and when they reduce their chromosome number in half, that means that sperm and egg have 23. So the number of different combinations created in metaphase alone is 2 to the 23rd power, or 8 million. (That doesn't count crossing over.) However, the man can only give the traits that he has in his genes. Once paired with mom's chromosomes, some of her genetic information can dominate over his and vice versa. There are certainly traits that run in families, but some may have been hidden (recessive) for generations. So to answer your question, there is a lot of luck involved.
Pretty much all people have 23 chromosome pairs, each chromosome pair consisting of one half from their father (sperm) and one from their mother (egg).
Once that person grows into an adult and produces sperm (or eggs if female), they can contribute one half of each chromosome pair to each egg or sperm.
To illustrate, let's simplify and show the case if there were only 3 chromosome pairs, labelling them A through C, and -1 or -2 depending on if it came from their parent's mother or father.
That person will contribute an A, B, and C to each sperm (or egg if female), but whether each of those chromosomes is their -1 or -2 is random. So the possibilities are:
A1 B1 C1
A1 B1 C2
A1 B2 C1
A1 B2 C2
A2 B1 C1
A2 B1 C2
A2 B2 C1
A2 B2 C2
So you can see with only 3 chromosomes there are already (2 possibilities for each of 3 chromosomes, =2x2x2) many (8) unique possibilities for each sperm or egg.
For 23 chromosomes, the number is 2^23= 8388608, so basically you could make over 8 million sperm without having any 2 alike. Sperm counts are often around 50 million per milliliter, so there are likely identical copies for many of them, but your odds of finding those matching pairs are pretty small.
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Update: June 2012