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Name: Tony C.
Status: Other
Age: 40s
Location: N/A
Country: N/A
Date: February 2003


Question:
For any given human female, is each and every egg cell she produces unique (carrying different genetic information)? Likewise for any given human male, are each of his sperm cells unique?



Replies:
Yes, they are unique. There are several factors that contribute to this. In order of importance, they are:

1. Independent assortment during meiosis. During the formation of the gamete (a gamete is a sperm or egg cell, for those wondering), each cell gets a copy of each chromosome. So, since you start out with 23 pairs of chromosomes in a human cell (i.e., one of each chromosome from mom, and one from dad), the reproductive cell gets randomly dealt one copy of each chromosome. If this were all that happened, then yes, on rare occasions you could get two cells with the same set of chromosomes (because on rare occasions, you could deal two hands containing the identical set of 23 chromosomes. Not likely, but possible.)

2. Now add in recombination. This is the heart of why the answer to this question is that you will not get two identical gametes. During the formation of gametes, there is an important stage where the pairs of chromosomes line up with their partners. In other words, the copy of chromosome 1 from that person's father pairs up with the copy of chromosome 1 from that person's mother. And so on down the line with all the chromosomes. Then the paired chromosomes physically break along the DNA strand and exchange DNA with their partner. This happens randomly along the length of the paired chromosomes. So there is in effect a mixing of the DNA from each chromosome. Since the parts of DNA that recombine, or exchange are unique to each cell, each gamete is different from any other.

3. Finally, and less important, there is always the issue of DNA mutation rates. Any time DNA is copied, there is a very small but certain amount of copying errors that occur. Some of these mutations may be insignificant, some may be beneficial, or some detrimental. But when over a billion bases of DNA are being copied to make a reproductive cell, there ARE going to be a few errors; it is the engine of natural variation and hence, natural selection. This too contributes to (minor) differences between gametes.

Paul Mahoney


Probability says yes. During meiosis and due to independent assortment at metaphase I, the homologous chromosome pairs (one from Mom and one from dad for each type of chromosome) line up together. The first time the cell divides, one of each chromosome will go into each new cell, so they will be separated from each other. Each pair lines up independently of every other pair. Just because in the first pair mom's chromosome lines up on the left, and dad's on the right, has no influence on the next and subsequent pairs. So each cell is a random combination of Mom and dad's chromosomes. There are 2 chromosomes in each pair and 23 pairs of chromosomes, so there are 2 to the 23rd different ways these chromosomes can be arranged for each meiosis event.

That's about 8 million different ways. But that is not the whole picture. Because early in meiosis, when the pairs first get together before metaphase, they trade pieces of themselves, i.e., a piece breaks off of each and switches places. This is known as crossing over and it occurs at least once in every meiosis in every pair. And, lastly it is random as to which gamete is paired with the other gamete, i.e., one sperm makes it to the egg at random and one egg is ovulated each month at random. So although it is POSSIBLE to have two gametes be identical, it so highly unlikely that we say that each is unique. Of course, each individual only has the genetic material passed down from their parents to work with, and do not re-create it each time. So it is still possible to look at an offspring and say for instance-he or she has dad's eyes and grandma's nose. But none of the offspring will be genetic copies of either of the parents.

vanhoeck



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