Are All Gametes Unique
Name: Tony C.
Date: February 2003
For any given human female, is each and every egg cell
unique (carrying different genetic information)? Likewise for any
given human male, are each of his sperm cells unique?
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
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
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.
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.
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Update: June 2012