[thedoc (OP)]

Wei  asked the Naked Scientists:
   
Hi Chris,

I have been puzzled by a question related to the fertilisation process for years.

We've all been told that in the fertilisation process, billions of sperms compete and only a few can reach the egg and usually only one sperm can penetrate the egg, and the reason for this is so that the most fitted sperm will win.

However, doesn't each sperm carry the EXACT same genetic information(DNA)?

In that case, why does the fitness of a sperm matters, when only the DNA information seems to decide the health of the baby?

Thanks!

Wei

What do you think?

[evan_au]

Actually, every sperm carries a different mix of genes.

A human male has 23 pairs of chromosomes (46 in total).
A sperm has 23 individual chromosomes, each drawn from one of these pairs. If this was the only mechanism involved, there would be about 8 million different sperm contents out of the typical 200 million sperm.

However, another mechanism called Chromosomal crossover ensures that the chromosomes in the sperm are not the same as either of the chromosomes carried by the father - it is actually a mix of the chromosomes from the father's mother & father.

The one exception is the X & Y chromosomes, which don't undergo this crossover process in males.

This means that every sperm carries a different set of DNA.
http://en.wikipedia.org/wiki/Chromosomal_crossover

[cheryl j]

The other reason for the large numbers is that the vagina is a surprisingly hostile environment (no jokes please) since there has to be a balance between allowing sperm to survive but discouraging the growth of micro-organisms. In addition there is the possibility of competition from someone else's sperm. I also read that although only one sperm gets inside the ova, many sperm are needed to initiate the chemical change in the outer layer that will allow one of them to enter. I hope I've stated that accurately.