Sex, heredity, etc.

It seems that each of us is the improbable result of a long series of chance events. The last one being when an egg is fertilized by a spermatozoon with either an x chromosome or a y chromosome.

Now then, several questions arise. First, are all of a woman’s eggs identical? If not, then why not. Also, do all of a man’s spermatozoa consist of two and only two types? If not, then why not.

I’m particularly interested in the question of the spermatozoa. If there are only two possibilities, then it seems that it is inevitable that any male or female born will be the same person regardless of which spermatozoon fertilizes the egg as long as it has either an x or y chromazome. But then fraternal twins, as long as they are the same sex, would be identical. They’re not. And all brothers should be identical also and they’re surely not. If they’re all somehow different, then there is the possibility any one of a couple of hundred million different people being conceived.

Maybe each conception of identical eggs and spermatazoa do not result in identical zygotes. I suspect this is true, but why is it so?

Standard disclaimer – I am not a geneticist.

The ovum and the sperm each contain thousands of genes, each of which can merge in different ways. It is not simply a case of the XY chromosome drags in one set of traits and the XX chromosome drags in another.

So yes, you’re right. The union of sperm and ovum carries the possibility of thousands of different gene combinations developing. In other words, the red-haired gene may come over in one set of mergers, while the blond gene comes over in another.


All of a womans eggs and a mans sperm are not identical. In fact there is almost an infinite number of different genetic combinations posible. The reason for this is that during the formation of eggs and sperm the cells undergo a phenomenon called homologus recombination. During this process two homologus chromosomes (one originally from the male parent, the other from the female parent of the individual producing the sperm/egg) align themselves and swap genetic information. The swap is mostly random resulting in new arrangements of the original genetic information. Since no two cells undergo the exact same recombination the resulting sperm/eggs are all different.

I am probably not doing the topic any justice. If you are still confused then a general biology text should have a section on homologus recombination.


galen, you missed out on a couple of genetics classes there didn’t ya, big fella? that’s ok, bryan your friendly neighborhood geneticist is here to remedy the situation.

Each of your cells has a copy of your approximately three billion base-pairs of DNA. All this genetic information is split up into 46 chromosomes. 44 of those chromosomes are actually 22 pairs carrying copies of the same genes. One of each pair of chromosomes came from your mother, and one from your father. So, if you have type AB blood, for instance, that means that one parent gave you a chromosome with a gene that makes the A blood protein, and the other parent gave you a chromosome with the B version of the gene on it. (Confusing, I know.)

So every gene you have (with the exception of those on the sex chromosomes, which I’ll get to shortly), you actually have two copies of: one from your mother and one from your father. If you are female, you have an aditional pair of chromosomes; you have two X’s. If you are male, instead of a 23rd pair, you have two chromosomes that are different from each other; an X and a Y. (Incidentally, these are just arbitrary names. They have nothing to do with chromosome shape or anything like that. We could just as easilly call 'em Q’Bert and Zoey.)

Anywho, when sperm and egg cells are made, the chromosome pairs line up next to each other before the cell divides, and they do so randomly. One member of each pair (or, if it’s the sex chromosomes, the XX or XY pair) goes into each daughter cells. So, the total number of different sperm or egg cells an individual can produce is 2^23, or 8,388,608. Square this number to get the possible number of different offspring produced by two people: 7.04x10^13, or about 70 trillion different possibilities.

The number is actually much higher than that due to an event called “crossing over” where chromosome pairs switch and swap pieces while they’re lined up next to each other.

Hope that helps :slight_smile: