What makes a male of female?

Denerick · 19-07-2010 9:48am #1

I assume that during sex both sexes are essentially equal and the chances of getting a male or female child are equal. So what determines what the child is going to be?

Improbable · 19-07-2010 11:26am

Normal human cells have 23 pairs of chromosomes, 1 of those pairs being the sex chromosomes. Women have 2 X chromosomes (XX). Men have an X chromosome and a Y chromosome (XY). The sex cells (eggs and sperm) are haploid which means that for eggs, they have a single X chromosome and the sperm has either an X or a Y chromosome. What determines the sex of the baby is whether the sperm which fertilizes the egg has an X or a Y chromsome. If it's an X, you get XX (an X from the egg and an X from the sperm) making the child female. If it's a Y, you get XY (an X from the egg and a Y from the sperm) making the child male.

seamus · 19-07-2010 11:41am

What improbable says there is the biological reality of it. XY = male, XX = female.

However, gestation is a complex process and there are a number of things that can go wrong in the process that cause someone who is genetically male to develop the appearance of a female, and vice-versa.

Society has also fairly recently accepted that even if everything forms biologicially as it should, that some males can "feel" mentally female (and vice-versa) and consider themselves to be female, despite their XY genetics.

These are more sociological grey areas than biological ones though.

It should be noted that errors in gestation of this nature are very uncommon and the vast majority of the time, someone who is genetically XY will look and act male, and someone who is genetically XX will look and act female.

Denerick · 19-07-2010 12:12pm

I only really ask because humanity requires a relative equal number of males and females to reproduce. How come roughly half and half are male and female? Is just biological chance?

Improbable · 19-07-2010 12:19pm

Denerick wrote: »

I only really ask because humanity requires a relative equal number of males and females to reproduce. How come roughly half and half are male and female? Is just biological chance?

I don't really know that humanity does require an equal number of females and males. I can't see why it would matter as long as you had enough of a population to prevent inbreeding why there couldn't be a large disparity between the sexes.

As far as I know though, it is chance. I'll just refer to the male sperm as that is the sex determining gamete. When sperm are being formed, the "base cell" that they are formed from have both an X and a Y. The mature sperm cell has either an X or a Y and then its all down to which is the better swimmer :P

Rubecula · 19-07-2010 2:53pm

Just to confuse things a bit further, there is such a thing as an XYY man.

I remember a drama series a long time ago where the lead character had this make up.

It was said that he is a more violent and more 'Alpha' type of male.

I have no idea if any of this is true however and I assume someone will correct me if I am wrong.

fontanalis · 19-07-2010 3:18pm

Rubecula wrote: »

Just to confuse things a bit further, there is such a thing as an XYY man.

I remember a drama series a long time ago where the lead character had this make up.

It was said that he is a more violent and more 'Alpha' type of male.

I have no idea if any of this is true however and I assume someone will correct me if I am wrong.

That was Criminal Minds wasn't it? And George Costanza played the baddie.
I also remember reading baout a case of semi identical twins, where I think two sperm cells fertilised the egg but the egg split and one of the twins was a hermaphrodite.
http://news.bbc.co.uk/2/hi/health/6498215.stm

Improbable · 19-07-2010 3:22pm

individuals with XXY, also called Klinefelter Syndrome, can live with no symptoms at all. I know they do show symptoms such as smaller testicles. This can result in testosterone imbalances so I don't know how much of the violent alpha male thing is true.

seamus · 19-07-2010 4:03pm

Wikipedia.

There are all sorts of chromosomal disorders. Typically we should have 23 pairs, but sometimes the numbers are off, known as Aneuploidy.

You can get XXY people & XYY people, who seem to have few if any differences to normal XY people. Problems start appearing when there are more than 3 or only 1 chromosome in the "pair".

The general rule is that if there is a Y chromosome present, the person is male. XXY males are no less "masculine" than XY males and likewise XYY males are not prone to aggressiveness or to being more "alpha" than other males. Awesomely though, having three chromosomes at the XY position tends towards increased height - about 7cm (3 inches) above the population mean.

GLaDOS · 19-07-2010 4:13pm

Always a subject I was interested in. Usually variations in chromosome number in sex chromosomes is relatively harmless compared to what happens in autosomes.

Here's a quick table giving some common examples and symptoms of abnormalities in sex chromosomes, obtained from here.

Capt'n Midnight · 21-07-2010 1:23am

Denerick wrote: »

I only really ask because humanity requires a relative equal number of males and females to reproduce. How come roughly half and half are male and female? Is just biological chance?

battle of the sexes really. Even in species with one male per herd having breeding rights the sexes are roughly equal in number even though you'd think that females have a better chance of breeding than males. But when a male does get to breed he has more offspring so it balances up.

in some species of fish (amphibians / lizards ?) females can become males , if there aren't enough males around

Catari Jaguar · 21-07-2010 1:30am

I have read that even though there are more women than men in the world, there are more male babies born because (evolutionary wise) they are more likely to die hunting or at war when they get older.

Piste · 21-07-2010 1:27pm

Denerick wrote: »

I only really ask because humanity requires a relative equal number of males and females to reproduce. How come roughly half and half are male and female? Is just biological chance?

Gametes (sex cells) are derived from somatic cells (body cells). In a diploid male (two copies of each chromosome, no copy number variation), his body cells will have on X and one Y chromosome. When these body cells divide to become sex cells the X chromosome goes into one cell and the Y chromosome goes into the other, yielding two cells, one of which can fertilise an egg to produce a female, the other of which can fertilise an egg to produce a male, which is why there are roughly 50% of each sex in the human population.

seamus · 21-07-2010 1:31pm

cherub rock wrote: »

I have read that even though there are more women than men in the world, there are more male babies born because (evolutionary wise) they are more likely to die hunting or at war when they get older.

That's the conventional thinking. It's no so much that male babies are more likely to be conceived, it's that the miscarriage/natural abortion rate for female offspring is higher than for male.
This has in the past worked in the species's favour because of the factors you mention about the number of males who die post-birth.

fontanalis · 21-07-2010 2:46pm

Has anyone heard of the idea put forward in this book?
http://www.amazon.com/Adams-Curse-Future-without-Men/dp/0393326802/ref=sr_1_3?s=books&ie=UTF8&qid=1279719899&sr=1-3

Tree · 21-07-2010 6:59pm

Which idea? Please quote the relevant part. Thanks

fontanalis · 21-07-2010 7:35pm

Sorry the link was meaningless on its own, below in bold is the part I meant. I think he also refers to the "puniness" of the y chromosome. it does seem sensationalist to say the least.

Sykes concludes by noting that, as evidenced by declining sperm counts and high percentages of abnormal sperm, among other variables, the Y chromosome is a genetic mess and is deteriorating so quickly that men could become extinct.

GLaDOS · 21-07-2010 8:27pm

fontanalis wrote: »

Sorry the link was meaningless on its own, below in bold is the part I meant. I think he also refers to the "puniness" of the y chromosome. it does seem sensationalist to say the least.

Sykes concludes by noting that, as evidenced by declining sperm counts and high percentages of abnormal sperm, among other variables, the Y chromosome is a genetic mess and is deteriorating so quickly that men could become extinct.

Doesn't make sense to me, sperm is essential to life whether it's contributing an X or Y chromosome.

Kurtosis · 21-07-2010 10:24pm

fontanalis wrote: »

Sorry the link was meaningless on its own, below in bold is the part I meant. I think he also refers to the "puniness" of the y chromosome. it does seem sensationalist to say the least.

Sykes concludes by noting that, as evidenced by declining sperm counts and high percentages of abnormal sperm, among other variables, the Y chromosome is a genetic mess and is deteriorating so quickly that men could become extinct.

I've read a bit of a book that seems to cover a similar topic by Steve Jones, it seems good so far anyway!

Improbable · 22-07-2010 1:03am

Evolutionarily speaking, if a certain population of males were to start experiencing declining sperm counts and abnormal sperm, natural selection would favor those males who do not exhibit that, or who exhibit it to a lesser degree. I don't really like the word favor but I couldn't think of a better word.

There's an interesting paper that you can read if you have a university with access to it or if you feel like purchasing it:

http://www.springerlink.com/content/ek33086788453663/

Alternatively, there is another for free here:

http://www.cell.com/retrieve/pii/S0092867406002418

What makes a male of female?

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