Science 1.3
Describe aspects of biology
Sex Determination
eBook
Science 1.3 Sex Determination
Sex Determination
In this audio we’re going to talk about sex determination; what determines whether offspring is
male or female. If you haven’t yet listened to the sexual reproduction audio we suggest you do so
before you listen to this one.
Ok how many pairs of chromosomes does the human genome have? 23 right? And these pairs are
all the same right? Answer – no! The 23rd pair is different for males and females. The 23rd
chromosome pair in males is made up of an X chromosome and a Y chromosome, but in females
the 23rd pair is made up of two X chromosomes. Now, if you remember, meiosis produces gametes
which have half the normal number of chromosomes, meaning gametes only inherit one
chromosome from each pair of homologous chromosomes. This means that of the gametes
produced by human males, that is sperm cells, half of them contain an X chromosome, and the
other half contain a Y chromosome. And in female gametes, or egg cells, they all contain one X
chromosome. So because all egg cells can only ever have an X chromosome, whether the offspring
is male or female depends on the sperm that fertilises the egg; If a sperm that contains a Y
chromosome fertilises the egg, then the offspring will be male, but if a sperm that contains an X
chromosome fertilises the egg, then the offspring will be female.
The easiest way to understand how this works is to look at a punnett square. If you can, take a look
at the punnett in the corresponding eBook. In the pink boxes are the possible female gametes, X
and X, as egg cells can only ever have an X chromosome. In the blue boxes are the possible male
gametes, X and Y. After figuring out the possible offspring, you should have 2 possible zygotes with
the genotype XX, and 2 with the genotype XY. So what’s the genotypic ratio? 50% males : 50%
females, or 1:1. In theory this means that in the world there should be a 1:1 ratio of men and
woman, and if you have four children two of them should be boys and two of them should be girls.
As we know this doesn’t always happen. This is purely because sex determination is totally
random, as the sex of offspring ultimately comes down to what sperm happens to fertilise the egg.
Whether a baby is a boy or a girl is really just a coin toss!
X
How we get boys and girls
Boy
Y
Y
Girl
X
X
X
X XX XY
X
© Sound Classroom 2009
XX XY
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