Genes and Inheritance [3]: Sex Chromosome (A Level Biology)


Uploaded by freeeschool on 02.09.2012

Transcript:
A Level Biology: Genes and Inheritance 3 – Sex Chromosome
Hi! Welcome to my third video in this series about Genes. In this video, we are going to
focus on the 23rd chromosome, which is the sex chromosome.
You probably remembered from the last video, you get one of each chromosome from each parent.
On the list of 23 that you have here, one of these will have come from your mother and
the other one will have come from your father and the same with all of the chromosomes.
The same is also true with the 23rd or the X and Y chromosome.
In this PowerPoint, this shows us the key difference between the X and Y chromosome.
What you will notice with the X is it is a lot longer. It appears to have a lot more
genetic information on the X chromosome than there is on the Y.
Like I said before, one of each chromosome comes from each of your parents and the same
is true with the 23rd chromosome or the sex chromosome. This is the code that determines
whether you are either a male or a female.
In this case, XX is a female and XY is the male. So therefore, if XX is the female, then
the only chromosome that a female can give across is the X and it is the male is the
one that actually determines the sex of the offspring. The male can either give across
X or Y. It is in fact the male that determines the sex of the offspring.
This square diagram shows us the different permutations that are possible. Down here,
this represents the female and along here, this represents the male. Obviously, these
are the sperm and these are the eggs. What you notice here is these are both X chromosomes
because they are the long ones and hence, sex of the offspring is female. This one,
however, you have one X chromosome and one Y chromosome, hence a male. Here as well,
you have two X’s hence female and on this one, an XY and therefore male.
In the general population, there is an even split of sperms carrying the X and carrying
the Y chromosome. In reality, I think there are more females on the planet than there
are males. Generally speaking, there is an even distribution of X-carrying sperm and
Y-carrying sperm. You notice that all egg cells must carry the X chromosome. It is on
the Y chromosome that there is a gene that basically determines that the embryo will
turn into a male. If you receive 2 X chromosomes, therefore, there is in this gene present and
therefore the embryo continues to develop as a female.
In summary, the 23rd chromosome is the sex chromosome. You get one chromosome from your
mom and one for your dad. In the case of your mother, she always donates the X chromosome.
You father can either donate either the X or the Y. In that respect, it is always the
male sperm that determines the sex of the baby. On the Y chromosome, there is a gene
that causes the embryo to develop into a male. If there is no Y chromosome present, this
will cause the embryo to develop into a female.
[end of audio – 03:37] A Level Biology: Genes and Inheritance 3 – Sex
Chromosome Page…1