Chromosomes and Sex: Grade 9 Understanding for IGCSE Biology3.26 3.27

Having spent the last day or two writing material about one of the hardest topics in the IGCSE Biology specification (DNA and Protein Synthesis), I am going to write today about something much simpler.  You need to understand how the sex of a human is determined at the moment of fertilisation.  But this is a topic which can confuse students so I am going to try to explain it for you as best I can.

The sex of a human (whether male or female) is determined by the 23rd pair of chromosomes.  Please remember that just because humans determine their sex this way, this doesn’t mean that other species have to be the same.  In fact other species use a variety of ways to ensure the correct proportion of male and offspring are born.


As you can see from the picture above, the 23rd pair of chromosomes in humans are called the sex chromosomes.  The person whose chromosomes are shown above is male because he has one X and one Y chromosome in his 23rd pair.  If we looked at a picture of a human female set of chromosomes, pairs 1 to 22 would be exactly as above, but the 23rd pair would be different.  There would be two large X chromosomes rather than one large X and one tiny Y chromosome as shown above.

So a human female has XX as her 23rd pair of chromosomes, a human male has XY as his 23rd pair.

Gametes (Sperm and Egg cells) are made in a process called Meiosis.  Remember that meiosis produces daughter cells that are haploid (this means they only have one member of each pair of chromosomes and so half the genetic material)

When a female cell undergoes meiosis in her ovary, the daughter cells produced (egg cells) will contain one of each of the 23 pairs of chromosomes.  For the 23rd pair this will always be an X chromosome since both chromosomes in the 23rd pair are X chromosomes.

When a male cell undergoes meiosis in the testis, the daughter cells produced (sperm cells) will contain one of each of the 22 pairs of chromosomes exactly as above.  But the 23 pair are different to each other and so half the sperm cells will contain an X chromosome as the 23rd chromosome and half the sperm cells will contain a Y chromosome as the 23rd chromosome.


If you understand the picture above, you understand sex determination in humans.  You also need to be able to draw a genetic diagram to show this.

Phenotype:                       Mum                                Dad

23rd pair:                          XX                                    XY

Gametes:                            X                            ½X            ½Y


Screenshot 2019-07-17 at 11.26.06

Offspring 23rd pair of chromosomes:       ½ XX and ½ XY

Offspring phenotypes:                                  ½ female and ½ male

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