1.
When a baby is born, he or she develops rapidly in their childhood, growing in size, learning to walk, to speak, moving their hands, legs, etc.

2.
This fantastic development involves many organ systems such as the skeleton, the cardiovascular system, the brain, the nerves, etc etc.

3.
But, there is one organ system that does not take part at all in this beautiful growth during childhood; the genital system!

4.
In contrast to all other systems, the testes and the ovaries, the penis and the vagina remain relatively quiet and underdeveloped.

5.
We don’t really know why this is the case but the fact is that it is only after a number of years that sexual development, called puberty, really starts.

6.
In females this ‘transition’ (puberty) occurs somewhere at the age of 11 to 14 years, in boys often 1-2 years later.

4.
Also the development of secondary sexual traits such as hair development in the pubic zone and in the axilla (=armpits).

5.
The pituitary hormones also influence the size and distribution of (skeletal) muscles and of fat. This makes for broader hips, thighs and the development of her breasts.

4.
Testosterone will also induce hair growth in many locations along the male body, but especially around the pubic area, along the chest, the arms, the armpits, the chin (a beard!) etc.

5.
And, of course, it will also affect the vocal chords, making these thicker which leads to a lowering (‘deepening’) of the male voice (‘breaking the voice’).

The next part is a repetition of a panel from Chapter A, A.2.5. Mitosis and Meiosis.

In that chapter, we discussed how cell divide (mitosis) and how our gametes develop (meiosis). This process actually starts at puberty, which is why I think it is useful to repeat this part in this section.

1.
There is one type of mitosis which is ‘slightly’ different from mitosis but crucial for our survival! This is the cellular division of our ‘sex’ cells; our gametes. These are the sperm cells (in males) and the oocytes (in females).

2.
As you may know/remember, life starts when a sperm cell and an oocyte ‘merge’ together to form an embryo. If their chromosomes where put together into the new child, his (or her!) cells would contain 2 x 2 x 24 chromosomes! That is impossible.

3.
Therefore, the number of chromosomes has to be reduced before fertilization occurs. This is (one of) the purpose of meiosis.

4.
Meiosis actually consists of two parts, or two steps, called “meiosis I” and “meiosis II”. Both meiosis contain the same phases as during mitosis: prophase, metaphase, anaphase and telophase.

5.
Meiosis I is very similar to a normal mitosis whereby the number of chromosomes are duplicated. Then, when the cells divide into two daughter cells, the number of chromosomes is still 2 x 24.

6.
The purpose of meiosis I is to exchange the genes (=DNA) across their chromosomes during the actual duplication. This is called “crossing-over” or “recombination”. In this way, the daughter cells are actually genetically different from each other!

7.
After Meiosis I, obviously, Meiosis II will start. Now, in contrast to all other divisions, the chromosomes are NOT duplicated.

8.
So, when the daughter cells are created, each cell, in its nucleus, contains only one set of chromosome: 1 x 24!!

9.
This is SO important that we have names for cells that have two or one sets of chromosomes; “diploid” and “haploid”.

10.
Diploid means two sets of chromosomes (‘di’= two) while Haploid means one set. Therefore, sperm cells and oocytes are haploid, while all other cells in the body are diploid.