IB Biology Sub-topic D3.1 Notes

You previously learned that every human at birth is either biologically male (XY) or female (XX), characterised by distinct anatomy known as the primary sexual characteristics. The combination of genetic and chemical changes that influence this outcome are collectively termed sex determination. Beyond this, when puberty is reached, there are further anatomical changes known as secondary sexual characteristics.

In males:

1. The SRY gene on the Y chromosome codes for DNA binding protein TDF.
2. TDF stimulates the embryonic glands to develop into testes.
3. The testes secrete the hormone testosterone in weeks 8-15 of pregnancy, stimulating the development of male primary sexual characteristics: the presence of a male reproductive system.
4. During childhood, gonadotropin-releasing hormone (GnRH) is increasingly released by the hypothalamus.
5. This causes an increase in LH and FSH levels throughout childhood.
6. Eventually, this initiates puberty by increasing testosterone levels rise and cause the development of male secondary sexual characteristics, including:
   1. Sperm production
   2. Penis enlargement
   3. Growth of pubic hair
   4. Voice deepening

In females:

1. The lack of the SRY gene due to the absence of a Y chromosome does not stimulate TDF.
2. This causes the embryonic glands to develop into the ovaries and placenta.
3. The ovarie secrete the hormones estrogen and progesterone in weeks 8-15 of pregnancy, stimulating the development of female primary sexual characteristics: the presence of a female reproductive system.
4. During childhood, gonadotropin-releasing hormone (GnRH) is increasingly released by the hypothalamus.
5. This causes an increase in LH and FSH levels throughout childhood.
6. Eventually, this initiates puberty by increasing estrogen and progesterone levels and cause the development of female secondary sexual characteristics, including:
   1. Menstruation
   2. Breast enlargement
   3. Growth of pubic hair