6.6.4 Reproduction: Sex Determination in Males

• In sexual reproduction in humans, a sperm from a male fuses with, or fertilises, an egg from a female to form a zygote, which then develops into an embryo
• To begin with the embryo develops in the same way regardless of its sex, and embryonic gonads develop that will either become ovaries in females or testes in males
• The factor that determines whether the embryonic gonads will develop into ovaries or testes is the presence or absence of a single gene known as the SRY gene
  • The SRY gene is located on the Y chromosome, meaning that is only present in roughly 50% of embryos
  • The SRY gene codes for a DNA-binding protein known as TDF, or testis determining factor, which stimulates the expression of further genes responsible for the development of testes
• If the SRY gene is present in the embryo's DNA, the embryonic gonads will develop into testes
• If the embryo has two X chromosomes, and therefore the SRY gene is not present in its DNA, the embryonic gonads will develop into ovaries

• During embryonic development, at the time when the embryo is developing into a foetus, the testes develop testosterone-secreting cells which produce and secrete testosterone
• This testosterone causes pre-natal development of male genitalia
  • This testosterone secretion declines in the latter stages of pregnancy so that, at birth, the testes are inactive
• During puberty in males, testosterone secretions increase once again
• This leads to:
  • The stimulation of sperm production in the testes; a primary sexual characteristic of males
  • The development of male secondary sexual characteristics e.g.
    • The penis gets larger
    • Growth of facial hair
    • Deepening of the voice

Secondary sexual characteristics

• Primary sexual characteristics are the features of reproductive organs that differ between males and females
  • They are present during development in the uterus
• Secondary sexual characteristics are the changes that occur during puberty as children grow into adults
• They are controlled by the release of hormones
  • Oestrogen and progesterone in females
  • Testosterone in males

• Some changes occur in both males and females, including:
  • The further development of sexual organs
  • The growth of body hair

• Emotional changes also occur at this time due to the increased levels of hormones in the body

Secondary sexual characteristics of a human male