IB DP Biology: SL复习笔记6.6.4 Reproduction: Sex Determination in Males

SRY Gene

  • 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