IB Biology SL

Revision Notes

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

Author: Ruth

Ruth graduated from Sheffield University with a degree in Biology and went on to teach Science in London whilst also completing an MA in innovation in Education. With 10 years of teaching experience across the 3 key science disciplines, Ruth decided to set up a tutoring business to support students in her local area. Ruth has worked with several exam boards and loves to use her experience to produce educational materials which make the mark schemes accessible to all students.

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