• 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