• Question: Why are people either a boy or a girl? I've heard about XX and XY genes, but don't understand the rest. Thanks! :)

    Asked by AnimalLover to Stéphane on 13 Mar 2018.
    • Photo: Stéphane Berneau

      Stéphane Berneau answered on 13 Mar 2018:

      At 6 week of gestation, the gonad is not sex differentiated yet.
      There is a gene called SRY (Sex-determining Region Y) present on the Y chromosome. This gene will lead to the male sexualisation of the gonads into testes and the production of testosterone during embryo development. Testosterone will leading to the masculinization of organs (reproductive, but also brain), the acquisition of male phenotype and the male gender at birth.

      In absence of SRY, the hormonal balance will differentiate the gonads into ovaries. It is considered as a default function.

      In absence of SRY on the Y chromosome (mutation or deletion), this will lead to the creation of a baby girl but she is most likely to be sterile with poorly developed ovaries and uterus.

      I hope I have answered your question. Comment if you want

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