No, that is not true. The sex of humans is determined from fertilization, that is, as soon as it becomes a zygote: if it is XX, it will be a female, while, if it is XY, a male. However, at the beginning, human embryos have the precursors of both female and male gonads.
Taking this into account, if there is no Y chromosome, the embryo will proceed with the development of Müllerian ducts (which give rise to the ovaries). Also, when there is no Y chromosome, the embryo lacks the SRY region: the sex determining region.
To sum up: it is true that one could say that, in mammals, the initial plan for all embryos is female, which is only altered if the SRY region is present.
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