Personally, I am particularly interested in genes that do something with us during embryonic development. And hardly any genes make anything more crucial than SRY during our embryonic development. The acronym stands for Sex determining Region on Y-chromosomes and in fact, the name says almost everything.

Humans have 23 pairs of chromosomes, 22 pairs of which have a relatively similar shape. The 23rd chromosome pair, however, is different: while women have two X-shaped chromosomes, men have an X- and a Y-shaped chromosome. When germ cells, i.e. egg or sperm cells are produced, the set of chromosomes is halved so that two halves will make another full set again when the germ cells fuse during fertilization. Therefore, a mother always passes on an X chromosome, whereas a father can pass on an X- or a Y-chromosome, leading to a girl or a boy developing, respectively. But how can that Y-chromosome induce the making of a boy?

Well, genetic testing has shown that it is really only about SRY: if this region is missing on the Y chromosome, a girl develops, even though an X and a Y are recognizable in the karyogram. Conversely, if the SRY gets attached to an X chromosome, a boy will develop, even though the chromosome pair represents XX.

So how can this one gene do that? SRY encodes a transcription factor, i.e. a protein that binds to DNA and can turn transcription of other genes on or off. Thus, the presence of SRY regulates several other genes. SRY is first transcribed in the seventh week of embryonic development. At this time, the gonadal glands (that is, the rudimentary precursors of the gonads) are still in a so-called pluripotent state, that is, they have the potential to form different types of organs. If SRY is absent and all of SRY's target genes are not activated or suppressed, ovaries develop from these precursors. When SRY exerts its effect on the genome, the gene products of the target genes cause testicles to form instead. We still don’t know the full complement of genes being up- or downregulated by SRY. Also, how these gene products contribute to the different development of the sex apparatus is the subject of intensive research and would go beyond the scope of this article.