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doing both of these things. To discriminate among the possibilities, it is necessary to know more about the next level of genes in the developmental cascade, SRY's targets. Several possible direct targets of SRY have been proposed. Most notable is the SOX9 gene, which also encodes a high-mobility group protein that is known to promote male differentiation and which is expressed in the developing male gonad immediately after SRY is first expressed.

Female Development as the Default. When Jost removed the gonads from embryonic rabbits, the embryos that were XX as well as those that were XY developed as females, though they lacked internal genitalia. This finding emphasized that gonads are critical to secondary sex determination and in the absence of male-specific hormones, female characteristics develop even in an XY individual.

As noted above, individuals with only a single copy of the X chromosome in each cell can survive, and they develop as females. Their ovaries develop normally at first but degenerate around the time of birth, resulting in a sterile adult. Hence a single X chromosome suffices for sex determination, but two copies are needed for ovary maintenance.

One explanation is that the female developmental program is the default, with embryos developing as females unless there are alternative instructions. SRY is a major switch gene required for male development, and only a gonad whose cells contain a copy of SRY will differentiate into a testicle.

Despite intensive searching, no major switch gene has been identified for the female developmental pathway. One candidate, DAX1, was proposed as the main ovarian differentiation gene principally because it was found to be duplicated on the X chromosomes of two XY siblings who developed as females. However, experimentally disrupting the mouse homolog of DAX1 had no effect on the sex determination, maturation, or fertility of female XX mice.

Instead of having a positive regulatory role, DAX1 appears to be antagonistic to SRTs function. When DAX1 is present in two copies, as in the XY sisters, it apparently disrupts SRY function sufficiently to prevent initiation of the male developmental program. These observations are consistent with the idea that the female sex determination pathway is the default option.

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