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Homeobox genes, or hox genes, are the part of the genome that codes for where each part of the body goes. Scientists frequently manipulate hox genes in drosophila to give them legs where the antennae should be, or more than one set of wings, or some other deformity.

The mechanism of homeobox genes is well understood. It involves localized expression of the genes where an expression gradient is formed at very early embryonic stages. This initial gradient then regulates the expression of other genes which are also locally expressed creating a cascade of expression gradients. The location of totipotent cells within blastula determine cell fate by regulating the morphogens which are expressed. Morphogens then come into play which generate specific structures such as abdominal segments, thoraces, fingers etc.

It has been show that the initiation of the first gradient is dependant upon the point on the ovum where the sperm enters. This point is directly correllated to the animal end of the blastula where mitosis occurs more quickly than the vegetal end which in turn initiates the invagination process. Homeobox regulating proteins in the surface of the sperm are the cause of all of this.

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