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Naturally Occurring Polymers—Animals 367

homologous pairs again line up across the center or equator of the cell in preparation for separation
of the chromatids, chromosomes. The second meiotic division produces four haploid daughter cells
that can act as gametes. Each cell has three chromosomes, half the number of the diploid cell. The
chromosomes have resorted and recombined.
We have just considered mitosis in general. We can take a simplistic look at the determination of
whether a given embryo is a male or female. Females have two X chromosomes while males have
one X and one Y so that the ability of the X chromosome to overwhelm the Y chromosomes and
give only female embryos is favored on a statistical basis. On a size basis, the Y chromosome is
the smallest of all the chromosomes while the X is among the largest. Further, the Y chromosome
is largely composed of noncoding DNA giving few targets for the X chromosome to interact with.
The gene on the Y chromosome that makes men men is called the SRY gene. The SRY gene inter-
acts with the DAX gene on the X chromosome. In some sense, these two genes are antagonistic to
one another where two DAX genes overcome the single SRY gene but one SRY gene overcomes one
DAX gene so that depending upon the particular course of evens the outcome is a male or female.
The SRY gene, when activated, ignites a whole cascade of events that leads to the maculation of the
embryo. The SKY gene is peculiar in that it is remarkably consistent between men with essentially
no variations in the coding regardless of race. Further, the human SRY gene is very different from
those of other primates.
For many species this XY battle greatly favors one sex, generally the female, over the other. For
instance in the butterfl y Acrea encedon, 97% of the butterflies are female. But in humans the com-

petition is such that the ratio of males to females is about, not exactly, one to one.
We will look at another example where the “equal” splitting of chromosome information is not
entirely true. Some families have members that exhibit two related diseases. Those with the Prader–
Willi syndrome are characterized with small hands and feet, underdeveloped sex organs, they are
generally obese, and are also often mildly mentally retarded. Those with the Angelman’s syndrome
are taut, thin, insomnic, small-headed, move jerkily, have a happy disposition, always smiling, and
are generally unable to speak and are mentally retarded. In both cases a section of chromosome 15
is missing. In the Prader–Willi syndrome the missing part is from the father’s chromosome but in
the Angelman’s syndrome the missing part is from the mother’s chromosome. Thus, the two dis-
eases differ depending whether it is transmitted through the male or female.
Related to this are recent attempts to produce unisex children—that is embryos that are from
male–male and those from female–female as sources of the chromosomal material. In both cases
the attempt failed. The “two-mother-derived” chromosomes could not make a placenta and the
“two-father-derived” chromosomes could not make a discernible head so that chromosomes from
both sexes are required to give a healthy successful embryo.
We now move to what makes a single egg/sperm combination grow into a child. It is a combina-
tion of special events of which we will look at only one aspect. The machinery to construct a person
is found about our chromosomes. One cluster of these developmental genes is found in the middle of
chromosome 12. Within these genes is a grouping of homeotic genes that reside in the same general
area. These genes are called the Hox genes and affect the parts of the body in the exact sequence
that they appear in the fruit fly—mouth, face, top of head, neck, thorax, front half of abdomen, rear

half of abdomen, and finally the other parts of the abdomen. Also, found in each of these homeotic
genes is the same sequence of about 180 base-pairs long that is believed to act as a switch to turn
on or off each gene referred to as the homeobox. Mice were examined and also found to have such
homeotic genes and homeoboxes. Mice have 39 Hox genes in four clusters with some differences,
but many similarities, with the fruit fly. We have the same Hox clusters as mice with one such

cluster, Cluster C, on chromosome 12. A practical implication is that all the work done with other

species, such as the fruit fly, may be useful as we look at our own genome with at least such devel-
opmental genes.
The similarity of the embryo genes between us and other species allows developmental scientists

a filled table of data to sort over. It does not eliminate nor confirm a so-called master designer since

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