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Sex determination
Sex determination
A sex-determination system is determines the development of sexual characteristics in an
organism. Most sexual organisms have two sexes. In many cases, sex determination is
genetic: males and females have different alleles or even different genes that specify their
sexual morphology. In animals, this is often accompanied by chromosomal differences. In
other cases, sex is determined by environmental variables (such as temperature) or social
variables (the size of an organism relative to other members of its population). The details of
some sex-determination systems are not yet fully understood.
I. Genetic sex-determination systems
1. The XX/XY sex-determination system
This system is found in most mammals (including humans), as well as some insects. In the
system, females have two of the same kind of sex chromosome (XX), while males have two
distinct sex chromosomes (XY). The XY sex chromosomes are different in shape and size
from each other, unlike the autosomes.
The sex as a genetic program
The female as the default program Embryogenesis of males follows the female program as
long as it is deviated from this default program by a single gene of Y chromosome. This is the
SRY (sex determining region Y) gene encoding the TDF (testis determining factor), which is a
transcription factor. Mutation of SRY gene results in female phenotype (not perfect sexual
characters, e.g. sterility: Swyer syndrome). In the case of the so called XX male syndrome,
the SRY gene (to be more precise, a chromosomal segment containing the SRY gene) has been
translocated to an autosome, and despite the fact that the affected person has XX genotype,
the phenotype will be male (but sterile). In a general aspect, what happens in sex
determination is that in males the operation of the genome will be diverted from the female
default program. Thus, the genome is like a piano, we can play different pieces on it.
Male as a phenotype Switching the TDF on, induces a chain reaction, of which one of the
most important steps is the initiation of testosterone production (this hormone is expressed in
females, too!). Testosterone exerts its effect in two steps: at 8 week after conception and at
puberty, resulting in the development of primary and secondary sexual characters of males.
Testosterone induces significant changes in brain development, too: A decrease of
communication centers in the brain, and an increase in sexual brain centers in the
hypothalamus. In Y-centered sex determination, the SRY gene is not the only gene to have an
influence on sex. In XY mice, lack of the gene DAX1 on the X chromosome results in sterility,
but in humans it causes adrenal hypoplasia congenita. However, when an extra DAX1 gene is
placed on the X, the result is a female, despite the existence of SRY. Evidence from
both Dax1 knockout mice and a limited number of patients with AHC, suggests that
mutations in DAX1 may directly cause abnormalities in spermatogenesis.
2. The XX/X0 sex determination system
In this system the females have two copies of the sex chromosome (XX) while the males have
only one (X0). The 0 denotes the absence of a second sex chromosome. This system is
observed in a number of insects, including the grasshoppers and crickets and in cockroaches.
The nematode C. elegans is male with one sex chromosome (X0); with a pair of
chromosomes (XX) it is a hermaphrodite. A small number of mammals also lack a Y
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Sex determination
chromosome. These include the spiny rats. Voles also have a form of X0 determination in
which both genders lack a second sex chromosome. The mechanism of this sex determination
is not yet understood.
3. ZW sex chromosomes
The ZW sex-determination system is found in birds, some insects and other organisms. The
ZW sex-determination system is reversed compared to the XY system: females have two
different kinds of chromosomes (ZW), and males have two of the same kind of chromosomes
(ZZ). In the chicken, this was found to be dependent on the expression of DMRT1. However,
not all species depend upon the W for their gender. For example, there are moths and
butterflies that are ZW, but some have been found female with Z0, as well as female with
ZZW. Also, while mammals inactivate one of their extra X chromosomes when female, it
appears that in the case of Lepidoptera, the males produce double the normal amount of
enzymes, due to having two Z's. Because the use of ZW sex determination is varied, it is still
unknown how exactly most species determine their sex. Despite the similarities between ZW
and XY, the sex chromosomes do not line up correctly and evolved separately. In the case of
the chicken, their Z chromosome is more similar to humans' autosome 9. The chicken's Z
chromosome also seems to match with the X chromosomes of the platypus. The platypus,
presents a particular sex determination scheme that in some ways resembles that of the ZW
sex chromosomes of birds, and also lacks the SRY gene, whereas some rodents, such as voles
and lemmings, are also noted for their unusual sex determination systems. The platypus has
ten sex chromosomes; males have an XYXYXYXYXY pattern while females have ten X
chromosomes. Although it is an XY system, the platypus' sex chromosomes share no
homologues with eutherians sex chromosomes.[ Instead, homologues with eutherian sex
chromosomes lie on the platypus chromosome 6, which means that the eutherian sex
chromosomes were autosomes at the time that the monotremes diverged from the therian
mammals (marsupials and eutherian mammals). However, homologues to the avian DMRT1
gene on platypus sex chromosomes X3 and X5 suggest that it is possible the sex-determining
gene for the platypus is the same one that is involved in bird sex-determination. However,
more research must be conducted in order to determine the exact sex determining gene of the
platypus.
4. Haplodiploidy
Haplodiploidity is found in insects such as ants and bees. Unfertilized eggs develop into
haploid individuals, which are the males. Diploid individuals are generally female but may be
sterile males.
II. Non-genetic sex-determination systems
Many other sex-determination systems exist. In some species of reptiles, including alligators,
some turtles, sex is determined by the temperature at which the egg is incubated. Some
species, such as some snails, practice sex change: adults start out male, then become female.
In tropical clown fish, the dominant individual in a group becomes female while the other
ones are male, and bluehead wrasses are the reverse. In the marine worm larvae become males
if they make physical contact with a female, and females if they end up on the bare sea floor.
Some species, however, have no sex-determination system. Hermaphrodites include the
common earthworm and certain species of snails. A few species of fish, reptiles, and insects
reproduce by parthenogenesis and are female altogether.
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Do bacteria and viruses have sex? Although bacteria and archaebacteria replicate
asexually, and viruses are cloned asexually by their host cell, rampant horizontal gene transfer
(from one cell to another, not from parent to offspring) also shapes their genomes. In bacteria
we see plasmid-mediated transfer of genes that confer antibiotic resistance and virulence, and
in the influenza A virus we see mixing of genes and gene segments when both an avian and a
mammalian strain infect an intermediate mammal host simultaneously. Such mixing of genes
in bacteria and viruses, called “parasexuality,” is as effective as any form of sexual
reproduction in generating genotypic diversity and can occur instantaneously, not having to
wait for a new generation of progeny. Microbes enjoy much more rapid evolution and
adaptation than their larger hosts.
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