All
animals have a
genome made of
DNA, which forms
chromosomes during cell division. In humans, most mammals, and some other species, two of the
chromosomes, called the
X chromosome and
Y chromosome, contain genes which code for sex. In these species, one or more
genes are present on their
Y chromosome that trigger development of the male phenotype . In this system, the
X chromosome and the
Y chromosome determine the karyotypic sex of offspring, while genes located on the Y chromosome trigger development of the male phenotype. Offspring usually have two sex chromosomes: an offspring with two X chromosomes (XX) will usually develop the female phenotype, and an offspring with an X and a Y chromosome (XY) will usually develop the male phenotype. Variations such as individuals with
Swyer syndrome, that have an XY karyotype yet a female phenotype, and
de la Chapelle Syndrome, that have XX chromosomes and a male phenotype, though less common, are exceptions. Additionally, there are several cases of phenotypic females with an XY karyotype (Swyer syndrome), who have successfully hosted a
pregnancy.
Mammals In most mammals, karyotypic sex is determined by presence of the Y chromosome. This makes individuals with
XXY and
XYY karyotypic males, and individuals with
X and
XXX karyotypic females. In the 1930s,
Alfred Jost determined that the presence of
testosterone was required for
Wolffian duct development in the male rabbit. SRY is a sex-determining gene on the Y chromosome in the
therians (placental mammals and marsupials). Non-human mammals use several genes on the Y chromosome. Not all male-specific genes are located on the
Y chromosome. The
platypus, a
monotreme, use five pairs of different XY chromosomes with six groups of male-linked genes,
AMH being the master switch.
Humans A single gene (
SRY) present on the Y chromosome acts as a signal to trigger male phenotypic development. Presence of this gene starts off the process of
virilization. This and other factors result in the
sex differences in humans. In individuals with two X chromosomes, cells undergo
X-inactivation, in which one of the two X chromosomes is inactivated. The inactivated X chromosome remains within a cell as a
Barr body.
Other animals Some species of
turtles have convergently evolved XY sex determination systems, specifically those in
Chelidae and
Staurotypinae. Other species (including most
Drosophila species) use the presence of two X chromosomes to determine femaleness: one X chromosome gives putative maleness, but the presence of Y chromosome genes is required for normal male development. In the fruit fly individuals with XY are male and individuals with XX are female; however, individuals with XXY or XXX can also be female, and individuals with X can be males.
Plants Angiosperms While very few species of
dioecious angiosperm have XY sex determination, making up less than 5% of all species, the sheer diversity of angiosperms means that the total number of species with XY sex determination is actually quite high, estimated to be at around 13,000 species. Molecular and evolutionary studies also show that XY sex determination has evolved independently many times in upwards of 175 unique families, with a recent study suggesting its evolution has independently occurred hundreds to thousands of times. Many economically important crops are known to have an XY system of sex determination, including kiwifruit, asparagus, grapes and date palms.
Gymnosperms In sharp contrast to angiosperms, approximately 65% of
gymnosperms are dioecious. Some families which contain members that are known to have a XY system of sex determination include the cycad families
Cycadaceae and
Zamiaceae,
Ginkgoaceae,
Gnetaceae and
Podocarpaceae.
Other systems Whilst XY sex determination is the most familiar, since it is the system that humans use, there are a range of alternative systems found in nature. The inverse of the XY system (called
ZW to distinguish it) is used in birds and many insects, in which it is the females that are heterogametic (ZW), while males are homogametic (ZZ). Many insects of the order
Hymenoptera instead have a
haplo-diploid system, where the females are full
diploids (with all chromosomes appearing in pairs) but males are
haploid (having just one copy of all chromosomes). Some other insects have the
X0 sex-determination system, where just the sex-determining chromosome varies in ploidy (XX in females but X in males), while all other chromosomes appear in pairs in both sexes. ==Influences==