The fruit fly,
as well as other species, use the number of Xs to determine sex, they are
nonviable with an extra X. SRY-reliant species can have conditions such as XXY and still live.Human
Some fish also have
variants of this, as well as the regular system. It has an XY format,
the Xiphophorus
variatus also has a second Y chromosome, known
as Y', that creates XY' females and YY' males.
The platypus has ten sex
chromosomes; males have an XYXYXYXYXY pattern while females have ten X
chromosomes. Although it is an XY system.
XX/X0 sex determination
In this variant of the XY
system, females have two copies of the sex chromosome (XX) but males have only
one (X0). The 0denotes the absence
of a second sex chromosome. Generally in this method, the sex is determined by
amount of genes expressed across the two chromosomes. This system is observed
in a number of insects, including the grasshoppers and crickets of order Orthoptera and in cockroaches (order Blattodea). A small number
of mammals also lack a Y chromosome.
ZW sex chromosomes
The ZW sex-determination system is found in birds, some reptiles, and
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. In birds,
the genes FET1 and ASW are found on the W chromosome for females, similar to
how the Y chromosome contains SRY.However, not all species depend upon the W
for their sex. For example, there are moths and butterflies that are ZW, but
some have been found female with ZO, as well as female with ZZW.
UV sex chromosomes
In some Bryophyte and some algae species, the gametophyte stage of the life cycle, rather than
being hermaphrodite, occurs as separate male or female individuals that produce
male and female gametes respectively. When meiosis occurs in the sporophyte generation of the life cycle, the sex
chromosomes known as U and V assort in spores that carry either the U
chromosome and give rise to female gametophytes, or the V chromosome and give
rise to male gametophytes.
Non-genetic
sex-determination systems
Temperature-dependent sex determination
Many other sex-determination
systems exist. In some species of reptiles, including alligators, some turtles,
and the tuatara, sex is determined
by the temperature at which the egg is incubated during a temperature-sensitive
period. There are no examples of temperature-dependent sex determination (TSD)
in birds. Megapodes had formerly been thought to exhibit
this phenomenon, but actually exhibit temperature-dependent embryo mortality.For
some species with TSD, sex determination is achieved by exposure to hotter
temperatures resulting in the offspring being one sex and cooler temperatures
resulting in the other. For others species using TSD, it is exposure to
temperatures on both extremes that results in offspring of one sex, and
exposure to moderate temperatures that results in offspring of the opposite
sex. These systems are known as Pattern I and Pattern II, respectively. The
specific temperatures required to produce each sex are known as the
female-promoting temperature and the male-promoting temperature. When the temperature stays near the
threshold during the temperature sensitive period, the sex ratio is varied between the two sexes.Some
species' temperature standards are based on when a particular enzyme is
created.
Sex Change system.
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 (Thalassoma bifasciatum) are the reverse. In the marine worm (Bonellia viridis), larvae become males if they make physical
contact with a female, and females if they end up on the bare sea floor. This
is triggered by the presence of a chemical produced by the females, bonellin.
Parthenogenesis
Some species,
however, have no sex-determination system. Hermaphrodite species include the
common earthworm and certain species of snails. A few species of fish,
reptiles, and insects reproduce by parthenogenesis and are female altogether.
There are some reptiles, such as the boa constrictor and komodo dragon that can reproduce both sexually and asexually, depending on
whether a mate is available.
All alligators determine the sex of their offspring by the temperature of the nest.
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