Everything about Sex Chromosome totally explained
A
sex-determination system is a
biological system that 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.
Chromosomal determination
XX/XY sex chromosomes
The
XX/XY sex-determination system is one of the most familiar sex-determination systems and is found in
human beings and most other
mammals, although at least one
monotreme, the
platypus, presents a particular sex determination scheme that in some ways resembles that of the ZW sex chromosomes of birds, and it also lacks the
SRY gene.
In the XY sex-determination system, females have two of the same kind of sex
chromosome (XX), while males have two distinct sex chromosomes (XY). Some species (including humans) have a gene
SRY on the Y chromosome that determines maleness; others (such as the
fruit fly) use the presence of two X chromosomes to determine femaleness.
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
0 denotes the absence of a second sex chromosome. This system is observed in a number of
insects, including the grasshoppers and crickets of order
Orthoptera and in cockroaches (order
Blattodea).
The
nematode C. elegans is male with one sex chromosome (X0); with a pair of chromosomes (XX) it's a
hermaphrodite.
ZW sex chromosomes
The
ZW sex-determination system is found in
birds 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).
Haplodiploidy
Haplodiploidy is found in
insects belonging to
Hymenoptera, 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. Thus, if a queen
bee mates with one drone, her daughters share ¾ of their genes with each other, not ½ as in the XY and ZW systems. This is believed to be significant for the development of
eusociality, as it increases the significance of
kin selection.
This is common also in wasps that are parasitic and in the male greenflies.
Non-genetic sex-determination systems
Many other exotic 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. Other 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.
Some species have no sex-determination system.
Earthworms and some snails are
hermaphrodites; a few species of lizard, fish, and insect are all female and reproduce by
parthenogenesis.
In some
arthropods, sex is determined by infection, as when
Bacteria of the genus
Wolbachia alter their sexuality; some species consist entirely of ZZ individuals, with sex determined by the presence of
Wolbachia.
Other unusual systems [thissection still being researched]:
- Swordtail fish?
- The Chironomus midge species
- The Platypus lacks the mammalian sex-determining gene SRY, meaning that the process of sex determination in the Platypus remains unknown.
Further Information
Get more info on 'Sex Chromosome'.
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