Sex chromosome
Nettie Stevens and Edmund Beecher Wilson both independently discovered sex chromosomes in 1905.
The deactivated X chromosome is silenced by repressive heterochromatin that compacts the DNA and prevents expression of most genes.
It can also occur when two fertilized embryo fuse, producing a chimera that might contain two different sets of DNA one XX and the other XY.
[6] It could also result from exposure, often in utero, to chemicals that disrupt the normal conversion of the allosomes into sex hormones and further into the development of either ambiguous outer genitalia or internal organs.
[8] This gene produces a testis-determining factor ("TDF"), which initiates testis development in humans and other mammals.
Many lower chordates, such as fish, amphibians and reptiles, have systems that are influenced by the environment.
This is because even flowering plants have a variety of mating systems, their sex determination primarily regulated by MADS-box genes.
[15] Only a few pseudoautosomal regions normally remain once sex chromosomes are fully differentiated.
Due to this recency, most plant sex chromosomes also have relatively small sex-linked regions.
The insertion of retrotransposons is probably the major cause of y-chromosome expansion and plant genome size evolution.
Retrotransposones contribute in size determination of sex chromosomes and its proliferation varies even in closely related species.
LTR and tandom repeats play dominant role in the evolution of S. latifolia sex chromosomes.
[18] Athila is new family of retroelements, discovered in Arabidopsis thaliana, present in heterochromatin region only.
Athila retroelements overrepresented in X but absent in Y while tandem repeats enriched in Y-chromosome.
This reduced selection leads to insertion of transposable elements and accumulation of deleterious mutation.
The Y become larger and smaller than X due to insertion of retroelement and deletion of genetic material respectively.
Unlike seed plants, where gametophytes are always unisexual, in bryophytes they may produce male, female, or both types of gamete.
Some gymnosperms, such as Johann's Pine (Pinus johannis), have homomorphic sex chromosomes that are almost indistinguishable through karyotyping.
Cytogenetic data from about 100 angiosperm species showed heteromorphic sex chromosomes in approximately half, mostly taking the form of XY sex-determination systems.
[16] Sex chromosomes have arisen independently multiple times in angiosperms, from the monoecious ancestral condition.
The move from a monoecious to dioecious system requires both male and female sterility mutations to be present in the population.
This corresponds with three sexes: females with XX chromosomes, males with XY, and hermaphrodites with XYh.
Most of the Y chromosome genes are involved with essential cell house-keeping activities and sperm production.
[28] In a large number of organisms, the sex-determination systems presently observed are products of sex chromosome turnover.
[30] There is one experimentally documented case of sex chromosome turnover occurring during a 30-year evolutionary experiment involving teleost fish (specifically the swordtails), in which hybridization experiments resulted in a translocation of the sex-determiner region of a sex chromosome into an autosome.
