Y chromosome
The Y chromosome is one of two sex chromosomes in therian mammals and other organisms. Along with the X chromosome, it is part of the XY sex-determination system, in which the Y is the sex-determining because it is the presence or absence of Y chromosome that determines the male or female sex of offspring produced in sexual reproduction. In mammals, the Y chromosome contains the SRY gene, which triggers development of male gonads. The Y chromosome is passed only from male parents to male offspring.
Human Y chromosome
62,460,029 bp (CHM13)
Acrocentric[2]
(10.4 Mbp[3])
Overview[edit]
Discovery[edit]
The Y chromosome was identified as a sex-determining chromosome by Nettie Stevens at Bryn Mawr College in 1905 during a study of the mealworm Tenebrio molitor. Edmund Beecher Wilson independently discovered the same mechanisms the same year, working with Hemiptera. Stevens proposed that chromosomes always existed in pairs and that the smaller chromosome (now labelled "Y") was the pair of the X chromosome discovered in 1890 by Hermann Henking. She realized that the previous idea of Clarence Erwin McClung, that the X chromosome determines sex, was wrong and that sex determination is, in fact, due to the presence or absence of the Y chromosome. In the early 1920s Theophilus Painter determined that X and Y chromosomes determined sex in humans (and other mammals).[4]
The chromosome was given the name "Y" simply to follow on from Henking's "X" alphabetically.[5][6] The idea that the Y chromosome was named after its similarity in appearance to the letter "Y" is mistaken. All chromosomes normally appear as an amorphous blob under the microscope and only take on a well-defined shape during mitosis. This shape is vaguely X-shaped for all chromosomes. It is entirely coincidental that the Y chromosome, during mitosis, has two very short branches which can look merged under the microscope and appear as the descender of a Y-shape.[5]: 65–66
Origins and evolution[edit]
Before Y chromosome[edit]
Many ectothermic vertebrates have no sex chromosomes.[9] If these species have different sexes, sex is determined environmentally rather than genetically. For some species, especially reptiles, sex depends on the incubation temperature.[10] Some vertebrates are hermaphrodites, though hermaphroditic species are most commonly sequential, meaning the organism switches sex, producing male or female gametes at different points in its life, but never producing both at the same time. This is opposed to simultaneous hermaphroditism, where the same organism produces male and female gametes at the same time. Most simultaneous hermaphrodite species are invertebrates, and among vertebrates, simultaneous hermaphroditism has only been discovered in a few orders of fish.[11]
Origin[edit]
The X and Y chromosomes are thought to have evolved from a pair of identical chromosomes,[12][13] termed autosomes, when an ancestral animal developed an allelic variation (a so-called "sex locus") and simply possessing this allele caused the organism to be male.[14] The chromosome with this allele became the Y chromosome, while the other member of the pair became the X chromosome. Over time, genes that were beneficial for males and harmful to (or had no effect on) females either developed on the Y chromosome or were acquired by the Y chromosome through the process of translocation.[15]
Until recently, the X and Y chromosomes were thought to have diverged around 300 million years ago.[16] However, research published in 2008 analyzing the platypus genome[17] suggested that the XY sex-determination system would not have been present more than 166 million years ago, when monotremes split from other mammals.[18] This re-estimation of the age of the therian XY system is based on the finding that sequences that are on the X chromosomes of marsupials and eutherian mammals are not present on the autosomes of platypus and birds.[18] The older estimate was based on erroneous reports that the platypus X chromosomes contained these sequences.[19][20]
Recombination inhibition[edit]
Most chromosomes recombine during meiosis. However, in males, the X and Y pair in a shared region known as the pseudoautosomal region (PAR).[21] The PAR undergoes frequent recombination between the X and Y chromosomes,[21] but recombination is suppressed in other regions of the Y chromosome.[14] These regions contain sex-determining and other male-specific genes.[22] Without this suppression, these genes could be lost from the Y chromosome from recombination and cause issues such as infertility.[23]
The lack of recombination across the majority of the Y chromosome makes it a useful tool in studying human evolution, since recombination complicates the mathematical models used to trace ancestries.[24]
Degeneration[edit]
By one estimate, the human Y chromosome has lost 1,393 of its 1,438 original genes over the course of its existence, and linear extrapolation of this 1,393-gene loss over 300 million years gives a rate of genetic loss of 4.6 genes per million years.[25] Continued loss of genes at the rate of 4.6 genes per million years would result in a Y chromosome with no functional genes – that is the Y chromosome would lose complete function – within the next 10 million years, or half that time with the current age estimate of 160 million years.[14][26] Comparative genomic analysis reveals that many mammalian species are experiencing a similar loss of function in their heterozygous sex chromosome. Degeneration may simply be the fate of all non-recombining sex chromosomes, due to three common evolutionary forces: high mutation rate, inefficient selection, and genetic drift.[14]
With a 30% difference between humans and chimpanzees, the Y chromosome is one of the fastest-evolving parts of the human genome.[27] However, these changes have been limited to non-coding sequences and comparisons of the human and chimpanzee Y chromosomes (first published in 2005) show that the human Y chromosome has not lost any genes since the divergence of humans and chimpanzees between 6–7 million years ago.[28] Additionally, a scientific report in 2012 stated that only one gene had been lost since humans diverged from the rhesus macaque 25 million years ago.[29] These facts provide direct evidence that the linear extrapolation model is flawed and suggest that the current human Y chromosome is either no longer shrinking or is shrinking at a much slower rate than the 4.6 genes per million years estimated by the linear extrapolation model.