Discovery[ edit ] The Y chromosome was identified as a sex-determining chromosome by Nettie Stevens at Bryn Mawr College in during a study of the mealworm Tenebrio molitor. Edmund Beecher Wilson independently discovered the same mechanisms the same year.
Stevens proposed that chromosomes always existed in pairs and that the Y chromosome was the pair of the X chromosome discovered in 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. Stevens named the chromosome "Y" simply to follow on from Henking's "X" alphabetically.
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. Males have one Y chromosome and one X chromosome , while females have two X chromosomes.
In mammals, the Y chromosome contains a gene, SRY , which triggers embryonic development as a male. The Y chromosomes of humans and other mammals also contain other genes needed for normal sperm production. There are exceptions, however. For example, the platypus relies on an XY sex-determination system based on five pairs of chromosomes. For related phenomena, see Androgen insensitivity syndrome and Intersex.
Origins and evolution[ edit ] Before Y chromosome[ edit ] Many ectothermic vertebrates have no sex chromosomes. If they have different sexes, sex is determined environmentally rather than genetically. For some of them, especially reptiles , sex depends on the incubation temperature; others are hermaphroditic meaning they contain both male and female gametes in the same individual.
Origin[ edit ] The X and Y chromosomes are thought to have evolved from a pair of identical chromosomes,   termed autosomes , when an ancestral animal developed an allelic variation, a so-called "sex locus" — simply possessing this allele caused the organism to be male. 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 through the process of translocation.
As a result, genes beneficial to males accumulated near the sex-determining genes, and recombination in this region was suppressed in order to preserve this male specific region. Only the tips of the Y and X chromosomes recombine. The tips of the Y chromosome that could recombine with the X chromosome are referred to as the pseudoautosomal region. The rest of the Y chromosome is passed on to the next generation intact.
It is because of this disregard for the rules that the Y chromosome is such a superb tool for investigating recent human evolution. Degeneration[ edit ] By one estimate, the human Y chromosome has lost 1, of its 1, original genes over the course of its existence, and linear extrapolation of this 1,gene loss over million years gives a rate of genetic loss of 4.
Degeneration may simply be the fate of all non-recombining sex chromosomes, due to three common evolutionary forces: High mutation rate[ edit ] The human Y chromosome is particularly exposed to high mutation rates due to the environment in which it is housed. The Y chromosome is passed exclusively through sperm , which undergo multiple cell divisions during gametogenesis. Each cellular division provides further opportunity to accumulate base pair mutations.
Additionally, sperm are stored in the highly oxidative environment of the testis, which encourages further mutation. These two conditions combined put the Y chromosome at a greater risk of mutation than the rest of the genome. Deleterious alleles are allowed to "hitchhike" with beneficial neighbors, thus propagating maladapted alleles in to the next generation. Conversely, advantageous alleles may be selected against if they are surrounded by harmful alleles background selection.
Due to this inability to sort through its gene content, the Y chromosome is particularly prone to the accumulation of "junk" DNA. Massive accumulations of retrotransposable elements are scattered throughout the Y.
However, the Y chromosome has no way of weeding out these "jumping genes". Without the ability to isolate alleles, selection cannot effectively act upon them. A clear, quantitative indication of this inefficiency is the entropy rate of the Y chromosome. Whereas all other chromosomes in the human genome have entropy rates of 1. Genetic drift[ edit ] Even if a well adapted Y chromosome manages to maintain genetic activity by avoiding mutation accumulation, there is no guarantee it will be passed down to the next generation.
Thus, genetic drift is an exceptionally strong force acting upon the Y chromosome. Through sheer random assortment, an adult male may never pass on his Y chromosome if he only has female offspring. Thus, although a male may have a well adapted Y chromosome free of excessive mutation, it may never make it in to the next gene pool. They found that human Y chromosome is able to "recombine" with itself, using palindrome base pair sequences.
In the case of the Y chromosomes, the palindromes are not noncoding DNA ; these strings of bases contain functioning genes important for male fertility. Most of the sequence pairs are greater than The extensive use of gene conversion may play a role in the ability of the Y chromosome to edit out genetic mistakes and maintain the integrity of the relatively few genes it carries.
In other words, since the Y chromosome is single, it has duplicates of its genes on itself instead of having a second, homologous, chromosome. When errors occur, it can use other parts of itself as a template to correct them. Findings were confirmed by comparing similar regions of the Y chromosome in humans to the Y chromosomes of chimpanzees , bonobos and gorillas. The comparison demonstrated that the same phenomenon of gene conversion appeared to be at work more than 5 million years ago, when humans and the non-human primates diverged from each other.
Future evolution[ edit ] In the terminal stages of the degeneration of the Y chromosome, other chromosomes increasingly take over genes and functions formerly associated with it.
Finally, the Y chromosome disappears entirely, and a new sex-determining system arises. The Transcaucasian mole vole , Ellobius lutescens, the Zaisan mole vole , Ellobius tancrei, and the Japanese spinous country rats Tokudaia osimensis and Tokudaia tokunoshimensis , have lost the Y chromosome and SRY entirely. The wood lemming Myopus schisticolor, the Arctic lemming , Dicrostonyx torquatus, and multiple species in the grass mouse genus Akodon have evolved fertile females who possess the genotype generally coding for males, XY, in addition to the ancestral XX female, through a variety of modifications to the X and Y chromosomes.
Hamilton gave the following basic explanation in his paper on "Extraordinary sex ratios",  given the condition that males and females cost equal amounts to produce: Suppose male births are less common than female.
A newborn male then has better mating prospects than a newborn female, and therefore can expect to have more offspring. Therefore parents genetically disposed to produce males tend to have more than average numbers of grandchildren born to them.
Therefore the genes for male-producing tendencies spread, and male births become more common. The same reasoning holds if females are substituted for males throughout. Non-mammal Y chromosome[ edit ] Many groups of organisms in addition to mammals have Y chromosomes, but these Y chromosomes do not share common ancestry with mammalian Y chromosomes. Such groups include Drosophila , some other insects, some fish, some reptiles, and some plants.
In Drosophila melanogaster , the Y chromosome does not trigger male development. Instead, sex is determined by the number of X chromosomes. There are some species of Drosophila in which X0 males are both viable and fertile. For example, female birds, snakes, and butterflies have ZW sex chromosomes, and males have ZZ sex chromosomes. Non-inverted Y chromosome[ edit ] There are some species, such as the Japanese rice fish , the XY system is still developing and cross over between the X and Y is still possible.
Because the male specific region is very small and contains no essential genes, it is even possible to artificially induce XX males and YY females to no ill effect. Some cells, especially in older men and smokers , lack a Y chromosome.
It has been found that men with a higher percentage of hematopoietic stem cells in blood lacking the Y chromosome and perhaps a higher percentage of other cells lacking it have a higher risk of certain cancers and have a shorter life expectancy. This has been interpreted as a sign that the Y chromosome plays a role going beyond sex determination and reproduction  although the loss of Y may be an effect rather than a cause. And yet women, who have no Y chromosome, have lower rates of cancer.
Male smokers have between 1. These regions are relics of ancient homology between the X and Y chromosomes. The bulk of the Y chromosome, which does not recombine, is called the "NRY", or non-recombining region of the Y chromosome. For details, see human Y-chromosome DNA haplogroup. Genes[ edit ] Number of genes[ edit ] The following are some of the gene count estimates of human Y chromosome.
Because researchers use different approaches to genome annotation their predictions of the number of genes on each chromosome varies for technical details, see gene prediction.
Among various projects, the collaborative consensus coding sequence project CCDS takes an extremely conservative strategy. So CCDS's gene number prediction represents a lower bound on the total number of human protein-coding genes.