Sex chromosomes (also referred to as allosomes , heterotypical chromosome, gonosomes , heterochromosomes , or idiochromosomes ) are chromosomes that carry the genes that determine the sex of an individual. The human sex chromosomes are a typical pair of mammal allosomes. They differ from autosomes in form, size, and behavior. Whereas autosomes occur in homologous pairs whose members have the same form in a diploid cell , members of an allosome pair may differ from one another.
41-533: The Legbar is a rare British auto-sexing breed of chicken . It was created in the early twentieth century by Reginald Crundall Punnett and Michael Pease at the Genetical Institute of Cambridge University . They cross-bred American barred Plymouth Rock birds with brown Leghorns and created the gold and silver colour varieties . Pease created a cream Legbar by cross-breeding these with white Leghorns; later crossing with Araucanas caused this to have
82-428: A chimera that might contain two different sets of DNA one XX and the other XY. 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 . There is a gene in the Y chromosome that has regulatory sequences that control genes that code for maleness, called
123-511: A WZ system. Some gymnosperms, such as Johann's Pine ( Pinus johannis ), have homomorphic sex chromosomes that are almost indistinguishable through karyotyping . Cosexual angiosperms with either monoecious or hermaphroditic flowers do not have sex chromosomes. Angiosperms with separate sexes (dioecious) may use sex chromosomes or environmental flowers for sex determination. Cytogenetic data from about 100 angiosperm species showed heteromorphic sex chromosomes in approximately half, mostly taking
164-468: A Y. Females in such species receive an X chromosome from each parent while males receive an X chromosome from their mother and a Y chromosome from their father. It is thus the male's sperm that determines the sex of each offspring in such species. However, a small percentage of humans have a divergent sexual development, known as intersex . This can result from allosomes that are neither XX nor XY. It can also occur when two fertilized embryo fuse, producing
205-483: A crest and to lay blue or blue-green eggs. The Legbar was the second auto-sexing chicken breed created by Reginald Crundall Punnett and Michael Pease at the Genetical Institute of Cambridge University , after the Cambar , which was created in 1929 by crossing barred Plymouth Rock with gold Campine birds. The Legbar arose from cross-breeding of Plymouth Rock birds with brown Leghorns , which at that time were two of
246-501: A different sex-determination system compared to mammals (X and Y). Male birds have therefore two Z-chromosomes while female birds have a Z- and a dwarfed w-chromosome. This means that phenotypically barred cocks can either have the B/B or the B/b+ genotype , while a barred hen always has to have a B/- genotype. The colour-sexing of Legbar chicks, however, is only possible because the male chicks have
287-563: A double dose of the sex-linked barring gene (genotype B/B), while the female chicks only have a single dose (genotype B/-), resulting in the observed down colours. The concept of an auto-sexing breed of chicken is due to the geneticist Reginald Punnett , who during the First World War had already proposed the technique of cross-breeding chickens carrying the barred gene (B) with others to produce sex-linked chicks with plumage differences that could easily be distinguished. Working at
328-420: A standards for the adult birds, the down colour and patterns are also standardised. Day-old male chicks can be distinguished from day-old female chicks by the down colour and the pattern they form. Female Legbar chicks in general have a broad very dark brown stripe extending over the head, neck and rump and a clear eye barring. The edges of the stripe are clearly defined and should not be blurred and blending with
369-500: A typical XX individual (traditional female) and a sex-reversed XX man was that the typical individuals lacked the SRY gene. It is theorized that in sex-reversed XX men, the SRY mistakenly gets translocated to an X chromosome in the XX pair during meiosis . Diverse mechanisms are involved in the determination of sex in animals. For mammals, sex determination is carried by the genetic contribution of
410-454: Is a process defined as when the type of the sex chromosome changes as a product of a change in the identity of the sex-determining genes (such as by mutation) or by a change in their location. In other cases, sex chromosomes may grow substantially with respect to their ancestral forms as a result of fusion events with autosomes, and autosome-sex chromosome fusions result in what are called neo-sex chromosomes. Five examples of this are now known in
451-434: Is also used as model for the study of sex chromosomes evolution. Based on the phylogenetic topology distribution there are three regions on sex chromosomes. One region that stops recombining in the ancestor of H. lupulus , second that stops recombining in modern H. lupulus and the third region called pseudoautosomal region. H. lupulus is the rare case in plants in which Y is smaller than X, while its ancestor plant has
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#1732876307228492-404: Is always present as the 23rd chromosome in the ovum , while either an X or Y chromosome may be present in an individual sperm . Early in female embryonic development, in cells other than egg cells, one of the X chromosomes is randomly and permanently partially deactivated : In some cells, the X chromosome inherited from the mother deactivates; in other cells, it is the X chromosome inherited from
533-399: Is determined in a single XY system. In a more complicated system, the sandalwood species Viscum fischeri has X1X1X2X2 chromosomes in females, and X1X2Y chromosomes in males. Amplification of transposable elements, tandom repeats especially accumulation of long tandom repeats ( LTR ) retrotransposones are responsible for plant sex chromosome evolution. The insertion of retrotransposons
574-502: Is estimated to have arisen only 4000 years ago, post-domestication of the plant. The genetic architecture suggests that either the Y chromosome has an X-inactivating gene, or that the Y chromosome has an X-activating gene. Allosomes not only carry the genes that determine male and female traits, but also those for some other characteristics as well. Genes that are carried by either sex chromosome are said to be sex linked . Sex linked diseases are passed down through families through one of
615-404: Is more complex than that in humans. This is because even flowering plants have a variety of mating systems, their sex determination primarily regulated by MADS-box genes. These genes code for proteins that form the sex organs in flowers. Plant sex chromosomes are most common in bryophytes , relatively common in vascular plants and unknown in ferns and lycophytes . The diversity of plants
656-708: 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. 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. Some chloroplast sequences have also been identified in
697-557: Is reflected in their sex-determination systems, which include XY and UV systems as well as many variants. Sex chromosomes have evolved independently across many plant groups. Recombination of chromosomes may lead to heterogamety before the development of sex chromosomes, or recombination may be reduced after sex chromosomes develop. Only a few pseudoautosomal regions normally remain once sex chromosomes are fully differentiated. When chromosomes do not recombine, neutral sequence divergences begin to accumulate, which has been used to estimate
738-444: Is when both matching genes must be abnormal to cause disease. If only one gene in the pair is abnormal, the disease does not occur, or is mild. Someone who has one abnormal gene (but no symptoms) is called a carrier. A carrier can pass this abnormal gene to his or her children. X chromosome carry about 1500 genes, more than any other chromosome in the human body. Most of them code for something other than female anatomical traits. Many of
779-553: The Genetical Institute of Cambridge University , he and Michael Pease cross-bred Golden Campines with barred Plymouth Rocks , resulting in the creation of the Cambar in 1928. About ten years later they produced the Legbar by crossing brown Leghorns with barred Plymouth Rocks. Other "Cambridge" breeds later developed were: Many other breeds were created in the same way, all making use of barred Plymouth Rocks to impart
820-449: The SRY gene . This gene produces a testis-determining factor ("TDF"), which initiates testis development in humans and other mammals. The SRY sequence's prominence in sex determination was discovered when the genetics of sex-reversed XX men (i.e. humans who possess biological male-traits but actually have XX allosomes) were studied. After examination, it was discovered that the difference between
861-462: The Legbar is an auto-sexing breed. Several other auto-sexing breeds or auto-sexing varieties of breeds exist, such as Plymouth Rock, Bielefelder Kennhuhn , Niederrheiner , and Norwegian Jærhøns . Most breeds that end with -bar, such as Welbar , Rhodebar , Brussbar or Wybar , are auto-sexing as well. The importance that auto-sexing plays in the Legbar breed is also reflected in the fact that, next to
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#1732876307228902-499: The X or Y chromosomes. Since usually men inherit Y chromosomes, they are the only ones to inherit Y-linked traits. Men and women can get the X-linked ones since both inherit X chromosomes. An allele is either said to be dominant or recessive . Dominant inheritance occurs when an abnormal gene from one parent causes disease even though the matching gene from the other parent is normal. The abnormal allele dominates. Recessive inheritance
943-696: The Y-chromosome of S. latifolia . S. vulgaris has more retroelements in their sex chromosomes compare to S. latifolia . Microsatellite data shows that there is no significant difference between X and Y-chromosome microsatellites in both Silene species. This would conclude that microsatellites do not participate in Y-chromosome evolution. The portion of Y-chromosome that never recombine with X-chromosome faces selection reduction. 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. The genus Humulus
984-460: The age of sex chromosomes in various plant lineages. Even the oldest estimated divergence, in the liverwort Marchantia polymorpha , is more recent than mammal or bird divergence. Due to this recency, most plant sex chromosomes also have relatively small sex-linked regions. Current evidence does not support the existence of plant sex chromosomes more ancient than those of M. polymorpha . The high prevalence of autopolyploidy in plants also impacts
1025-477: The autosomes. There is variation even within this system, including UU/V and U/VV chromosome arrangements. In some bryophytes, microchromosomes have been found to co-occur with sex chromosomes and likely impact sex determination. Dioecy is common among gymnosperms , found in an estimated 36% of species. However, heteromorphic sex chromosomes are relatively rare, with only five species known as of 2014. Five of these use an XY system, and one ( Ginkgo biloba ) uses
1066-415: The barred gene: Allosome Nettie Stevens and Edmund Beecher Wilson both independently discovered sex chromosomes in 1905. However, Stevens is credited for discovering them earlier than Wilson. In humans, each cell nucleus contains 23 pairs of chromosomes, a total of 46 chromosomes. The first 22 pairs are called autosomes . Autosomes are homologous chromosomes i.e. chromosomes which contain
1107-596: The bryophytes, including liverworts, hornworts and mosses, sex chromosomes are common. The sex chromosomes in bryophytes affect what type of gamete is produced by the gametophyte, and there is wide diversity in gametophyte type. Unlike seed plants, where gametophytes are always unisexual, in bryophytes they may produce male, female, or both types of gamete. Bryophytes most commonly employ a UV sex-determination system, where U produces female gametophytes and V produces male gametophytes. The U and V chromosomes are heteromorphic with U larger than V and are frequently both larger than
1148-634: The creation of the cream Legbar, a crested layer of coloured eggs for which a standard was drawn up in 1958. Within a short time it became very rare, but has since recovered. It is treated by the Poultry Club of Great Britain as a colour variety of the Legbar, but is considered by the Rare Breed Survival Trust to be a separate breed. Both the Legbar and the Cream Legbar are listed among the UK breeds on
1189-608: The father. This ensures that both sexes always have exactly one functional copy of an X chromosome in each body cell. The deactivated X chromosome is silenced by repressive heterochromatin that compacts the DNA and prevents expression of most genes. This compaction is regulated by PRC2 (Polycomb Repressive Complex 2). All diploid organisms with allosome-determined sex get half of their allosomes from each of their parents. In most mammals, females are XX, and can pass along either of their Xs; since males are XY they can pass along either an X or
1230-523: The form of XY sex-determination systems. Their Y is typically larger, unlike in humans; however there is diversity among angiosperms. In the Poplar genus ( Populus ) some species have male heterogamety while others have female heterogamety. 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
1271-528: The ground colour, which is dark brown. A light head spot should be visible but is usually small. The male Legbar chicks in contrast have a much paler down shade and the pattern is blurred and washed out from head to rump. The marked difference between male and female chicks is due to gene dosage of the sex-linked barring gene ('barring' (B), 'nonbarring' (b+)). This gene is located on the Z-Chromosome of birds. Birds have different sex-chromosomes (Z and w) and
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1312-403: The non-sex determining X-linked genes are responsible for abnormal conditions. The Y chromosome carries about 78 genes. Most of the Y chromosome genes are involved with essential cell house-keeping activities and sperm production. Only one of the Y chromosome genes, the SRY gene, is responsible for male anatomical traits. When any of the 9 genes involved in sperm production are missing or defective
1353-442: The population. Male sterility likely arises first as an adaptation to prevent selfing. Once male sterility has reached a certain prevalence, then female sterility may have a chance to arise and spread. In the domesticated papaya ( Carica papaya ), three sex chromosomes are present, denoted as X, Y and Y . This corresponds with three sexes: females with XX chromosomes, males with XY, and hermaphrodites with XY . The hermaphrodite sex
1394-691: The principal egg-laying breeds. As with the Cambar, they set out to breed a bird that would both have brown down and carry the barred gene (B) , so that chicks would have sex-linked plumage differences that could easily be distinguished. Standards for the gold and silver colour varieties were drawn up in 1945 and 1951 respectively. The cream Legbar was created by chance. Through cross-breeding of gold Legbars with white Leghorn stock, Pease had obtained some cream-coloured birds; their eggs were white, and they had no crest. An experimental crossing of these with some cream-coloured Araucanas from Punnett's laboratory led to
1435-404: The result is usually very low sperm counts and infertility. Examples of mutations on the X chromosome include more common diseases such as the following: Other complications include: Sex chromosomes evolve from standard pairs of autosomal chromosomes. In a large number of organisms, the sex-determination systems presently observed are products of sex chromosome turnover. Sex chromosome turnover
1476-415: The same genes (regions of DNA) in the same order along their chromosomal arms. The 23rd pair of chromosomes are called allosomes. These consist of two X chromosomes in females, and an X chromosome and a Y chromosome in males. Females therefore have 23 homologous chromosome pairs, while males have 22. The X and Y chromosomes have small regions of homology called pseudoautosomal regions . An X chromosome
1517-406: The same size of both X and Y chromosomes. This size difference should be caused by deletion of genetic material in Y but that is not the case. This is because of complex dynamics like the larger size of X than Y-chromosome may be due to duplication or retrotransposition and size of Y remains same. Ferns and lycophytes have bisexual gametophytes , so there is no evidence for sex chromosomes. In
1558-489: The sex of newly-hatched chicks can be determined from the colour and markings of the down . Some breeds of chicken , of goose and of domestic pigeon have this characteristic. The idea of such a breed is due to Reginald Punnett , who created the first auto-sexing chicken breed, the Cambar , at the Genetical Institute in Cambridge in 1928. Unlike sex-linked hybrids, such as 'red sex-links' or 'black sex-links' ,
1599-492: The spermatozoon. Many lower chordates, such as fish, amphibians and reptiles, have systems that are influenced by the environment. Fish and amphibians, for example, have genetic sex determination but their sex can also be influenced by externally available steroids and incubation temperature of eggs. In some reptiles, e.g. sea turtles , only the incubation temperature determines sex ( temperature-dependent sex determination ). Many scientists argue that sex determination in plants
1640-413: The structure of their sex chromosomes. Polyploidization can occur before and after the development of sex chromosomes. If it occurs after sex chromosomes are established, dosage should stay consistent between the sex chromosomes and autosomes, with minimal impact on sex differentiation. If it occurs before sex chromosomes become heteromorphic, as is likely in the octoploid red sorrel Rumex acetosella , sex
1681-737: The watchlist of the trust; neither is considered "priority". The Legbar has three colour varieties : gold, silver and cream. The cream variant has a crest and lays blue, olive or green eggs. The Legbar is considered a rare breed by the Poultry Club of Great Britain and, until the Autosexing Breeds Association was re-formed, fell under the Rare Poultry Society . The Legbar was bred as an auto-sexing layer breed. Hens may give 180 eggs or more per year. Auto-sexing Auto-sexing breeds of poultry are those in which