The pseudoautosomal regions, PAR1, PAR2, [1] are homologous sequences of nucleotides on the X and Y chromosomes.
The pseudoautosomal regions get their name because any genes within them (so far at least 29 have been found for humans) [2] are inherited just like any autosomal genes. PAR1 comprises 2.6 Mbp of the short-arm tips of both X and Y chromosomes in humans and great apes (X and Y are 154 Mbp and 62 Mbp in total). PAR2 is at the tips of the long arms, spanning 320 kbp. [3]
The locations of the PARs within GRCh38 are: [4]
| Name | Chromosome | Basepair start | Basepair stop | Band [5] |
|---|---|---|---|---|
| PAR1 | X | 10,001 | 2,781,479 | Xp22 |
| Y | 10,001 | 2,781,479 | Yp11 | |
| PAR2 | X | 155,701,383 | 156,030,895 | Xq28 |
| Y | 56,887,903 | 57,217,415 | Yq12 |
The locations of the PARs within GRCh37 are:
| Name | Chromosome | Basepair start | Basepair stop |
|---|---|---|---|
| PAR1 | X | 60,001 | 2,699,520 |
| Y | 10,001 | 2,649,520 | |
| PAR2 | X | 154,931,044 | 155,260,560 |
| Y | 59,034,050 | 59,363,566 |
Normal male mammals have two copies of these genes: one in the pseudoautosomal region of their Y chromosome, the other in the corresponding portion of their X chromosome. Normal females also possess two copies of pseudoautosomal genes, as each of their two X chromosomes contains a pseudoautosomal region. Crossing over between the X and Y chromosomes is normally restricted to the pseudoautosomal regions; thus, pseudoautosomal genes exhibit an autosomal, rather than sex-linked, pattern of inheritance. So, females can inherit an allele originally present on the Y chromosome of their father.
The function of these pseudoautosomal regions is that they allow the X and Y chromosomes to pair and properly segregate during meiosis in males. [6]
Pseudoautosomal genes are found in two different locations: PAR1 and PAR2. These are believed to have evolved independently. [8]
in mice, some PAR1 genes have transferred to autosomes. [10]
Pairing (synapsis) of the X and Y chromosomes and crossing over (recombination) between their pseudoautosomal regions appear to be necessary for the normal progression of male meiosis. [13] Thus, those cells in which X-Y recombination does not occur will fail to complete meiosis. Structural and/or genetic dissimilarity (due to hybridization or mutation) between the pseudoautosomal regions of the X and Y chromosomes can disrupt pairing and recombination, and consequently cause male infertility.
The SHOX gene in the PAR1 region is the gene most commonly associated with and well understood with regards to disorders in humans, [14] but all pseudoautosomal genes escape X-inactivation and are therefore candidates for having gene dosage effects in sex chromosome aneuploidy conditions (45,X, 47,XXX, 47,XXY, 47,XYY, etc.).
Deletions have also been associated with Léri-Weill dyschondrosteosis [15] and Madelung's deformity.
Meiosis is a special type of cell division of germ cells and apicomplexans in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately result in four cells with only one copy of each chromosome (haploid). Additionally, prior to the division, genetic material from the paternal and maternal copies of each chromosome is crossed over, creating new combinations of code on each chromosome. Later on, during fertilisation, the haploid cells produced by meiosis from a male and a female will fuse to create a cell with two copies of each chromosome again, the zygote.
Chromosomal crossover, or crossing over, is the exchange of genetic material during sexual reproduction between two homologous chromosomes' non-sister chromatids that results in recombinant chromosomes. It is one of the final phases of genetic recombination, which occurs in the pachytene stage of prophase I of meiosis during a process called synapsis. Synapsis begins before the synaptonemal complex develops and is not completed until near the end of prophase I. Crossover usually occurs when matching regions on matching chromosomes break and then reconnect to the other chromosome.
Genetic recombination is the exchange of genetic material between different organisms which leads to production of offspring with combinations of traits that differ from those found in either parent. In eukaryotes, genetic recombination during meiosis can lead to a novel set of genetic information that can be further passed on from parents to offspring. Most recombination occurs naturally and can be classified into two types: (1) interchromosomal recombination, occurring through independent assortment of alleles whose loci are on different but homologous chromosomes ; & (2) intrachromosomal recombination, occurring through crossing over.
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.
A couple of homologous chromosomes, or homologs, are a set of one maternal and one paternal chromosome that pair up with each other inside a cell during fertilization. Homologs have the same genes in the same loci, where they provide points along each chromosome that enable a pair of chromosomes to align correctly with each other before separating during meiosis. This is the basis for Mendelian inheritance, which characterizes inheritance patterns of genetic material from an organism to its offspring parent developmental cell at the given time and area.
Y linkage, also known as holandric inheritance, describes traits that are produced by genes located on the Y chromosome. It is a form of sex linkage.
Gene conversion is the process by which one DNA sequence replaces a homologous sequence such that the sequences become identical after the conversion event. Gene conversion can be either allelic, meaning that one allele of the same gene replaces another allele, or ectopic, meaning that one paralogous DNA sequence converts another.
Madelung's deformity is usually characterized by malformed wrists and wrist bones and is often associated with Léri-Weill dyschondrosteosis. It can be bilateral or just in the one wrist. It has only been recognized within the past hundred years. Named after Otto Wilhelm Madelung (1846–1926), a German surgeon, who described it in detail, it was noted by others. Guillaume Dupuytren mentioned it in 1834, Auguste Nélaton in 1847, and Joseph-François Malgaigne in 1855.
The short-stature homeobox gene (SHOX), also known as short-stature-homeobox-containing gene, is a gene located on both the X and Y chromosomes, which is associated with short stature in humans if mutated or present in only one copy (haploinsufficiency).
Léri–Weill dyschondrosteosis or LWD is a rare pseudoautosomal dominant genetic disorder which results in dwarfism with short forearms and legs and a bayonet-like deformity of the forearms.
A sex chromosome (also referred to as an allosome, heterotypical chromosome, gonosome, heterochromosome, or idiochromosome is a chromosome that differs from an ordinary autosome in form, size, and behavior. The human sex chromosomes, a typical pair of mammal allosomes, carry the genes that determine the sex of an individual created in sexual reproduction. Autosomes differ from allosomes because autosomes appear in pairs whose members have the same form but differ from other pairs in a diploid cell, whereas members of an allosome pair may differ from one another and thereby determine sex.
Protein SOX-15 is a protein that in humans is encoded by the SOX15 gene.
Protein sprouty homolog 3 is a protein that in humans is encoded by the SPRY3 gene.
Prader-Willi/Angelman region-1, also known as PWAR1, is an exon of the lncRNA Small nucleolar RNA host gene 14 (SNHG14).
Serine/threonine-protein kinase PRKY is an enzyme that in humans is encoded by the PRKY gene.
Dehydrogenase/reductase X-linked also known as DHRSX is an enzyme which in humans is encoded by the pseudoautosomal DHRSX gene. DHRSX is a member of the short-chain dehydrogenase family of oxidoreductase enzymes.
GTP binding protein 6 also known as GTPBP6 is a protein which in humans is encoded by the pseudoautosomal GTPBP6 gene.
Arylsulfatase L is an enzyme that, in humans, is encoded by the ARSL gene.
Non-allelic homologous recombination (NAHR) is a form of homologous recombination that occurs between two lengths of DNA that have high sequence similarity, but are not alleles.
Short-stature homeobox 2, also known as homeobox protein Og12X or paired-related homeobox protein SHOT, is a protein that in humans is encoded by the SHOX2 gene.
