Autosome - Wikipedia
The autosomes and the sex chromosomes differ in their evolutionary . than the relative difference normally found between males and females (36,37). . Work by D.H.S. has been funded by the Wellcome Trust, the National. Transcriptional silencing of the sex chromosomes during male meiosis (MSCI) is Furthermore, whereas the autosomes exhibit abundant transcription during . ( D and E) Relationship between YY synapsis and Uty transcription. .. respectively) acquired from the Wellcome Trust Sanger Institute, Cambridge, UK and. The sex chromosomes determine the biological sex of the child. Humans have 23 pairs of chromosomes; 22 pairs are the autosomes and the remaining pair.
Sexually dimorphic processes could be influenced by several mechanisms. First, a small number of X-linked genes are apparently expressed differently in male and female brains in mouse models.Autosomes V Sex Chromosomes - Mr H
Secondly, many human X-linked genes outside the X—Y pairing pseudoautosomal regions escape X-inactivation. To date, little is known about the process but clues can be gleaned from the study of X-monosomic females who are haploinsufficient for expression of all non-inactivated genes relative to 46,XX females. Finally, from studies of both X-monosomic humans 45,X and mice 39,Xwe are learning more about the influences of X-linked imprinted genes upon brain structure and function.
Difference Between Autosomes and Sex Chromosomes | Definition, Function, Related Genetic Disorders
Surprising specificity of effects has been described in both species, and identification of candidate genes cannot now be far off. There are estimated to be genes on the X-chromosome Ensembl version Why should there be such a concentration on this particular chromosome 1?
In particular, males are more likely than females to be influenced by haplotypes that are associated with exceptionally high abilities. For an equivalent reason, they are also more likely to show deficits in mental abilities than females because of the impact of deleterious mutations carried in haploid state. The hypothesis offers an explanation for the higher male variance in many aspects of cognitive performance 3. Genes on the X-chromosome not only influence general intelligence, but also have relatively specific effects on social—cognition and emotional regulation.
Subsequently, another family has been identified with similar phenotypic associations 6.
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We, therefore, have a potential explanation for the male preponderance of mental retardation in general, and for isolated heritable cases of autism in males, in particular.
But, we are still some way from understanding the wider male predisposition to a range of neurodevelopmental disorders including reading disabilities 7Asperger syndrome, which may be 10 times as common in males as in females 8 and attention deficit hyperactivity disorder 9.
This review shall consider the accumulating evidence that there are several genetic and epigenetic mechanisms that could influence the role of X-linked genes in sexual dimorphism, not only in humans but also in mice, and thus potentially in other mammalian species too.
Mechanisms of sexual dimorphism involving X-linked genes Genes on the X- and Y-chromosomes are of particular importance in the development of differences between the sexes, a fact that might at first sight appear self evident, because the mechanism for mammalian sex-specific differentiation involves the Y-linked gene SRY, but nature is not so transparent The Y-chromosome does indeed contain a substantial proportion of genes that are involved in spermatogenesis 11 We might reasonably suppose these are on the Y-chromosome because this is evolution's way of ensuring they are expressed only in males.
Surprisingly, many genes involved in spermatogenesis in mice are X-linked 13 and are expressed exclusively in males. How has this extraordinary situation evolved?
Hurst 10 proposes that an X-linked locus is at least three times more likely to be involved in sexual development than is a locus on an autosomal chromosome, especially if that locus is advantageous to males. Accordingly, the X-chromosome could function as a filter for sexually antagonistic alleles.
X-linked genes and mental functioning | Human Molecular Genetics | Oxford Academic
As the male-advantageous allele frequency increases on the X-chromosome, the proportion of females who are homozygous for that allele which is disadvantageous to them will also increase. Accordingly, deleterious gene-function will become suppressed in females.
Logically, we should not be surprised to find a male-biased expression of X-linked genes in clearly sexually dimorphic processes such as spermatogenesis. The same mechanism may apply to specific higher cognitive functions, if they are associated with some male advantage in adaptation 1. Similarly, if there are mutations in such specialized genes associated with impaired function, these will be manifested more commonly in males than in females.
Skewed patterns of X-inactivation may arise, which will influence the expression of recessive X-linked disease mutations in females. Skewing could also influence expression patterns of common allelic variants in genes that are subject to X-inactivation. There is some limited and controversial evidence to suggest that skewing of X-inactivation normally becomes greater with advancing age, but the implications of that observation if true are unknown.
The potential impact of Y-linked genes on sexual dimorphism is limited, because few different proteins are encoded by the Y-chromosome 12 All three have human homologues, but their role in human neural development is presently unclear, although worthy of investigation Fig.
These regions contain relatively few genes about 12 and are known as pseudoautosomal PAR. They show the monophyly of major taxonomic groups, including Collyrinae, Omina, and Cicindelini, the sister relation of Iresina and Theratina, and the paraphyly of Prothymina with respect to Cicindelina. The analysis also narrows down the sister relationships of Cicindelina, near the African genera Dromica and Prothyma.
Similar to what has been established in recent studies Vogler and Pearson ; Arndt and Putchkov 1the 18S rRNA data revealed two major deviations from the traditional taxonomy: Most nodes were well supported, as indicated by high Bremer Support values. But changes in gap costs resulted in substantial changes in topology.
Under high gap costs, basal groups were included within the derived taxa e. Removing these parts from the tree resulted in a set of nodes labeled with asterisks in fig.
Inclusion of 14 carabid outgroups recovered the Cicindelidae as monophyletic under all the gap costs tested, in an unexpected derived position within the Harpalini and Scaritini but consistent with more extensive studies of carabid phylogeny based on this gene Maddison, Baker, and Ober The inclusion of outgroups had a moderate effect on cicindelid relationships, in particular with respect to the Collyrinae, which appear in a more basal position under some alignment schemes.
But because of the great length differences of the outgroups, we use the outgroup information only as the basis for rooting the ingroup tree at the node separating the Mantichorini plus Omini from all the others fig.
Conflict on the Sex Chromosomes: Cause, Effect, and Complexity
This is the basal node in all the analyses with or without the inclusion of carabid outgroups, and it is always strongly supported. Using the new karyotype data and all existing information from the literature table 1major karyotypic features were optimized on the phylogenetic tree.
This analysis revealed a single inferred gain of the multiple sex chromosome system at the node that groups Collyrinae and Cicindelini fig. Two or possibly three independent losses of the multiple system can be inferred within Cicindela to accommodate C.
Because the latter two taxa group together in some of the most parsimonious trees, it is possible that their karyotypic similarity is due to common ancestry and, hence, the loss of the multiple system may be a trait of the wider Pentacomia-Odontocheila clade a large group of South American tiger beetles associated with tropical forest.
Further, this analysis revealed a continuous reduction of chromosome numbers from the basal Mantichora and Omini to the derived taxa in the genus Cicindela with 9, 10, or 11 or as few as 7, in the case of C. This coincides with a reduction in the number of rRNA gene clusters from the basal to the derived groups and a switch in localization from autosomes to heterosomes. But the position of rDNA appears very unstable and involves repeated switches between the autosomal and heterosomal locations in Cicindelina and appears confined to the autosomes in taxa, which are secondarily single-X.
In summary, the phylogenetic analysis favors a single origin of the multiple sex chromosome system in a common ancestor of Collyrinae and Cicindelini.
Several species with a single system contained in the multiple-system clade likely represent reversals to the original state, but the gross karyotype morphology resembles the derived type lower autosome counts, symmetrical karyotype, only two rDNA clusters. Denser taxon sampling still has to confirm that no species with multiple systems exist in the basal groups of Cicindelidae.
Accepting the single-origin hypothesis, the multiple-X system is remarkable within beetles for its evolutionary antiquity. Autosomal recessive diseases, however, require two copies of the deleterious allele for the disease to manifest.
Because it is possible to possess one copy of a deleterious allele without presenting a disease phenotype, two phenotypically normal parents can have a child with the disease if both parents are carriers also known as heterozygotes for the condition. Autosomal aneuploidy can also result in disease conditions. Aneuploidy of autosomes is not well tolerated and usually results in miscarriage of the developing fetus.
Having three copies of an autosome known as a trisomy is far more compatible with life, however. A common example is Down syndromewhich is caused by possessing three copies of chromosome 21 instead of the usual two.
If the duplication or deletion is large enough, it can be discovered by analyzing a karyogram of the individual.