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CHROMOSOMAL SEX DETERMINATION & HUMAN Y CHROMOSONE.
Term Paper ID:28997
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Essay Subject:
Scientific discussion of genetic material of & Chromosone.... More...
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7 Pages / 1575 Words
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Paper Abstract:
Scientific discussion of genetic material of Y Chromosone. Research on human genetics. Concept of sexual selection. Male & female traits. Genes on the Y Chromosone. & their crucial role. Implications of mechanisims of chromosomal sex determination.
Paper Introduction:
Review of Functional Coherence of the Human Y Chromosome
Introduction
In the current rush to map the human genome it is sometimes easy to assume that the modern science of genetics understands all there is to know concerning the functions of the human Y chromosome. This is far from reality. Because there has been such tremendous effort for an extended period of time on molecular biology, i.e.; research into AIDS, cancer, and auto immune diseases, certain aspects of human genetics have been under explored. This no longer is the case. The article by Lahn and Page make some salient points:
the wasteland model of Y chromosome content was based largely on anectodotal evidence (Lahn and Page, 1997, p 676);
at
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Vol. Third, different males in a population would have Ychromosomes with a different number of copies of each gene, or withdifferent sequence 'repeats', or even different combinations of copynumbers and sequences (Yen P.H., Chai N.N. Because there has been such tremendous effort for an extendedperiod of time on molecular biology, i.e.; research into AIDS, cancer, andauto immune diseases, certain aspects of human genetics have been under-explored. et al. Therefore, sexual selectioncould have favored genes on the Y chromosome that enhance malefertilization success because they spread unaltered through the male line(Fisher, 1981).Significance of Research The concept of sexual selection, vis-a-vis X and Y chromosomes, wasdeveloped to explain the evolution of traits that confer advantages incompetition over mates, often at the expense of survival rates (Andersson,M., 1994). The work of Lahn and Page has begun touncover how this process works. A putative gene named GrowthControl Y (GCY), with a marked effect on stature, has been identified inthe most proximal region of the long arm of the human Y chromosome (Salo P.et al.,1995). HumanReproduction, 12:5 3-5 8. (1997) Does sex determination start at conception? (1997) Microdeletions in the Y chromosome ofinfertile men. 199 . Andersson, M. Just W. 1997). and Page, D. Journal of BiologicalChemistry, p 266, Vol. (1997) Sex chromosomes and human growth. TB4Y encodesa Y isoform of thymosin 841, which functions in actin sequestration (Sater,Elzinga, Nachmais. (1981). Mammalian Genome, 8:756-759. London Ser. BioEssays, 19:1 27-1 32. There are ten genes on the human Y chromosome expressed exclusively inthe testes (Lahn and Page, 1997). and Zeh D.W.,1996). American Journal of HumanGenetics, 5 :3 3-316. RPS4Y encodes a Yisform of an essential ribosomal protein (Fignee, E. et al.,1992). Salo P. Chai N.N., Salido E.C. the growth of enamel) anddentinogenesis (i.e. at least 12 novel genes can be identified from the non-recombinant (NRY) portion of the Y chromosome. Lahn, B. Vogt P.H. Zeh J.A. In somecases, females gain direct benefits, such as food, paternal care and accessto high quality territories. Among mammals, these traits influence a male's competitive ability inmale-male contests and in sperm competition. One important feature of the genes on the Y chromosome is that theyshow a high degree of conservation across mammals, including marsupials.The best example is the testis-determining gene SRY that, with theexception of two species (Just W. Human Genetics, 1 1:1-5. 1992). (1997) Multiple functional copiesof the RBM gene family, a spermatogenesis candidate on the human Ychromosome. Sater, D., Elzinga, M., Nachmai, V. A dentalapproach. The existence of these Y-specificgenes involved in the control of spermatogenesis indicate a uniquefunctional coherence of the Y chromosome not seen in autosomes, which carrygenes with no common function (Lahn and Page, 1997). 278. (1997) A theory of mate choice based on heterozygosity.Behavioral Ecology, 8:6 -65. First, gene copies might act together,increasing the 'efficiency' of spermatogenesis. These genes probably play a crucial rolein the control of proliferation and/or differentiation of the male germline to form spermatozoa. AMLX transcripts are more abundant than theAMELY ones. (1993) Sex chromosome aberrations and stature:deduction of the principal factors involved in the determination of adultheight. Traits assumed to be important in male contests includeweapons (such as horns, antlers and enlarged canines) and large body size. Second, products of these genes have been found in germ cells atcrucial stages of spermatogenesis 18D21 or in mature spermatozoa (HabermannB. Biological Review, p 345. In contrast, female choice has been a controversial subject.Reluctance to accept the theory has prompted not only numerous empiricalstudies, but also an intense effort to develop mathematical models.Although the available evidence supports the view that females activelychoose their sexual partners, the implications of such choices remain to beexplained. This region, which constitutes about 95% of thehuman Y chromosome, does not exchange genetic material with the Xchromosome, in contrast to the recombination seen between homologousautosomes and between the pseudoautosomal regions of the X and Ychromosomes (Vogt P.H., 1997). It has been proposed that the Y chromosome has a region controllingstature (Ogata T. Inmammals, such genes spread through the male line and are related toejaculate quality and competitive ability, whereas in birds such genes passon through the female line and are predicted to be related to the abilityto choose male handicaps. In severalmammalian species, XY embryos grow faster than XX ones. One important characteristic common to all the testis-specific geneson the Y chromosome is that they occur in multiple copies and can bepolymorphic in their sequences (Chai N.N., Salido E.C. The search for products of gene expression has been guided by theassumption that they would be found preferentially in the testis (Lahn andPage, 1997). R. et al., 1997).Conclusion In conclusion, mechanisms of chromosomal sex determination provide anopportunity for the heterogametic sex to accumulate genes that arebeneficial to that sex, even at the expense of the homogametic sex. (1996) The evolution of polyandry I:intragenomic conflict and genetic incompatibility.Proc. (1992) The human enamel protein gene amelogenin isexpressed from both the X and the Y chromosomes. et al. Ogata T. T. and Salido E.C. The XY individualsare also developmentally more advanced than their XX counterparts at thesame gestational age (reflected in the number of blastomeres or somites,weight, body length and head size) (Erickson R.P., 1997). et al. the wasteland model of Y chromosome content was based largely on anectodotal evidence (Lahn and Page, 1997, p 676); . (1997) Azoospermic men with deletion of the DAZgene cluster are capable of completing spermatogenesis: fertilization,normal embryonic development and pregnancy occur when retrieved testicularspermatozoa are used for intracytoplasmic sperm injection. the growth of dentine) (Alvesalo L.,1997). Measurements of enamel and dentine thickness of permanent incisors andcanines in normal females and males, and in individuals with sex-chromosomeabnormalities, have suggested that the Y chromosome influences dentalgrowth by promoting both amelogenesis (i.e. Kempenaers B. (1997) Report of the Third International Workshop onY Chromosome Mapping 1997. M. M. pp 675-679. A. Cell Genet., 79:2-16. Review of Functional Coherence of the Human Y ChromosomeIntroduction In the current rush to map the human genome it is sometimes easy toassume that the modern science of genetics understands all there is to knowconcerning the functions of the human Y chromosome. Cytogenet. New England Journal of Medicine, 336:534-539. and Matsuo N., 1993). et al.,1997). et al. et al. For many years, it was assumed that the Y chromosome was a wastelandcarrying no genetic information apart from the sex-determining gene SRY.The work of Lahn and Page (among others) has revealed that there areactually more than 2 genes or gene families in the nonrecombining regionof the human Y chromosome. More genes might be discovered on the Y chromosome ifexpression products are looked for in other tissues, such as brain(Burgoyne P.S., 1998). 199 ). et al., 1997). Genomics, 45:355-361. Burgoyne P.S. October 24, 1997. (1998) The mammalian Y chromosome: a new perspective.BioEssays, 2 :363-366. et al. References Alvesalo L. (1997) The human DAZ genes, aputative male infertility factor on the Y chromosome, are highlypolymorphic in the DAZ repeat regions. C. Such traits enhance reproductive success, either by improving amale's ability to win intrasexual contests or by making a male attractiveto females. However, the benefits in other cases remainobscure, particularly when females receive little more than sperm frommales (Zeh J.A. (1992) Extra-pair paternity results from femalepreference for high-quality males in the blue tit.Nature, 357:494-496. Salido E.C. Yen P.H., Chai N.N. First, patients with a defective sperm productionand, therefore, a reduced sperm number (severe oligospermia) or a totalabsence of spermatozoa in the ejaculate (azoospermia) have microdeletionsin regions of the Y chromosome carrying these genes (Pryor J.L. 1991). (1998) DAZ (Deleted in AZoospermia) genes encodeproteins located in human late spermatids and in sperm tails. B, 263:1711-1717. (1995) Deletion mapping of stature determinants on thelong arm of the Y chromosome. T. However, the AMELX-AMELY genes are functionally unique becauseX inactivation can act on the X locus so that males have potentially moregene product than do females. et al., 1998). (1995) Absence of Sry in species of the vole Ellobius,Natural Genetics., 11:117-118. Second, deletions ormutations in one copy would not completely block spermatogenesis (MulhallJ.P. HumanReproduction, 13:363-369. One particularly controversial hypothesis proposes that females lookfor 'good genes', but the nature of such genetic quality remains elusive.Whereas some authors argue that males in a population will differ in theirintrinsic genetic quality and that females should aim to be fertilized bysuperior males 2D8, other authors believe that it is the degree of geneticcompatibility between a male and a female that matters (Brown, J. It is likely that this GCY gene is different from the genedetermining the Y-effect on embryo growth (Growth factor Y) described above(Erickson R.P., 1997). etal.,1995). Brown J.L. Cell, p 63. L. As mentioned, the genes that have been identified in this region ofthe Y chromosome fall into two categories; genes that are ubiquitouslyexpressed, and with a high degree of homology to genes present on the Xchromosome; and genes expressed only in the testis (thought to be involvedin the control of spermatogenesis), and that are only present on the Ychromosome (Kempenaers B. Manyof the X-homologous genes appear to be involved in cellular housekeeping,as suggested by their ubiquitous expression and by the functions of theencoded proteins, which are well established in three cases. Mulhall J.P. et al., 1995), is present in allmammalian species studied so far, although, interestingly, its sequence isnot highly conserved (Vogt P.H. Hershey, J. Human Genetics, 95:283-286. Soc. 12 5 Fisher, R. Growthof tooth buds during human development is promoted by a gene on the Ychromosome (AMELY) and an homologous gene (AMELX) on the X chromosome(Salido E.C. 4 29. et al. This is far fromreality. and Salido E.C., 1997). 1991. Thisleads to the following question: is there a relationship between the numberof copies of genes controlling spermatogenesis in different individuals andthe number of sperm, or sperm abnormalities, in those individuals?Future Research Directions Other genes on the Y chromosome, whose presence has been eitherinferred or actually demonstrated, include several that might also beimportant in sexual selection, such as those related to the control ofembryonic growth, stature and the development of teeth. and Zeh D.W. Human Genetics, 91:551-562 Pryor J.L. The effectof the Y chromosome on tooth growth could explain the expression of sexualdimorphism in size, shape and number of teeth (Alvesalo L.,1997). Science, Vol. This no longer is the case. Male-male competition was intuitively accepted as an importantprocess, and perhaps for this reason has not been the focus of muchempirical research. It is possible that GCY is also involved intooth development, because individuals with deletions in the regioncontaining this putative gene also have smaller teeth (Salo P. et al. et al. This GrowthFactor Y-effect has been shown to persist to mid-gestation. and Yen P.H. Habermann B. and Yen P.H., 1997).This has three implications. (1994) Sexual Selection, Princeton University Press. Annual Review of Biochemistry, p.6 . W. Erickson R.P. Their involvement in spermatogenesis is deducedfrom two sets of data. every gene (found) is expressed in a wide range of human tissues and each gene appears to exist in a single copy on the NRY (Lahn and Page, 1997, p 678).Further Results Because Lahn and Page drew their genetic material from the testis, thegene pairs uncovered seem to express themselves primarily in testis. The article by Lahn and Page makesome salient points: . E1FA1Y encodes a Y isoform of elf-1A, anessential translation initiation factor (Hershey, 1991). Accordingly each gene (found) has a homolog on the X chromosome encoding a very similar but nonidentical protein isoform; . Functional Coherence ofthe Human Y Chromosome. and Matsuo N. In this battle of the sexes, there seems to belittle the homogametic sex can do. Further searches would benefit from clearpredictions as to which genes are likely to be found on the Y chromosome. Fignee, E. 1991.
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