Mutant

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"According to Goldschmidt, all that evolution by the usual mutations—dubbed "micromutations"—can accomplish is to bring about "diversification strictly within species, usually, if not exclusively, for the sake of adaptation of the species to specific conditions within the area which it is able to occupy." New species, genera, and higher groups arise at once, by cataclysmic saltations—termed macromutations or systematic mutations—which bring about in one step a basic reconstruction of the whole organism. The role of natural selection in this process becomes "reduced to the simple alternative: immediate acceptance or rejection." A new form of life having been thus catapulted into being, the details of its structures and functions are subsequently adjusted by micromutation and selection. It is unnecessary to stress here that this theory virtually rejects evolution as this term is usually understood (to evolve means to unfold or to develop gradually), and that the systematic mutations it postulates have never been observed. It is possible to imagine a mutation so drastic that its product becomes a monster hurling itself beyond the confines of species, genus, family, or class. But in what Goldschmidt has called the "hopeful monster" the harmonious system, which any organism must necessarily possess, must be transformed at once into a radically different, but still sufficiently coherent, system to enable the monster to survive. The assumption that such a prodigy may, however rarely, walk the earth overtaxes one's credulity, even though it may be right that the existence of life in the cosmos is in itself an extremely improbable event."

- Mutant

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"It is a generally accepted assumption that sporadic pregnancy losses occurring before an embryo has developed represent a ‘physiological’ phenomenon, which prevents conceptions affected by serious structural malformations or chromosomal aberrations incompatible with life from progressing to viability. This concept is supported by clinical studies in which embryoscopy was used to assess fetal morphology prior to removal by uterine evacuation. Fetal malformations were observed in 85% of cases presenting with early clinical miscarriage. The same study also demonstrated that 75% of the fetuses had an abnormal karyotype. Fetal chromosomal aneuploidies arising from non-inherited and non-disjunctional events are common. Indeed, in a recent study using comparative genomic hybridization to study the chromosomal complement of all blastomeres in preimplantation human embryos, more than 90% were found to have at least one chromosomal abnormality in one or more cells. The clinical implications of minor, mosaic and possibly ‘transient’ aneuploidies remain unclear. However, while most fetuses with severe developmental defects will die in utero some aneuploidies can be compatible with survival to term. The most commonly encountered is trisomy 21, although 80% of affected embryos perish in utero or in the neonatal period. In most cases, the extra chromosome is of maternal origin and caused by a malsegregation event in the first meiotic division. The risk of this increases with maternal age and may be considered to be a biological rather than pathological phenomenon. Although fetal chromosomal aberrations may be identified in 29% to 60% of cases in women with RM, the incidence decreases as the number of miscarriages increases suggesting other mechanisms as a cause of the miscarriage in RM couples with multiple losses."

- Mutant

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"6.2 Genetic factors The finding of an abnormal parental karyotype should prompt referral to a clinical geneticist. Genetic counselling offers the couple a prognosis for the risk of future pregnancies with an unbalanced chromosome complement and the opportunity for familial chromosome studies. Reproductive options in couples with chromosomal rearrangements include proceeding to a further natural pregnancy with or without a prenatal diagnosis test, gamete donation and adoption. Preimplantation genetic diagnosis has been proposed as a treatment option for translocation carriers. Since preimplantation genetic diagnosis necessitates that the couple undergo in vitro fertilisation to produce embryos, couples with proven fertility need to be aware of the financial cost as well as implantation and live birth rates per cycle following in vitro fertilisation/preimplantation genetic diagnosis. Furthermore, they should be informed that they have a higher (50–70%) chance of a healthy live birth in future untreated pregnancies following natural conception than is currently achieved after preimplantation genetic diagnosis/in vitro fertilisation (approximately 30%). Preimplantation genetic screening with in vitro fertilisation treatment in women with unexplained recurrent miscarriage does not improve live birth rates. Preimplantation genetic screening in conjunction with in vitro fertilisation has been advocated as a treatment option for women with recurrent miscarriage, the rationale being that the identification and transfer of what are thought to be genetically normal embryos will lead to a live birth. The live birth rate of women with unexplained recurrent miscarriage who conceive naturally is significantly higher than currently achieved after preimplantation genetic screening/in vitro fertilisation (20–30%)."

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"Genetic bases of defective chromosome content or structure In a biological sense, genetic defects leading to abortion may originate from two partners: The conceptus and the uterus. From the conceptus side, it is well established that chromosomal anomalies lead to implantation defects. However, they occur mostly stochastically; to be the genetic cause of a recurrent loss, a genetic defect leading to systematic chromosome anomalies has to be found, as nicely reviewed recently by Kurahashi and coworkers. Abnormal chromosome structure may originate from defects in the checkpoint mechanisms that constitute a quality control factory of the cell, which is able to detect meiosis anomalies and is called Spindle Assembly Checkpoint (SAC), constituted by pro teins such as Mad1, Mad2, Bub1, Bub3, BubR1, and Mps1. A very recent study addressed this question on the general issue of defective reproduction performance, by screening two candidate genes, aurora kinase B (AURKB) and synaptonemal complex protein 3 (SYCP3). In the AURKB gene, the authors identified low frequency (0.5%) non synonymous variants (c. 155C.T, c. 236T.C, and c. 880G.A) inducing the changes p.A52V, p.I79T, and p.A294T in the polypeptidic chain, respectively. 236T >C and 880G >A were associated with antecedents of pregnancy losses. This preliminary study is one of those that pave the way toward evaluating genes playing a role in the quality of the meiotic progression. Two other genes have been investigated to this respect in miscarriage, BUB1 and MAD2. The authors showed firstly that as generally assumed, half the embryos from spontaneous miscarriages have an abnormal karyotype. By western blotting, the authors quantified the two proteins and estimated that they were under expressed in pathological cases. The authors also isolated chorionic villi where they targeted BUB1 and MAD2 expression by short hairpin RNA (shRNA) and showed that this induces an abnormal karyotype at a ~ sixfold increased frequency, compared to controls (indicating that mitotic defects can also be induced when these genes are dysfunctional). The role of MAD2 in such defects was independently confirmed by another study on trisomic abortuses. Clearly, therefore, anomalies in spindle assembly can lead to aneuploidies, and infertilities in the most severe cases, such as in the case of the recently identified mutation of the cohesin STAG3, leading to premature ovarian failure, but less severe mutations in the SAC genes could lead to variants that have more subtle effects. There is clearly space for studying this aspect of spindle stability on a wider basis than what has been performed up to now in the context of recurrent miscarriage."

- Mutant

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