Potential Bias Regarding Birth Weight in Historical and Contemporary Twin Data Bases

In this study we examine the hypothesis that monozygotic (MZ) twins in historical databases are less discordant for birth weight due to negative selection of severely discordant MZ twins. Furthermore, we test the hypothesis that MZ twins are less discordant for birth weight when comparing a volunteer based twin registry with a population based twin registry, due to selective registration. Data were available on 3927 twin pairs from the volunteer Australian Twin Registry born before 1964, 3059 volunteer twin pairs from the Netherlands Twin Register born 1987-1989 and 454 Belgian twin pairs from The East Flanders Prospective Twin Survey born 1987-1989. Intrapair relative birth weight differences (RBWD) were computed for MZ and dizygotic (DZ) twins from each twin registry. Comparing birth weight differences between MZ and DZ twins provides support for the hypothesis that MZ twins are subject to a negative selection in historical databases. Furthermore, Australian MZ twins have a lower RBWD compared to Dutch MZ twins when corrected for the RBWD of Australian and Dutch DZ twins, indicating circumstances which only affect MZ twins. Our hypothesis that MZ twins are less discordant for birth weight in a volunteer based twin registry compared to a population based twin registry had to be rejected. We suggest that investigators using historical databases to test the fetal origins hypothesis should be aware of this increased likelihood of selective exclusion of individuals with extreme morphometric parameters at time of birth

Number of X-linked androgen receptor gene CAG repeats and femininity in women.

Two studies used a sample of 300 female Australian monozygotic twins who had been genotyped for the X-linked androgen receptor gene and scored for the number of repeats of the triplet CAG. (A low number of repeats is associated with greater risk of prostate cancer in males and more effective transcription of androgens). In the first study, three measures of masculinity–femininity were constructed from the items of two personality questionnaires that had been taken by the members of a large twin sample (not, vert, similar3000 pairs). Two of the three measures, admitting to fears and worries and a willingness to break rules, were not significantly correlated with number of CAG repeats in the genotyped subsample. The third measure, confiding in others versus reserved, showed a small but significant correlation with CAG repeats in this female sample: fewer repeats went with scores in the reserved (i.e. masculine) direction. In the second study, CAG repeat scores were correlated with 90 questionnaire items related to female reproductive functions. Three items were associated with fewer repeats: age, having had a hysterectomy and length of labor at the birth of a second child. Because many items were screened, this was regarded as suggestive but not conclusive evidence of an association of CAG repeats with these reproduction-related traits

U-Pb LA-(MC)-ICP-MS dating of rutile: new reference materials and applications to sedimentary provenance

In response to the general lack of sufficiently abundant and high quality rutile UPb reference materials for in situ geochronology, we have characterised two new potential rutile ~ 1.8 Ga reference materials (Sugluk-4 and PCA-S207) from granulite facies belts of the Canadian Shield, namely the northern Cape Smith Belt of Quebec and the Snowbird Tectonic Zone (Sasatchewan). Characterisation includes ID-TIMS and LA-ICP-MS UPb dating, imaging, and trace element analysis. We compare these materials with existing rutiles used already (R19 and R10; and ) and show that the measured UPb compositions (i.e. including any common Pb) of our rutiles are considerably more homogeneous. This makes possible a UPb normalisation procedure (not reliant upon a common Pb correction) that results in a significant decrease in the uncertainty contribution from the common Pb correction and better reproducibility of reference materials and unknowns for provenance analysis and other applications. The reproducibility is 2–4% (2RSD) for 206Pb/238U and 207Pb/206Pb, only slightly greater than long-term data for zircon reference materials. We show in a rutile provenance study from young orogens (Bhutan Himalaya and Canadian Cordillera) that the sensitivity of our analytical set-up allows dating of ~ 90% of rutiles in a sediment using a 50 μm laser ablation spotsize within samples containing rutile as young as 10–20 Ma, and obviates the requirement for U concentration pre-screening, thus reducing or eliminating rutile selection bias. Unsuccessful analyses are due to poor quality rutiles with predominant common Pb, 207Pb signal below detection, or U content below ~ 1–2 ppm. We have used the ‘207Pb-method’ (using the Tera-Wasserburg diagram) to correct for substantial common Pb in very young and/or very low-U rutiles, rather than developing an on-line correction. Since rutile ages reflect mainly the time of cooling, rutile is a sensitive recorder of metamorphic thermochronological information and therefore is an excellent complement to detrital zircon U207Pb data. The contrast between zircon and rutile signatures in Himalayan samples with rutile as young as 10 Ma is shown to be very dramatic (most zircons from the same sample are > 480 Ma, with only a few grains or metamorphic rims reflecting Miocene metamorphism); as such rutile provides complementary information about the thermal events within the source regions of the grains. Rutile UPb dating is an underexploited provenance method with wide applicability to sedimentary provenance studies