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Leth of fingers and penis are related through fetal Hox gene expression

TO THE EDITOR: In their study of the size of the male external genital organs and their relations to other body measures, Spyropoulos and colleagues 1 reported statistically significant positive relations of penile shaft length, glanular length, and total penile length with index finger length (Pearson correlation coefficient r = 0.324, 0.330, and 0.339, respectively). The authors were unable to provide an explanation for this observation. In two editorial comments, the finding was discounted as being likely spurious 2 or of questionable relevance for urologic practice and counseling. 3 Here we present an explanation for their finding which renders it unlikely to be spurious and which further may have practical implications. Across vertebrate species, including humans, the Homeobox ( Hox ) genes regulate limb development, including fingers and toes, as well as urinogenital system development, including the penis. 4 Therefore morphologic patterns of the fingers may be related to morphologic patterns of the external genitalia. Specifically, there is evidence that the human 2D:4D, ie, the relative length of the second digit (2D, index finger) to the fourth digit (4D, ring finger), is an early established (end of first trimester) sexually dimorphic trait. 5 Cross-racially, men tend to have shorter index fingers relative to ring fingers (ie, lower 2D:4D) than do women. This is in keeping with evidence that 2D:4D is negatively related to prenatal testosterone concentrations and positively related to prenatal estrogen exposure. Penis growth is also influenced by prenatal testosterone. Since a low 2D:4D is a pointer to high fetal testosterone exposure levels, it seems counterintuitive that 2D is positively related to penis length. However, we expect that 4D, not measured by Spyropoulos and colleagues, is even more strongly positively related to penis length. It only requires this hypothesized difference in slope, ie, in the positive relations of penis length regressed on 2D compared with penis length regressed on 4D, for there to be a negative relation between 2D:4D and penis length. This is a testable hypothesis for future research. Prenatal sex steroids have effects on the heart, liver, and brain in addition to the urinogenital system. 2D:4D has been reported as being related to a growing number of sex-dependent variables, including behavioral traits (eg, autism, depression, handedness, homosexuality, impulsivity, musical talent, sporting prowess, and visuo-spatial ability), adult-onset diseases (eg, breast cancer, coronary heart disease, and myocardial infarction), and fertility-related measures (eg, birth weight, family size, offspring sex ratio, and sperm counts). 5 Thus relations of finger length patterns and penis length are not surprising and may have practical implications in urology.</P> http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Urology Elsevier
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