The Exchange of Impact Ejecta Between Terrestrial Planets (original) (raw)

models that assume stepwse mutaton of the STRP ~m p y that the varlaton of allele sizes Increases Inearly w~th tme (48) [D. 5. Godstein, A. Ruz Lnares, L. L. Cavali-Sforza, M. W. Feldman, Genetics 139, 463 (1995jl. Thus, we can est~mate R as the ratio of varances of allele szes (in base pa~rs) for alleles less than 110 bp (22 5/0.75 = 30.0), glvng a maxlmum age of 167,000 Y.B.P. Because we have considered only a slnge locus, the standard error on this varance-ratlo estmate IS high. However, the apparent lower boundary of 85 bp to the STRP allele slze results In an underestmate of the t~me of orlgn of the Au(-) chromosome in Afrca, so agaln ths estlmate of R is conservatve. 42. Despte a smaller effect~ve populat~on sze of A h -) chromosomes (because of a lower frequency), the mean varlance of STRP alleles on Alu(-j chromsomes among the 10 sub-Saharan Afr~can popua-t~ons is still substantial [64% of the mean varance of STRP alleles on Alu(+) chromosomes], suggest~ng that whle the orig~n of the Alu(-) chromosome IS more recent than the orig~n of the Alu(+j chromosome, it is st11 quite ancent. 43 C B. Str~nger, In The Origin and Evolution ofHumans and Humanness, D. T. Rasmussen Ed. (Jones and Bartett, Boston, 19931, pp. 75-94. 44 M. Slatk~n, Mol Blol. El/ol 12, 473 (1995). 45 Both the STRP and the Alu deletion polymorphism are located In noncodng regions of the CD4 gene and are unlkey to have any functional s~gn~f~cance. Except In the unlikely case of strong posltlve CISactng epstass of functorial varants flankng exons 1 and 2 of the 90-bp Au(-) chromosomes, seecton cannot expla~n the mantenance of the l~nkage d~sequlhbrum seen In non-Afr~can populatons because there would be nothing to prevent format~on of "non-90 bp" Alu(--) chromosomes by recombna-t~on between the markers or mutaton at the STRP, In addtion, the very low frequency of the Alu(-) chromosomes In Asa, the Pacifc islands, and the New World, as well as the very h~gh frequency of 85-bp Alu(+j and 11 0-bp Alu(+) chromosomes In all non-African populat~ons, argues aga~nst strong positive selecton for the Alu(-) chromosome.