change in alle frequency with selelction

[nena77]

I have a problem with the articel of Lebowitz in Theor Appl Genet, 1987
Trait based analysis for the detection of linkages between marker loci and quantitative trait loci in crosses between inbred lines.

in a F2 population with genotypes AA Aa aa at the QTL ( only one QTL is supposed to exist) the difference in allele frequency as a result of one step selection , carried out at both tail of the phenotypic normal distribution is
=iD/4
where i is the standardized selection differential
D= is the proportional effect of the QTL
why is it everythinf divided by 4?
for a BC population the same forumal would be divided by 8-.....why?
thanks

Elena

[CXCR4]

Do you think it can become natural selection