1. A method for detecting the presence or absence of a chromosomal abnormality in a target individual, the method comprising: (a) measuring the amount of genetic material at multiple loci on a chromosome or chromosome segment of interest in a sample comprising DNA from the target individual; (b) comparing the amount from step (a) to either (i) a threshold value or (ii) an expected amount for a particular copy number hypothesis; and (c) identifying the presence or absence of a chromosomal abnormality based on the comparison.