Table 1. Theoretical haplotype ratio calculations for a normal fetus according to the etiology of the non-disjunction error during meiosis.

Chromosome 21 (fetal)	Type of meiosis	HR = (|p1−p2)|/p3)
Normal/D21	---------------------	1
T21	Maternal meiosis I	0
T21	Maternal meiosis II, Paternal meiosis I	2
T21	Paternal meiosis II	0.5

HR, Haplotype Ratio; D21, Disomy21; T21, Trisomy 21.