Fig 2. The distribution of SNPs in maternal plasma.

The x-axis and y-axis show the depth of major (A/a) and minor alleles (B/b), respectively. y = D + 3δ − x and y = D − 3δ − x are simplified as the form of x+y = u+3*δ, where D denotes the depth of the SNP loci. (x+y)*e of Line 2 y = (x + y)e + 3δ represents the depth of minor alleles of category 1 (AAaa) caused by sequencing errors. The expectation (u) of the normal distribution is equal to the variance (δ²) when the Poisson distribution is approximately close to the normal distribution. Thus, δ² is approximately equal to the mean of (x+y) in the equation y = (x + y)e + 3δ. Similarly, δ² will be roughly equal to D in the functions y = D + 3δ − x and y = D − 3δ − x.