Methods for non-invasive prenatal ploidy calling
The present disclosure provides methods for determining the ploidy status of a chromosome in a gestating fetus from genotypic data measured from a mixed sample of DNA comprising DNA from both the mother of the fetus and from the fetus, and optionally from genotypic data from the mother and father. The ploidy state is determined by using a joint distribution model to create a plurality of expected allele distributions for different possible fetal ploidy states given the parental genotypic data, and comparing the expected allelic distributions to the pattern of measured allelic distributions measured in the mixed sample, and choosing the ploidy state whose expected allelic distribution pattern most closely matches the observed allelic distribution pattern. The mixed sample of DNA may be preferentially enriched at a plurality of polymorphic loci in a way that minimizes the allelic bias, for example using massively multiplexed targeted PCR.
1. A method for preparing a preparation of amplified DNA derived from a blood, serum, or plasma sample of a pregnant woman, the method comprising:
(a) isolating a mixture of free-floating maternal and free-floating fetal DNA from a blood, serum, or plasma sample from the mother of the gestating fetus;
(b) preparing a preparation of amplified DNA by performing targeted multiplex PCR amplification of at least 1000 loci from the free-floating maternal and fetal DNA using at least 1000 primer pairs in one reaction mixture, to obtain amplified products comprising a plurality of polymorphic loci that are likely to be informative based on the rate of single nucleotide polymorphism (SNP) heterozygosity in the human population for the locus, wherein a polymorphic loci is informative when the fetus carries an allele different from the mother or when the mother carries an allele different from the fetus, wherein each of the polymorphic loci is a SNP locus on a chromosome that is expected to be disomic;
(c) analyzing the preparation of amplified DNA by: (i) performing microarray analysis to measure the amount of amplified products derived from the fetal DNA using the measured amount of first and second alleles for each of the polymorphic loci on the chromosome that is expected to be disomic, without prior knowledge of fetal genotypes; (ii) performing microarray analysis to measure genetic data at a plurality of additional loci on the chromosome or chromosome segment of interest.
2. The method of claim 1 , wherein a maximum likelihood estimate is used to determine the amount of amplified products derived from the fetal DNA.
3. The method of claim 2 , wherein the maximum likelihood estimate is performed using a gradient descent method or a Newton-Raphson optimization method.
4. The method of claim 1 , wherein steps (c)(i) and (c)(ii) are performed in the same reaction.
5. The method of claim 1 , wherein the fetal DNA in the sample is not preferentially enriched over the maternal DNA before performance of the method.