PCR, polymerase chain reaction, is the fastest way to artificially (in a laboratory) multiply few copies of DNA into many, in order to create enough of a sample to use for other genetic testing.
It is used quite often in forensic science, such as magnifying a small recovered sample of DNA from body fluids or tissue samples at a crime scene, into a vastly greater quantity of DNA for identification of suspect(s) or victim(s).
You can determine the genotype of Maria and Juanita, even though you did not test their DNA is by looking at the traits of their parents. If they have other siblings, the ratio of the traits can give the actual genotype of their parents which in turn, can help determine the genotype of Maria and Juanita. This is assuming that the genotype in question corresponds to an observable physical trait.
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