I mean, how can you tell which gene in someone’s genome is gene TP53 when they’re all “mixed up” with thousands more?
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The genes usually all appear in their normal places on their normal chromosomes, so you where to find them. But since you can’t look at them with a microscope to do genotyping, that’s not very helpful!
For something like TP53, it requires some way of detecting. One is sequencing with PCR primers for the gene or elements right outside the gene. Another, if you’re looking for one of a set of known alleles, is to use labeled probes that will bind to only a particular allele. That works best when there are single nucleotide polymorphism, or SNPs. The probe can bind …CTTG… but not …CTAG… in the gene, which then produces some detectable signal like fluorescent dye. Actually a longer probe sequence, but that’s the idea.
For that process you don’t need to have any idea where the gene is. You dump all the genetic material from cells and then dump primers or probes in with it and they’ll bind to each other by randomly colliding.
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