Was Lewis just mapping them on a linkage map based on mutations?

Hi guys,

So I've spent a few days reviewing the genetic tools used in labs, but can't quite find the distinction between a few of the methods.

1. RT-PCR is used to determine if a specific gene is expressed; however, RNA-seq appears to do the same thing. I understand that RT-PCR uses a specific primer and thus tests the expression of one gene. Does RNA-seq use specific primers? How does this method amplify the cDNA through PCR? Also, given that mRNA is used, wouldn't the cDNA lack introns and thus not match to the DNA of the eukaryotic host? How could RT-PCR and RNA-seq be used to show the expression of genes if the cDNA doesn't match the original, intron laden, gene?

2. I was reading about Lewis, who worked with flies, and how he was able to map genes to specific chromosomes back in the 1930s. I'm curious how he was able to do this when the structure of DNA wasn't yet elucidated. From what I understand, he would use mutagenesis to find strange phenotypes and then use classical forward genetics. I still have some trouble understanding how he could map so many genes at such an early time. Was he just mapping them on a linkage map based on mutations?

Thank you everyone for your time!