Order of terms from small to large: Nucleotide, Allele (one variant of a gene), Gene, DNA, Chromosome, Nucleus.
QTL - Quantitative Trait Loci - locations where multiple genes are affecting a phenotype. (We don't know what makes the QTL effect - Week 4 - 5 video lecture). QTL mapping is just a hypothesis, it is not conclusive.
That which led to QTL solved the battle between the Mendelian and biometrician models. The Mendelians concluded new traits came from mutation, biometricians refuted that theory based on their bell curve of the various traits. They only see 1 gene as the cause.
SNP - Single Nucleotide Polymorphisms. Genetic markers.
Homozygote vs. Heterozygote = genotypes AA vs. AO blood types (for example).