Question

what are map units relative to?
distance between genes
gene expression
chromosome size
allele frequency

Explanation:

Map units, also known as centimorgans (cM), are used in genetic mapping. They are based on the concept of recombination frequency. Recombination frequency between two genes is related to the distance between them on a chromosome. The higher the recombination frequency (up to 50%), the farther apart the genes are. So, map units are relative to the distance between genes. Gene expression refers to the process of genes being transcribed and translated into proteins, which is not directly related to map units. Chromosome size is a physical characteristic of the chromosome as a whole, not what map units specifically measure. Allele frequency is about the proportion of alleles in a population, not related to the distance - based concept of map units.

Answer:

distance between genes