This makes sense because homologous chromosomes separate during Meiosis I, and the chromatids separate in Meiosis II, so ultimately there is only one allele going to any particular gamete. DNA is a molecule that can get altered. Sometimes, alterations are permanent. That is, its new sequence gets incorporated into new DNA when the DNA gets replicated, or some DNA gets moved. We'll talk more about this in the next section of the course.
Autosomes are the non-sex chromosomes.
How to make a genetic map. I it could exchange with its homolog and recieve a dominant allele for one particular gene. Now it would give rise to a different phenotype than it would have if no crossing over had occurred. By analyzing the rate of these allele exchanges, people can infer the order and often pretty well calculate the distances between genes. Crossing over should happen more often for genes further apart on a chromosome, than for genes adjacent to one another.