Recall the major symptoms and physical abnormalities associated with each. Recall the sex ratio associated with each. Before we go on, here is your chance to skip this introductory tutorial. If you have decided to do the tutorial, we will begin with consideration of genes and chromosomes. Genes cannot be seen with an optical microscope, but chromosomes can.
Chromosomes are strings of genes, covered by a protein coat.
Chromosomes are only visible during cell division, as shown in Fig.1. They appear as dense colored bodies, the particular color of which depends upon the stain used to prepare the cells for microscopy. The form of the chromosome is made up of a tangled mass of knotty strands. Each strand seen here is actually a protein coat which covers the individual genes, which are themselves made of long, coiled DNA molecules. It has been estimated that the untangled, extended DNA from a single human cell would be about one yard in length.