A Laboratory Exercise Demonstrating Chromosome Morphology and Number

Abstract

During the last ten years there have been many dramatic changes in the biological science course content at both the secondary school and undergraduate college level. There has also been a change in emphasis regarding the types of laboratory exercises which accompany the updated course work. In keeping with advances in biological research, increased emphasis is being placed on a molecular and cytological approach, while the anatomical-phylogenetic approach in biology teaching is being de-emphasized. The course content is advancing rapidly, but there is a very definite need for attention to investigatory problems that enhance the comprehension of this new subject matter. Both the BSCS Yellow Version, An Inquiry into Life (1), and the experimental edition of the Blue Version (2) contain laboratory exercises involving the use of Allium cepa (onion) root tip squashes to demonstrate mitosis and polyploidy. Both exercises neglect to suggest any additional uses to which such squashes may be put, namely, the demonstration of chromosome morphology and number. This paper will report specific directions for preparing onion root tip squashes in which chromosome morphology and number may be demonstrated. These directions are based on the authors' evaluation of pertinent literature and on their own investigations.