Molecular, cellular, organismal, population, and evolutionary studies all have genetic
components, and build on traditional knowledge about genes. For molecular research,
the DNA sequence of a gene and its presence and role within the genome are critically
important. The sequence itself determines how a gene functions and impacts on the final
characteristics of the organism. In larger-scale research such as population and evolutionary
studies, both the transcribed DNA within a gene and the DNA that falls outside genes
(spacer regions) may be used to describe population structure. Often in a comparative
study, the sequences within the genes are highly conserved, that is, too similar in
makeup, and are therefore noninformative with respect to deciphering genetic relatedness.
In this respect, variable genetic information outside the genes is often more useful for
large-scale population studies. These DNA sequences are often used in various types of
DNA fingerprinting procedures to elucidate differences between populations. It should
be noted that there are differences of opinion on basic definitions of critical terms
such as “gene.” Unlike our definition, some scientists/researchers refer to the gene as
simply the coding region (without the DNA responsible for regulating gene expression).
Others have a broader view of the gene to encompass nearly any stretch of DNA.
Genetics is a dynamic field whose terminology can be confusing—almost like a rapidly
evolving language.
For plant reproduction, the most important genetic level is the chromosome, since
chromosomes are the largest units of DNA passed from parents to offspring (progeny).
In other words, this chapter is the story of chromosomes. In plants as in all eukaryotes
(organisms with a nucleus), chromosomes are linear pieces of DNA that have a single
centromere and two arms (Fig. 2.2a). Thecentromereis the constricted region of the
chromosome and serves as a connection between the chromosome arms. Centromeres
also play an important role in cell division, which is discussed later in the chapter. The
genes exist mainly on the chromosome arms. Different plant species vary widely in
Figure 2.2.Chromosomes have several physical states during the life of a cell: (a) chromosome phys-
ical states; (b) chromosome conformations; (c) homologous chromosomes.
2.1. GENETICS OVERVIEW 23