2.1 Genetics Overview
The field of genetics impacts all aspects of the science of biology, but individual disciplines
within biology utilize different types of genetic information. In order to discuss plant repro-
duction specifically, several universal genetic definitions must be introduced. In its simplest
definition, the field ofgeneticsis the study of genes.DNA(deoxyribonucleic acid) is the
genetic material in organisms that stores all the information that encodes for life. The
sequence ofnucleotides(DNA building blocks: A, C, G, T) stores the instructions to
produce proteins and information that allows for the regulation of the genetic material.
TheDNA sequenceserves as a type of software or programming language that the cell
uses to produce and regulate all the necessary products for life. DNA exists as a double
helix, and each nucleotide is paired with its complementary base making abase pair
(adenine withthymine,cytosine withguanine). For this chapter, ageneis a contiguous
sequence of DNA that contains regulatory regions and a sequence that encodes for a
protein. Many sequences in the genome of an organism are outside this definition of the
gene, and in fact, much of a plant’s DNA would not be considered as part of a gene. At
the next level of genetic organization are thechromosomes, which are discrete molecules
of DNA and associated proteins that reside within the nucleus. The chromosome-associated
proteins help package and condense DNA for packing into the nucleus of a cell.
Thegenomeof an organism is the entire sequence of DNA inclusive of all the chromo-
somes. DNA is also present within certain cellular organelles: the mitochondria and
chloroplasts. Plants therefore contain three distinct genomes (the nuclear, mitochondrial,
and chloroplast genomes), and this chapter focuses specifically on the DNA contained
within the nucleus. If we draw an analogy comparing genetics to the structure of this
book, nucleotides are similar to letters that form three-letter words. Genes are similar to sen-
tences, and chromosomes are similar to chapters. The genome is similar to the complete
book, and a library would be a collection of different species (see Chapter 6 for detailed
explanation on molecular genetics).
Figure 2.1.In many ecosystems on Earth, plants change the color of the land to shades of green.22 MENDELIAN GENETICS AND PLANT REPRODUCTION