In the small reproductive structures (pollen grains and ovaries), the haploidgametophyte
stage is present and gives rise to haploid sex cells. Even with the diversity of chromosome
numbers observed among plant species, eukaryotic chromosomes function under the same
rules during cell division. During normal cell division (mitosis) in the sporophyte, the
chromosome number is maintained in the diploid state. During gametophyte production
(meiosis), the two copies of each chromosome separate from one another and produce
cells with half the normal number of chromosomes. All the variations of reproductive
mode are simply complexities of how the two homologous chromosomes come together
during the process of reproduction.
2.2 Mendelian Genetics
Gregor Mendel, a member of the Augustinian monastery in what is now the current Czech
Republic, was the first person to describe how chromosomes are transmitted between gen-
erations (Fig. 2.3). Mendel combined what are now considered typical plant breeding pro-
cedures, such as keeping accurate records of the characteristics that appeared in the
offspring of selected parents and the control of pollination of the experimental plants,
with statistics to describe how traits behave over generations. The molecular basis of gen-
etics was not understood in the 1800s, but Mendel observed and recorded the phenotypic
traits within the plants that he grew on the monastery grounds. Thephenotypeis the phys-
ical appearance of an organism, and thegenotypeis the underlying genetic makeup of an
organism. Using pea plants (Pisum sativum), Mendel was able to track the segregation of
traits over generations, and thus indirectly described the laws of how chromosomes act
within cells. He accurately described the cellular process of chromosomal segregation
Figure 2.3.Gregor Mendel was the father of genetics.2.2. MENDELIAN GENETICS 25