Page 27 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

2.1 The Puzzle of Inheritance 19

such as purple versus white flowers or yellow versus green Figure 2.8 The mating of parents with antagonistic
seeds antagonistic pairs, and he settled on seven such pairs traits produces hybrids. Note that each of the hybrids for the
for his study (Fig. 2.8). In his experiments, Mendel not seven antagonistic traits studied by Mendel resembles only one of
only perpetuated pure-breeding stocks for each member of the parents. The parental trait that shows up in the hybrid is known
a pair, but he also cross-fertilized pairs of plants to produce as the dominant trait.
hybrids, offspring of genetically dissimilar parents, for Antagonistic Pairs Appearance of Hybrid
each pair of antagonistic traits. Figure 2.8 shows the (dominant trait)
appearance of the hybrids he studied. Seed color (interior)
Fourth, being an expert plant breeder, Mendel care-
fully controlled his matings, going to great lengths to
ensure that the progeny he observed really resulted from Yellow Green
the specific fertilizations he intended. Thus Mendel Yellow
painstakingly prevented the intrusion of any foreign pol-
len and assured self- or cross-pollination as the experi- Seed shape
ment demanded. Not only did this allow him to carry out
controlled breedings of selected traits, but he could also
make reciprocal crosses. In such crosses, he reversed Round Wrinkled Round
the traits of the male and female parents, thus controlling
whether a particular trait was transmitted via the egg cell
within the ovule or via a sperm cell within the pollen. For Flower color
example, he could use pollen from a purple flower to
fertilize the eggs of a white flower and also use pollen
from a white flower to fertilize the eggs of a purple
flower. Because the progeny of these reciprocal crosses
were similar, Mendel demonstrated that the two parents
contribute equally to inheritance. “It is immaterial to the
form of the hybrid,” he wrote, “which of the parental
types was the seed or pollen plant.” Purple White Purple
Fifth, Mendel worked with large numbers of plants,
counted all offspring, subjected his findings to numerical Pod color (unripe)
analysis, and then compared his results with predictions
based on his models. He was the first person to study in-
heritance in this manner, and no doubt his background in
physics and mathematics contributed to this quantitative Green Yellow Green
approach. Mendel’s careful numerical analysis revealed
patterns of transmission that reflected basic laws of Pod shape (ripe)
heredity.
Finally, Mendel was a brilliant practical experimental-
ist. When comparing tall and short plants, for example, he
made sure that the short ones were out of the shade of the Round Pinched Round
tall ones so their growth would not be stunted. Eventually
he focused on certain traits of the pea seeds themselves, Stem length
such as their color or shape, rather than on traits of the
plants arising from the seeds. In this way, he could observe
many more individuals from the limited space of the mon-
astery garden, and he could evaluate the results of a cross
in a single growing season.
In short, Mendel purposely set up a simplified black- Long Short Long
and-white experimental system and then figured out how it
worked. He did not look at the vast number of variables Flower position
that determine the development of a prize ram nor at the
origin of differences between species. Rather, he looked at
discrete traits that came in two mutually exclusive forms
and asked questions that could be answered by observation
and computation.
Along stem At tip of stem Along stem