families’ questions about the probabilities of passing on a genetic disorder. When genetic
counselors sit down with families to discuss these types of questions, it’s amazing that their
answers are derived from the fundamentals of genetics discovered by a monk in the 1800s.
Figure 6.1: Gregor Mendel ( 9 )Mendel’s Experiments
The laws of heredity were first developed by an Austrian monk, Gregor Mendel (Figure6.1),
in the 1800s. To study genetics, Mendel chose to work with pea plants because they had
easily observable traits and a short generation time (Figure6.2). For example, pea plants
are either tall or short, which are easily identifiable traits. Furthermore, peas can either self
pollinate or be cross-pollinated by hand, by transferring the pollen from one flower to the
stigma of another. In this way, Mendel could carefully observe the results of crosses between
two different types of plants. He studied the inheritance patterns for many different traits in
peas, including round seeds versus wrinkled seeds, white flowers versus purple flowers, and
tall plants versus short plants. Because of his work, Mendel can be considered the father of
genetics.
During Mendel’s time, most people believed that traits were contributed from both parents
and blended together as they were passed down from generation to generation. For example,
if you crossed a short plant and a tall plant, they would expect the offspring to be medium-
sized plants. What Mendel observed, however, was that the offspring of this cross (called the
F1 generation,derived from the Latin termfilius,meaning sons and daughters) were all
tall plants. Based on the blending hypothesis, the result of all tall plants was unexpected.
Next, Mendel let the F1 generation self-pollinate. He then noted that 75% of the result-