Figure 6.2: Characteristics of pea plants. ( 11 )ing offspring, theF2 generation, were tall, while 25% were short. Therefore, shortness
appeared to have skipped a generation. Mendel found this same mathematical result over
and over again with all the traits he studied. In all, Mendel studied seven characteristics,
with almost 20,000 F2 plants analyzed. For example, purple flowers and white flowers were
crossed to produce plants with only purple flowers in the F1 generation. Then after self-
pollination, the F2 generation had 75% purple flowers and 25% white flowers. These results
did not reflect what you would expect if the blending model of inheritance was correct.
Dominance
Mendel had to come up with a new theory of inheritance to explain his results. His expla-
nation, thelaw of segregation, is still one of the fundamental laws of modern genetics.
He proposed that each pea plant had two hereditary factors for each trait. There were two
possibilities for each hereditary factor, such as short or tall. One factor isdominantto the
other, meaning it masks the effects of therecessivefactor. However, each parent could only
pass on one of these factors to the offspring. Therefore, during the formation ofgametes,
sperm or egg, the heredity factors must separate so there is only one factor per gamete.
When fertilization occurs, the offspring then have two hereditary factors.
This law explained what Mendel had seen in the F1 generation, because the two heredity
factors were the short and tall factors and each individual in the F1 would have one of each
factor, and as the tall factor is dominant to the short factor, all the plants appeared tall. In
the F2 generation, produced by self-pollination of the F1, 25% of the offspring could have
two short heredity factors, so they would appear short. 75% would have at least one tall
heredity factor and will be tall.
In genetics problems the dominant factor is labeled with a capital letter (T) while the
recessive factor is labeled with a lowercase letter (t). If we designate the letterT ort to
represent the heritable factor, as each individual has two factors for each trait, the possible
combinations areTt,TT, andtt. Plants withTTwould be tall while plants withttwould be
short. SinceTis dominant tot, plants that areTtwould be tall, as with the F1 generation