Introduction to Genetics
The first person to uncover how heredity works was Mendel
This may not seem like an important point to you now, but it was a huge scandal back in the 1800s. You see,
But Mendel continued his investigation anyway, choosing to study traits in pea plants. He specifically examined those pea plants traits that had only two possible versions-- like flower color (but no other flower colors besides white or purple were ever seen). Figure 9.5 in your book shows you some of the traits in pea plants he examined.
Nowadays, we talk about "Mendelian Genetics" as the topic within genetics that allows us to understand inheritance of traits. But in Mendel's day, his theories were not widely accepted (he was still considered a bit of a heretic), and he died never knowing that he would one day be called the "father of genetics."
Why bother with pea plants? Why not study humans?
It may seem like Mendel's studies would have been a lot more interesting if he had just studied humans. None of you probably care about pea plants. But, to study something, one has to be able to observe things objectively and precisely... and people themselves, when describing their own families and parentage, are not always so objective or precise. Also, in studying heredity, it is best if one can study an organism that reproduces quickly and easily-- pea plants are much better at that than people! Nowadays, many scientists study fruit flies for these reasons when studying genetics.
How does one understand the family trees of pea plants?
To do this, one has to be able to know who the parents of every organism are. That means, that you have to understand how plants reproduce (at least a little bit) to understand this. Pea plants have both male and female reproductive organs in their flowers. Mendel figured out that if a pea plant flower was left alone, the male reproductive part (the anther) would fertilize the female reproductive part (the carpel). The anthers contain pollen, which is actually just a package of sperm. People normally think of sperm as swimming cells, but plants are not in water and have to send their sperm through air. So plants package their sperm into pollen. When the pollen lands on the stigma of the carpel, the sperm within the pollen fertilize the eggs within the ovules at the base of the stamen. This fertilization is what leads to a new pea plant seed which will grow into a new pea plant organism. When pollen is transferred by bees or small animals or air or even by us, that is called pollination.
Therefore, if a pea plant flower is left alone, it will self-fertilize. That is, it will mate with itself. We talk about this mating as a cross. So a pea plant can cross with itself and produce new pea plants. Alternatively, Mendel was also able to cross-fertilize pea plants, by pollinating one plant's stamen with the pollen from a different plant. This is shown in Figure 9.4.
Understanding pea plant traits and general characteristics
When self-fertilization is allowed to occur for a long time, the pea plant ends up always producing offspring that look just like it. Mendel considered this kind of pea plant to be true-breeding. We sometimes use the term "purebred" to mean this today. But when cross-fertilization was carried out, Mendel could not initially predict what the next generation (the offspring) would look like. But as he figured out how heredity worked, he learned to predict certain things about the pea plants that would come in the next generation.
In the same way, you will learn to understand how heredity works enough to make some predictions about those you know and their children. For example, before my two nephews were born, I was able to tell my sister that there was a 50/50 chance that each child would have hazel eyes, and a 50/50 chance that they would be color blind. It turns out that both nephews, Elliot and Anthony, have brown eyes and are not color-blind. So, was I wrong? Read about the "Rules of Probability" (on page 151) to see.
© 2006 STCC Foundation Press, content by Dawn A. Tamarkin, Ph.D.
Last changed: September 05, 2007