1.1. Mendelās Laws
In the beginning ā¦
āGenesis 1:1
We suspect that people have been curious about how heredity works ever since they figured out where babies came from. It is important to note that our current sophistication in these matters is of fairly recent origin. There is an old saying that ālike begets like,ā but this seemingly obvious knowledge that children will be like their parents might have been surprising to some ancient Greeks who wrote about the progeny that resulted from mating members of different species, such as swans and sheep.
Farmers have long known that animal offspring often appeared to be a mixture of both of their parents. Thousands of years ago it was proposed that children resulted from a blending process, the mixing of maternal blood and paternal semen derived from blood, with all aspects of the organism being represented in the semen and the menstrual blood. Many others saw blending as involving some essential essence or particles coming from every part of each parent, but each offered a slightly different mechanism.
For long periods in our history, many people imagined that children were the offspring of only one parent (either the mother or the father). Some thought that babies were preformed in the father and sailed in sperm down the vaginal canal into awaiting uterine incubators. Drawings dating back to the seventeenth century show the tiny preformed individuals (now known as homunculi) that the early microscopists imagined they saw inside sperm. There were other schools of thought in which children were preformed only in their mothers; the father was thought to provide only a āvital sparkā (much like jump-starting a dead battery). By the mid-nineteenth century, most people were willing to accept the concept that the traits observed in children were some mix of those observed in both parents and in both sets of grandparents.
In many cases, this was thought of in terms of a blending model of inheritance. Although there are many situations in which blending is not the best model of what is happening, blending was a concept that was easy to understand. If you mix red paint and white paint, you get pink paint, so why would such a mechanism not explain intermediate skin tones in someone who has one dark-skinned parent and one light-skinned parent? People imagined that there was some kind of substance, such as blood, that blended in the offspring to produce a mixture of traits in the child. (Note the term āblood relative,ā which implies a shared ancestry, not relationship by marriage.)
Still, there were some surprises that blending did not explain: blue-eyed kids born to brown-eyed parents, blond children of raven-haired moms and dads, kids who are taller than either parent, and so on. Blending, although it made some sense for some traits such as height and weight, did not explain many traits.
It was into this rather curious intellectual environment that Gregor Mendel was born in 1822 (Box 1.1). Like Galileo, Newton, Darwin, and Einstein, Mendelās vision would change the course of human understanding. That vision results from one simple set of experiments. As we tell you about Mendel, we will describe one of those magical moments in human cognition when a new set of concepts became beautifully obvious and clear.
Box 1.1
Mendelās Original WorksBecause Mendel lived so recently (1822ā1884), much is known about his life, his education, the world he lived in, and just how his discoveries were brought to light again many years after his death. Although we will describe enough about Mendelās experiments to help you understand his conclusions, and the way they formed the basis of the science of heredity, we cannot begin to fully describe either Mendel himself or his work. If you want to know more about him, or if you want to read his original scientific writings (in English or in German), check out Mendelweb at www.mendelweb.org. This site does an excellent job of making both Mendel and his science more accessible by means of annotating his works and providing links to helpful items such as glossaries, reference materials, and related sites. Even if you donāt want to read Mendelās writings in detail, it is worth checking out this excellent site.
What Mendel Did
Mendel was a monk with a garden plot who carried out a set of simple experiments that revolutionized our understanding of genetics. He worked with the pea plant (
Figure 1.1). His experiments took years and involved more than 10,000 plants. He chose to study the inheritance (the passing of a characteristic from one generation to the next) of seven simple and obvious traits that could clearly be distinguished between different pea strains:
ā¢ Seed shape ā round vs. wrinkled
ā¢ Seed color ā yellow vs. green
ā¢ Flower color ā white vs. colored
ā¢ Seedpod shape ā inflated vs. constricted
ā¢ Color of the unripe seedpod ā green vs. yellow
ā¢ Flower position ā along the stem vs. at the ends
ā¢ Stem length ā short vs. tall
These were simple āyes-or-noā traits and not quantitative traits such as weight that can vary over a wide range of different values. Some traits, such as stem length, can vary under different conditions such as rich vs. poor soil, so Mendel focused on simple binary traits ā traits that were āyesā (wrinkled) or ānoā (smooth) ā traits that did not change classification between āyesā and ānoā if the environmental conditions changed. For instance, when comparing tall and short plants, the tall plants were not all the same height, but they were so much taller than the short plants that the two categories were never mistaken for each other.
But perhaps the MOST significant aspect of Mendelās experiments was that, unlike any of his predecessors, he began with pure-breeding populations of plants. For example, he had a strain of plants with yellow seedpods that produced only plants with yellow seedpods when bred to each other. Similarly, he had a strain of plants with green seedpods that produced only plants with green seedpods when bred to each other (Figure 1.2).
Mendel crossed two plants by using pollen from one plant to fertilize another plant, and then studying the characteristics of the progeny and comparing those characteristics to the characteristics of the parental plants. Please note that in the first generation, when Mendel crossed plants with green seedpods to plants with yellow seedpods, all he saw in the progeny were plants with seedpods identical in color to those of the green seedpod parent. None of the plants had seedpods of an intermediate color (Figure 1.3).
This experiment helped rule out several of the old ideas about inheritance. A real adherent to blending would ha...