A Gregor Mendel and Inheritance
This page is intended to assist in the basic understanding of the test cross experiments of Gregor Mendel (1822 - 1884)

The Mendelian Cross Experiments: Gregor Mendel conducted a series of seemingly simple test cross experiments with the garden pea pisum sativum though he also confirmed much of his data with beans as well. A test cross is a genetic tool to discover inheritance patterns. The procedure, with flowers, involves taking the male reproductive structures from one plant and pollinating another to determine genetic outcomes. The basic format for a test cross is as follows:

1. Cut the pedals from an unripened flower bud and remove the stamens (anthers). Cover the flower securely to prevent pollination. (see the generalized flower graphic below)
2. Once the ovary in the test flower has ripened, uncover the flower and pollinate with pollen from a desired plant. Recover the flower.
3. Harvest the seeds produced from the test cross for further experimentation.

The basic idea of a test cross is to control reproduction in order to observe the specific results without error. Pollen, landing on the sticky stigma, will grow a tube into the ovary to transfer sperm to the waiting egg. The fertilized egg, thereby becomes what we refer to as a seed.

One of the brilliant ideas Mendel had was to repeat these crosses in what is called a reciprocal cross. Mendel crossed one plant's female structure with another's males and also reversed the cross (male to female). Thus, he eliminated any idea that females, in terms of plants, were simple vassals and males were the ones producing all genetic components. Sounds simple, but in the 1850's this was cutting edge science. (see England's King Henry VIII for a fine example of genetic misunderstanding)

It could therefore be argued that Gregor Mendel was a pioneer feminist scientist.

Mendel's Data: The monastery in Abbey of Saint Thomas in Brno (currently in Slovakia) was surrounded by farm land. As farmer need to understand their crops, Mendel was able to build on a wealth of common knowledge about plant inheritance. So, as is often the case, Mendel used science to describe phenomena already understood by scientific laymen. Or, put another way, he described inheritance patterns discovered by a host of intelligent farmers who used science to understand their crops. For a farmer, there is money in understanding your crops.

Mendel used his understanding of statistics and probability to describe the reproduction of peas. As seen to the right, he found that specific traits were linked, acted independently, and showed an understandable dominance hierarchy.

Basically, two traits bred together would reveal one or the other. The revealed trait was called "Dominant" while the other hidden trait was called "recessive". When these second generation plants were self-crossed, they revealed a 3:1 ratio of Dominant to Recessive traits. Thus, as an example, when your grandmother tells you that you have your grandfather's eyes and they "skip generations", her knowledge is backed up by Mendel.

Making Sense of Mendel's Data: Understanding inheritance in peas is one thing, but we wouldn't be talking about these experiments if they didn't help us understand the world at-large. Thus, this is important because it works in peas, but much more important because it works within inheritance system throughout life on Earth. Mendel's work is therefore an important topic today, because it is a proven unifying theme in biology.

Mendel would go to his death in 1884 as his work sat unattended in science libraries throughout Europe. Thirty years would pass, after Mendel's work was finished until his papers would be rediscovered. At the end of the century and until 1908 a ragging scientific argument between supporters of Mendel's work (see W. Bateson) and supporters of eugenics would be settled in Mendel's favor.

Monohybrid Data: The basic data tends to set the theme here. Monohybrid crosses (looking at one trait) can be explained simply by using the format seen at right and a nifty tool called a Punnett Square. It helps to remember that Mendel worked with traits that were "Completely Dominant" over the recessive. Whether by luck or design, the complete dominance system does a good job of describing genetic inheritance writ-large.

Phenotypes are the structural expression of a genetic trait. The genotype is what causes the physical expression of that trait. The genotype was something unseen to Mendel.

Through careful experimentation, Mendel controlled the reproductive process. Two linked traits, called alleles, revealed a dominance relationship in the F1 generation. Mendel also discovered that the outcome of these experiments were the same without regard to the sexual make up of the plants. The F2 generation showed that the recessive trait remain hidden, but reemerged intact. Thus, there was no "mixing" of traits.

The Dihybrid Cross: Looking at two different traits confirms Mendel's ideas as well. As probability is a mathematical exercise in multiplication, it follows that adding another trait would multiply the possible outcomes. Thus, the monohybrid chance of four becomes the dihybrid chance of 16. The Phenotypic ratio of 3:1, squared, equals 9:3:3:1 and the genotypic ratio of 1:2:1 becomes 1:2:1:2:4:2:1:2:1. We can confirm this with by decifering the results of a punnett square.

It is important to remember that Mendel worked with a complete dominance system. Adding either an incomplete dominant (Red x White = Pink) or a codominant trait (Red x White = White w/Red) changes the phenotypic outcome, but the genotypic ratio remains.

Understanding the explanations that Gregor Mendel brought, tells us a great deal about the way organisms reproduce. The takes us a long way toward understanding the history of life of Earth. We still need to understand how these systems randomize genetic material and what exactly that genetic material is.

Science is a thing that makes us Human