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Mendel's Peas

Have you ever wondered why you have your mom's nose? Or your dad's eyes? Or your grandfather's hair? It turns out that, to find an answer, we need to start almost 200 years ago, with a monk in the Czech Republic.

Who was Gregor Mendel?

Gregor Mendel was born in what is now the Czech Republic in 1822. His family were farmers, and Mendel grew up gardening and studying beekeeping. As a young man, Mendel became an Augustinian friar, partially to continue his education without having to pay for his studies. While he was at the monastery, Mendel experimented with small plots of the common edible pea, and presented his work to a small group of unenthusiastic scientists in 1862. While he died in 1884 in relative scientific obscurity, his pea experiments gained appreciation over time, so much so that that now Gregor Mendel is considered the father of modern genetics.

If you want to read more about Gregor Mendel's early life and discoveries while at the monetary, please check out FamousScientists.org!

Why peas?

Although all of Mendel's experiments were carried out with pea plants, he wanted to understand something much more basic and common to all living organisms: the basis of inheritance, or what genetic information is passed between parents and offspring. Peas turned out to be a great model organism for studying inheritance for several reasons:

• Mendel could grow hundreds of plants at a time his small monastery garden

• There are several observable traits that peas can pass from parent to offspring like pea color, pea texture, and flower color

• Pea plants can actually self-breed, meaning that a single plant can act as both the mother and the father to generate its own offspring

Mendel began studying peas by observing traits like pea plant height, pea color, and pea shell smoothness. These characteristics are examples of phenotypes, a term used by geneticists to describe an observable trait. Human traits include phenotypes such as height, eye color, blood pressure, and blood type. In particular, Mendel was very interested in learning the ratio of plants with specific phenotypes. For example, one ratio he calculated was the humber of plants with yellow peas to the number of plants with green peas. In this case, yellow and green are two phenotypes for the trait pea color.

Let's calculate the ratio of yellow to green peas that Mendel observed.

You might notice that this number is very close to 3 (a ratio of 3:1 yellow:green peas). It turns out that this pattern was observed for many of the traits he measured.

Mendelian genetic inheritance

Based on this data, Mendel was the first to hypothesize what we now refer to as Mendelian genetic inheritance. He proposed that parents (mothers and fathers) each contribute half of their offspring's genetic material. We now know (although Mendel did not at the time), that this genetic material is deoxyribonucleic acid, or DNA. DNA makes up genes, which are units of heredity that can determine specific physical characteristics. That is, this genetic material controls the phenotype that we can observe; we refer to the genetic basis of the phenotype as the genotype. To put it simply, a genotype is the instructions in your DNA, while a phenotype is the trait you have because of those instructions.

In many organisms, including humans, there are two copies of every gene (one inherited from our mother, the other from our father). We refer to each version of a gene as an allele. For example, for a particular gene, let's say that one allele is A and the other allele is a. Then the genotype of that individual, for this particular gene, could be any combination of these alleles - AA, Aa, or aa. If an individual has two copies of the same allele (i.e. AA or aa), then that individual is said to be a homozygote, whereas an individual that has two different alleles (i.e. Aa) is called a heterozygote. These terms can be easily remembered, as "homo" means "same" and "hetero" means "different." Two different alleles can correspond to different phenotypes, e.g. green versus yellow.