Johann Mendel was born to a poor peasant family in Austria in 1822. He was bright and determined to pursue a higher education, even if it meant sacrificing everything, including food. His life was hard, as he grew up poor, then almost starved because he couldn’t afford food while paying for school. He joined a monastery and changed his name to Gregor Mendel. He tried to become a teacher twice, but both times was rejected because he had too much original thought! For eight years, he performed experiments with pea plants, breeding and crossbreeding them until he developed four conclusions, which he published in a paper. Unfortunately, most scientists of the time ignored Mendel’s groundbreaking paper, and then Mendel abandoned his scientific work to defend the religious freedom of the monasteries. In 1884, he died as a little-known monk who had written an obscure paper.
However, by the 1930s, Mendel’s paper was the foremost paper on genetics, the newest interest of the scientific community, and today Gregor Mendel is forever remembered as the father of modern genetics. So what were Mendel’s experiments, and what were the four conclusions that he listed in his groundbreaking paper?
Mendel’s scientific experiments with pea plants, spanning eight years, studied the traits of parents as compared to the traits of the offspring when the parents were crossed. Mendel used plants that bred true, which means that they always produced offspring with the same traits as themselves. For example, he had tall pea plants that always produced tall pea plants, and he had short pea plants that always produced short pea plants. However, when he crossed the true-breeding tall pea plants and the true-breeding short pea plants, they always produced tall pea plants. This was the same for other traits as well. Axial flowering plants crossed with terminal flowering plants always produced axial flowering plants. Plants with green pods crossed with plants with yellow pods always produced plants with green pods. Plants with yellow peas crossed with plants with green peas always produced plants with yellow peas. Plants with smooth peas crossed with plants with wrinkled peas always produced plants with smooth peas.
Mendel then performed another set of experiments using the property of self-pollination, which is when a plant sexually reproduces with itself. When the offspring from the first experiments self-pollinated, 75% of their offspring had the same trait as them. 75% of the tall plants’ offspring were tall, 75% of the axial flowering plants’ offspring were axial flowering, etc. However, the other 25% of the offspring had the trait not present in their parents. Instead, 25% of the tall plants’ offspring were short, 25% of the axial flowering plants’ offspring were terminal flowering, etc.
Mendel looked at these two sets of experiments containing three generations of pea plants, and he concluded that one trait was dominant over the other. He had started with a tall plant and a short plant, and all their offspring were tall, meaning that being tall was dominant over being short. The same thing was true for all the other traits. Axial flowers were dominant over terminal flowers, green pods were dominant over yellow pods, yellow peas were dominant over green peas, and smooth peas were dominant over wrinkled peas. But when Mendel took these new plants, the second generation, and self-pollinated them, the third generation was split. 75% of the offspring had the dominant trait, same as their parents, but 25% had the other trait that wasn’t present in their parents. Mendel was a smart guy, and so he made four conclusions about how traits are passed from parent to offspring.
Mendel’s four conclusions are these:
The traits of an organism are determined by packets of information called “factors.”
Each organism has not one, but two factors that determine its traits.
In sexual reproduction, each parent contributes ONLY ONE of its factors to the offspring.
In each definable trait, there is a dominant factor. If it exists in an organism, the trait determined by that dominant favor will be expressed.
These four conclusions accurately explain the results of Mendel’s pea experiments. Because of conclusion #2, Mendel concluded that each pea plant had two factors for each trait, such as height or flowers. Since conclusion #4 states that there is a dominant factor, Mendel knew that the tall pea plants had the dominant factor of being tall. Also, since conclusion #3 says that each parent only contributes one of its factors, and since all the offspring were tall in Mendel’s experiment, he knew that both factors of the tall plants must have been the dominant factor of being tall. And since the short plants had the non-dominant trait, or the recessive trait, that means that both their factors were being short. Looking at conclusion #3 again, Mendel knew that each of the offspring from the tall plant and the short plant had one factor of being short and one factor of being tall. Since the tall factor is dominant, all the offspring were tall.
So what about the self-pollination? 75% had the dominant trait, but 25% of the offspring ended up with the recessive trait. Mendel had this one figure out as well. Remember, he was self-pollinating the offspring from the first experiments, so each pea plant had one dominant factor and one recessive factor. If we represent the dominant factor with a “T” and the recessive factor with a “t”, then each pea plant was a “Tt”. When they self-pollinated, it was like being crossed with another, identical plant. Since only one factor from each parent can be passed down, each pea plant could pass down either a “T” or a “t”, but they would do it twice because the one plant was both parents. That means that its offspring through self-pollination could be either a “TT”, “Tt”, or “tt”. If it had four offspring, most likely there would be one “TT”, two “Tt”s, and one “tt”. Since both “TT” and “Tt” give the tall trait, 75% of the offspring would be tall, the dominant trait, and 25% would be short, the recessive trait.
Using his four conclusions, Mendel explained the results of his experiments in his paper, that, while ignored at first, became the foundation of modern genetics, or the study of characteristics inherited by organisms from their parents. Gregor Mendel is now known as the father of modern genetics, even though he received no fame or reward in his lifetime.
` As genetics became an increasingly important branch of biology, scientists started developing new terms to describe what they were studying. Mendel’s conclusions #1 and 2 said that each trait is determined by packets of information called factors, and there are two factors per trait. Scientists now call those factors “genes”, and to be even more specific, when there are two genes that determine a trait, each gene is called a “allele”. So in the pea plants experiments, “T” and “t” were the alleles. Scientists then came up with a name for a pairing of alleles that determine a trait; “genotype” is the term used to describe two alleles, one from each parent, that together define a unique trait. Looking at Mendel’s experiments, the genotypes were “TT”, “Tt”, and “tt”. The last new term scientists have come up with is “phenotype”, which is the expression of a genotype. A phenotype is how we see the results of the genotype; a phenotype is the trait determined by the genotype. In the pea plant experiments, there were two phenotypes for the three genotypes. Both the “TT” and “Tt” genotypes resulted in the phenotype of a tall pea plant, while the “tt” genotype resulted in the phenotype of a short pea plant.
With the updated terminology, scientists rephrased Mendel’s original four conclusions to create the four founding principles of genetics.
The traits of an organism are determined by its genes.
Each organism has two alleles that make up the genotype for a given trait.
In sexual reproduction, each parent contributes ONLY ONE of its alleles to its offspring.
In each genotype, there is a dominant allele. If it exists in an organism, the phenotype is determined by that allele.
These four principles, called Mendel’s principles, form the basis for understanding how characteristics are passed down from parents to offspring in all types of organisms, including humans. Thanks to Mendel, we can understand why we have our bodies. Thanks to Mendel, we can understand why we are who we are. Thanks to Mendel, we can understand how impressive and complex God’s creation is!
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