Planet Earth/7f. Heredity: Gregor Mendel’s Game of Cards

The Father of Genetics

Gregor Mendel

In 1849, the timid young friar named Gregor Mendel taught lessons in the small city of Znojmo, he was a popular teacher and enjoyed teaching nature and science subjects at the local school. Initially Gregor was training to be a priest, and run his own parish church, but the local abbot recognized his interests in nature and science, and encouraged him to teach at a local school, rather than serve at a parish. The teaching experience gave Mendel encouragement to study science more than previously allowed in the strict boarding school or Catholic monastery he attended. In 1849, however, a new law was passed that required that teachers at schools be certified in the subjects that they taught, and Mendel had to take a test to prove that he was qualified to teach science. The exam to get his certification was an intense examination, requiring written essays, oral questioning from university professors, and detailed reports written on subjects of science, including physics, chemistry, geology and biology. Mendel failed the exams, which were administered at the University of Vienna. The failure however came with an offer to study at the University of Vienna, and Mendel returned to school to pursue a degree in science. It was during his schooling that he enrolled in the botany courses offered by Franz Unger, who popularized the ideas of shared descent, and was particularly interested in the fossil record of plants, demonstrating in class how the Earth and its plants changed over time. Unger authored a popular book in 1851, entitled Die Urwelt in ihren verschiedenen Bildungsperioden (The Primitive World in its Various Transitional Periods), which feature lithographic reconstructions by Josef Kuwasseg and Leopold Rottman of ancient natural landscapes, as they may have looked like in Earth’s ancient past. These were primordial landscapes, featuring strange lizard-like creatures, and fern covered dense tropical forests. Mendel had never been introduced to these ideas from his religious Catholic teachings; ideas of a very old Earth and change over long spans of time featuring strange lifeforms that grew on Earth in the ancient past. This was all new and very exciting, and the botany course taught by Franz Unger excited his imagination regarding the study of plants. At the monastery, however, many of his superiors viewed his scholarly pursuits of science as too secular for the friar. There was talk of disbanding the St. Thomas Monastery for allowing too much freedom of its members. His schooling at the university was one contention frequently brought up in local discussions with the church. After attending courses and working hard however (and after encouragement from his friends, the abbot and fellow friars), Mendel was ready to take the certification exam again in 1856 at the University of Vienna. On arriving to take the exam, he was relieved to see the first question was an easy one, but then the second one was harder, and he started to panic. His hands started to shake, his stomach wanted to explode, and he felt feverous. He felt the need to vomit, and fled the room in devastation, embarrassed at his sudden illness and suffering a nervous breakdown. Back at the monastery he refused to leave his room, his family were called for, and Mendel realized that he had failed the exam once again. He would never teach in a classroom.

A Game of Cards

Devastated by his failures in school Mendel had during this time begun experimenting by growing peas, and carefully pollinating each flowering and recording the appearance of traits in the offspring. He was interested in studying how plants and animals inherit traits. He had begun the project by pollinating various garden peas so that they breed true in terms of a variable trait, like flower color. The expression of a variable trait in a population is referred to as a phenotype. Phenotype is the scientific term used for the observable characteristics or traits of an individual organism, as a result of genetic variability. Using these true breeding plants, for example plants that always produce pink flowers, and plants that always produce white flowers, he would cross pollinate the two with each other, and record the frequency in the offspring that resulted in pink and white flowers. He noticed that when he did the experiment the ratio of pink to white flowers remained nearly the same, about 25% of the flowers were white, while 75% were pink. None of the flowers appeared to be a blend of colors, rather they exhibited either pink or white. It was these ratios that made Mendel suspect that the inheritance of traits was not like blending paint together (as Darwin assumed), but more akin to a game of cards, for which he only knew the outcome of, and not the rules to the game.

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  Outcomes of two sets of heterozygous red flowers, each with a one red and one white allele. Note that on average 75% of flowers will be red, and 25% will be white.
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  Outcomes of two sets of flowers one a homozygous white flower paired with a heterozygous red flower, half the flowers will be red, with one dominate red allele, and half will be white with two recessive white alleles.
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  Outcomes of two sets of flowers both homozygous white flowers because all the alleles are white and recessive, all the offspring flowers will also be white.
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  Outcomes of two sets of flowers both red. One red flower is heterozygous while the other is homozygous. All the offspring flowers will also be red, but some will carry the recessive white flower alleles.]]

Imagine a deck of cards containing only red or white cards. Two cards are each dealt to each offspring pea plant player (we will call this hand of cards genotype) that will be used to determine the peas unique flower color (called a phenotype). The color will always be red, if any of the cards dealt are red. However, if the pea has no red cards, just white cards, then the plant will be white.

In this first game of cards (see above), the probability of a red card showing up is 75%, since the hand of cards could be two red cards (called homozygous since they are the same), one red and one white card (heterozygous since they are different), or one white and one red card (also heterozygous). All these hands of cards (called genotype) would result in a red flower.

The only way a flower would be white would be if the cards were both white (also homozygous). Hence red cards trump white cards in this game, and we referred these types of traits as being dominate. While white cards are recessive, meaning that you need both cards to be white in order for that trait to manifest white flowers as the phenotype. Each parent contributes one half of its cards (scientist called these hypothetical cards alleles), but only dominate traits are manifested when the pairs do not match, in this case red. This allows the diversity of traits to remain in following generations.

Mendel's pea experiment using pink and white flowers.

As a test, Mendel took pea plants of the first generation, white ones and pink ones, and self-fertilized them, so that they only drew alleles from themselves. The white peas only produced white flowers, because both cards were also white. However, pink peas produced both white and pink flowers in the subsequent generation, indicating that some offspring were heterozygous, containing both a pink and a white card (which again are called alleles, or heterozygous). Mendel did similar experiments with seven other traits with peas, ranging from the height of the plant, to seed shape and color, as well as pod shape and color. All appeared to be related to a key ratio or outcome that suggested a pairing of traits from unique alleles. Since inheritance appeared to be a probability distribution, variability within individuals can be preserved between generations, it is only the frequency of those traits that change. This discovery allowed the retention of variable traits that natural selection could work on. In 1863, Mendel read a German translation of Darwin’s Origin of Species book, which was all the discussion among a small group of scientists that regularly attended local meetings in Brno. Gregor Mendel in February and March of 1865 published his classic study, which he read before the Brno Natural History Society, and was published in 1866. The now classic study was never read by Darwin, and despite rumors of Mendel writing to Darwin during his lifetime, no evidence of correspondence exists between the two. Part of this lack of communication between the two scientists was likely a result of their differing languages, and the fact that Mendel was overly humble and fearful that his scientific experiments on peas would cast too much attention from those within the Catholic Church, who frowned upon his evolutionary experiments with plants. Mendel himself may have repressed his ambitions when he became abbot of the monastery in 1868, acknowledging his failures at the university, and avoiding attention to his scientific interests. Mendel’s ground-breaking research was to remain obscure to the greater scientific community, especially outside of Vienna, even among fellow botanists who could quickly understand the importance of his experiments.

Rediscovering Mendel’s Experiment

Carl Correns in 1910

In 1886, the Dutch botanist Hugo de Vries collected primrose seeds and planted them in his garden, noticing the great variety of flowers that the seeds produced, it was from this observation that he speculated that the plants must exhibit spontaneous mutations between generations, with the continued introduction of new traits, or phenotypes. In fact, the word mutant was coined by de Vries himself. This idea, which helped to explain increased variation between generations, was built on an older idea that there was some mechanism that changes between generations. He termed this mechanism or unknown molecule, pangenes, from Darwin’s idea of pangenesis. Later scientists shortening this term to simply gene. Hence de Vries viewed that genes mutate between generations, which introduce variation between each generation, helping to solve Darwin’s dilemma with blended inheritance. The problem with this idea was that it contradicted the experiments conducted by Mendel on peas. Mendel’s experiments were also ignored by the Swiss botanist Carl Nägeli, who despite numerous correspondences with Gregor Mendel never mentioned his work in his own publications, however, one of his students, Carl Correns became aware of the experiments and retested them while at the University of Tübingen, his verification was published in 1900, using Gregor Mendel’s name in the title of his paper. Overnight Mendel’s experiment became well known in biology, revealing an apparent pattern in how traits are inherited from parent to offspring.

Chromosomes and the complexity of cellular division in Eukaryotes

Chromosomes are found within the nucleus of eukaryotic cells. They are composed of long strains of DNA. This set of chromosomes are the ones that are found in humans.

Study of individual cells of eukaryotic organisms highlighted a complex process of cellular division, which invokes not only the copying of the nucleus of the cell, but also the many organelles within each cell. In bacteria, cellular division (called binary fission) is a simple process of DNA replication and cytokinesis, which divides the cell into two equal halves each containing a complete DNA molecule. In eukaryotic cells, cellular division is more complex, because the cell contains a nucleus in which sets of DNA are arranged into a series of chromosomes. Chromosomes in eukaryotes are held together by chromatin fibers, which prevent the long molecules from becoming entangled within the nucleus of the cell. Unlike prokaryotes, in which the DNA forms a ring or circle and floats in the cell freely, chromosomes in eukaryotes nucleus are extremely long tangles of DNA molecules paired into a series of rod-like chromosomes. During cellular replication in eukaryotes, the chromosomes condense such that they can be seen under a microscope, as tiny X shaped structures.

Mitosis

The major stages of Mitosis cell division

Most eukaryotic cells reproduce by mitosis. Mitosis is the cellular replication that results in two daughter cells each having the same number and kind of chromosomes as found in the parent nucleus. In multicellular organisms, mitosis cellular reproduction is responsible for ordinary tissue growth. Cells within a multicellular organism will continue to grow and be replaced through mitosis throughout the life of the organism, until all cells expire with death and no longer replicate. While mitosis in single celled eukaryote organisms produces two copies (or clones) of the cell, with nearly identical copies (asexual reproduction).

There are a number of stages in mitosis cellular division in eukaryotic cells, the end result of which is that each chromosome is paired up (Prophase,) hung on a mitotic spindle (Metaphase), and then pulled apart into individual chromatids (Anaphase), replicating the missing paired chromosome to form a new nucleus (Telophase), and finally the cell will pitch off in the middle (Cytokinesis) forming a new copy of the parent cell. This type of replication is occurring all the time in multicellular organisms, producing new tissue and growth over time. This cellular replication allows a multicellular organism to heal and repair itself, and is happening throughout your body right now. Single celled eukaryotic organisms mitosis is a method of asexual reproduction. Each cell produced during mitosis is said to be a diploid (meaning that each cell has two copies of each chromosome).

Meiosis

Major stages of meiosis cell division.

The other type of cellular reproduction is called meiosis. Meiosis is much rarer, and only occurs in sperm and egg cells, called gametes in multicellular organisms. These cells rather than being identical copies of the original parent cell, produce genetically unique cells. First the cells produce four genetically unique gamete cells, each with half the number of chromosomes as found in the parent cell (haploid cells). These cells then come together through sexual reproduction between individuals. This process shuffles the genes forming new pairs, one received from each parent gamete cell, producing recombinant chromosomes with unique genetic combinations.

The best way to think of meiosis is that it is like shuffling two decks of cards (one from each parent) and splitting each deck to make a total of four piles, and then combining two of the piles to make a new deck with the same number of cards. In many primitive multicellular plants (such as ferns and many alga), these gamete cells can form gametophytes, or multicellular individual plants that contain only haploid cells. This results in a strange life cycle that starts with meiosis by reducing the number of chromosomes in each cell by half to make abundant haploid spores. These spores are released to the environment and develop into a gametophyte. The mature gametophyte produces male or female gametes (or sometimes both) by mitosis. These male and female gametophytes then come together in the water to produces a complete diploid zygote/seed (a cell with paired chromosomes). The fertilized seed then develops into a new sporophyte that will produce haploid spores and start the process again. Most gametophytes are small and associated with only the reproductive stage of a life span. Spores on the other hand can be protected for long periods of time (such as dinoflagellate cysts), and only become active when conditions become ideal for the organism to reproduce. Such a life cycle is employed by dinoflagellates, which are single celled eukaryotic organisms that make up much of the Earth’s phytoplankton in the oceans. Dinoflagellates, when they bloom during ideal conditions can result in toxic red tides for fish and other animals in the oceans. These organisms have evolved these strategies to help them survive harsh natural conditions.

Genetics

The discovery of how variation is maintained in a population of individual life forms through the continued recombination of chromosomes has revolutionized science. Many diseases and traits are inherited through genetic material either directly by recessive traits or by incorrectly copying chromosomes (such as chromosome duplications), or by mistakes or mutations that result in maladapted cells during cellular reproduction, such as in cancer. A clear overview of genetics is beyond the goal of this class, but knowledge of its existence explains the wide variety of life on Earth.

The Importance of Population Size and Genetic Diversity

One of the important factors that leads to new varieties of life on Earth is something called the founder effect. The founder effect is the loss of genetic variation that occurs when small populations are isolated. These small samplings from the larger population will statistically be limited in their genetic variation. This will cause them to exhibit fixed states due to the random sampling of the original larger population. This can sometimes be caused by a population bottleneck, when populations drop to small numbers. These tiny populations may not genetically represent the original population by their characteristics, and over time become unique, with increased inbreeding, and low variation. These isolated populations might become new species, but they are also prone to extinction. New species often originate from such isolated pockets in a geographically distributed population.

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