Genetics and the Study of Heredity

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On a spring day in 1859, in what is now the Czech Republic, an Augustinian monk tended his peas. The monk, Gregor Mendel, was also a scientist with a hypothesis. His garden was his

laboratory. A few years before, Mendel had noticed that there were distinct traits that could be identified among the pea plants in his garden. Mendel’s hypothesis was that those traits were inherited intact from one generation to the next. This was important because most scientists of his time thought that inherited traits from parents blended together in their offspring. So, for instance, if a pea plant with purple flowers crossed with a pea plant with white flowers, it was expected that the next generation would have light purple flowers. Mendel thought differently, and dedicated his work to proving his belief. Mendel identified seven distinct trait pairs that seemed to occur in pea plants. These were flower color (purple or white), flower position (side of the stem or top of the stem), stem length (long or short), seed shape (round or wrinkled), seed color (yellow or green), pod shape (rounded or bumpy), and pod color (yellow or green). Mendel carefully crossed plants with one version of a trait with plants with the other version of the trait. He made notes about the traits of the plants’ offspring. He pollinated the plants he was studying himself, using a small brush to gather the pollen from one plant and place it in another. In the first generation, Mendel discovered that all of the offspring had only one version of the traits he was studying. For instance, if he crossed plants with round seeds with plants with wrinkled seeds, all the offspring in the first generation would have round seeds. But when those offspring were crossed with other plants of the same generation, the offspring were a mix of plants with round seeds and plants with wrinkled seeds. The traits appeared in a three-to-one ratio of yellow to green.

Mendel used controlled pollination in his experiments.

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Genetics and the Study of Heredity

Mendel regarded this as proof that there was something in the plant that could be passed from one generation to the next. He realized that even if a particular trait did not show up in a plant, that trait could be passed to the plant’s offspring. Today the two inherited versions of a trait are called genotypes. The visible version of the trait is called a phenotype. Mendel called these inherited traits factors. He also realized that each offspring received one copy of a given factor from each parent. He was the first to describe how heredity worked and how dominant and recessive traits were passed from one generation to the next. Work That Built on Mendel’s Theory Gregor Mendel did not know that his factors were what we now call genes. He did not realize how his work was related to that of Charles Darwin. Darwin’s book On the Origin of Species was published at about the same time that Mendel was working in his garden. In fact, Mendel almost faded into history without any recognition at all. It was more than 30 years after his death before other scientists re-examined his work and combined it with Darwin’s to create the field of genetics. Since that time, genes and chromosomes and DNA have been identified. These discoveries greatly expanded the understanding of genetics and the complex ways in which inheritance happens. It is now known that genes do not combine as simply as Mendel thought and that several different genes may influence a particular trait. As the result of genetics research, some of the genes that cause disease have been identified. Genetic counseling can be offered to people before they become parents so that they can evaluate the risk of passing on a particular disease to their children.

Mendel’s work helped explain the nature of dominant and recessive genes.

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Human Genetics Blue eyes are a recessive trait. This means that if a child has blue eyes, both of that child’s parents had to be carrying one copy of the gene for blue eyes. The parents may not have blue eyes themselves, but they passed them on to their offspring. Brown eyes are the dominant trait in eye color. Red hair is also a recessive trait. There are some other common recessive traits that can be easily tested for. The ability to roll one’s tongue is a recessive trait, as is being double jointed. The human genome, the map of where each gene in human DNA can be found on a person’s 23 pairs of chromosomes, was

Genetics and the Study of Heredity

finished in 2003. Current research indicates that there are between 20,000 and 25,000 genes on those chromosomes. By comparison, the Epstein-Barr virus, which causes mononucleosis, has only 80 genes. Gregor Mendel would probably be very impressed with the ways in which scientists have used his work. Now that the location of each human gene is known, it may be possible to treat or prevent harmful genetic mutations.

This chart shows an example of the inheritance of a recessive gene in people.