Module I – Genetics According to Mendel
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Educators Lesson Module I – Genetics According to Mendel his teaching license because of a disagreement with Genetics According examiners about the nature of heredity. Ironically, Mendel’s failure to become a teacher is what drove him to Mendel to discover the principal rules of genetics. I Bent on proving his ideas correct, Mendel began his famous pea experiments. In 1865, he presented the Part I results of his experiments to the Brunn Society of Natural History, one of the most prestigious scientific BACKGROUND INFORMATION societies in the world. In 1866, the society published his paper in their proceedings. Even though the journal reached 120 universities and other societies, Introduction Mendel’s paper was mostly ignored by the science It has been known for thousands of years that biological community of the time. It was not until 1900 that traits are passed to offspring by their parents who Mendel’s paper was rediscovered and viewed as a received these traits from past generations. This fact significant scientific achievement. was most successfully applied by farmers who recog- nized the importance of saving the seeds of the best How did Mendel succeed where earlier scientists plants for the next year’s crop and using the best failed to understand heredity? Mendel kept his animals for breeding. But until recent times, the experiments simple. scientific mechanisms that drive inheritance were not understood. Today, we call the scientific study of Mendel had many years of plant breeding experience. inheritance genetics. As a result, he selected the garden pea because it grew well in small gardens, produced a large number of In 1866, an Augustinian priest named Gregor Mendel seeds, and was easily pollinated. Pea plants are self- proposed the first theory about the units of inheritance, pollinating because the anthers (male, pollen producer) that we call genes. He described two basic rules that and the stigma (female, receives pollen) are enclosed in govern how traits are transmitted from one generation the same flower. This selfing produces strains of to another. Mendel’s work is recognized as one of the pea plants that are identical for many generations, greatest breakthroughs in the history of science and called true-breeding. marked the beginning of the science of genetics we know today. From his past experiences, Mendel knew how to cross- pollinate plants and how the characteristics of peas The science of genetics has served as the foundation for could be used in his studies. For his experiments, today’s advancements in crop and animal production, Mendel selected only true-breeding strains and studied for detecting and treating inherited human diseases, only a few traits at a time. He grew his chosen plants and for the production of medicines that treat human for two years to make sure he had a true-breeding strain and animal afflictions. Since the early part of the of peas. When he began crossing true-breeding strains, 20th century, genetics has been at the forefront of Mendel made sure that the two parents differed only in research in biology and is today the cornerstone of the few traits he had chosen. Scientists working before biological research. Mendel failed in their similar experiments because they did not use true-breeding strains and did not limit the The Nature of Mendel’s Discovery traits they were testing. In 1842, after completing all the formal education his Mendel was methodical in recording and applying family could afford, 21-year-old Gregor Mendel entered mathematical analysis to his results. His careful a Austrian monastery as a means to continue his numerical analysis had never been done in true- education. In 1851, with the assistance of the monas- breeding experiments before and was an essential factor tery, Mendel entered the University of Vienna. There in Mendel’s success. he studied physics, chemistry, zoology, botany, and mathematics, including the new field of statistics. Mendel’s Rule of Segregation Mendel returned to the monastery and began teaching physics and natural history at the local high school. In In his first series of experiments, Mendel made crosses 1856, Mendel walked out of the oral examination for between true-breeding pea strains that differed in one trait, seed shape. Parents either had round or wrinkled 8 Iowa State University Extension and ISU Office of Biotechnology Lesson Module I – Genetics According to Mendel Educators seeds. These crosses are referred to as monohybrid Mendel theorized that each parent plant passed one crosses. The plants involved in the original cross are allele for each trait to its offspring through their call the parental or P generation. The progeny (off- gametes (sperm or egg). Based on the fact that reces- spring) seeds from this cross are referred to as the first sive traits reappear in the offspring of heterozygotes, filial or F1 generation. Mendel proposed that alleles do not mix or blend in heterozygotes. This idea was contrary to the “blending Mendel found that all the F1 generation seeds resulting theory” that was the thinking of the day. Mendel’s from his original cross of parents with round and results suggested that dominant and recessive alleles do wrinkled seeds were round. (See p. 10-11 to view the not blend, but are passed intact from heterozygotes to results of Mendel’s crosses.) He concluded that the their offspring. wrinkled trait seemed to be masked or “dominated” by the round trait in the F1 seeds. Mendel called the round Mendel hypothesized that the two alleles segregate trait dominant and the wrinkled trait recessive. Mendel (move apart) from one another during gamete forma- planted the F1 seeds, raised the plants, and allowed tion. From a heterozygote (Rr), two types of haploid them to self-pollinate to produce a second filial or F2 gametes formed. One type of gamete contains the generation of seeds. He found that both round and round allele (R), and one contains the wrinkled allele wrinkled seeds appeared in the F2 generation in a ratio (r). Homozygous individuals produce only one type of of three round (dominant trait) to one wrinkled gamete. Homozygous individuals with round seeds (recessive trait). (RR) would produce only gametes containing the round allele (R), and wrinkled (rr) individuals would produce Mendel continued each cross for another generation by only gametes containing the wrinkled allele (r). collecting F2 seeds, planting them, rearing the plants, and allowing them to self-pollinate to produce a third From this hypothesis, Mendel formulated his first rule filial or F3 generation. Mendel discovered that F2 of genetics, the Rule of Segregation. This rule states individuals with the recessive trait always produced that pairs of factors (alleles) segregate or separate progeny with the recessive trait. Some F2 individuals during the formation of gametes. with the dominant trait produced only progeny with round seeds, while other F2 individuals with the dominant trait had progeny with both round and Mendel’s Rule of Independent wrinkled seeds. From these results, Mendel hypoth- Assortment esized that alternative traits, round or wrinkled seeds, Mendel wanted to understand how two or more traits are determined by “factors.” Today, we call these factors are inherited simultaneously. He hypothesized two genes. Mendel proposed that genes exist in different possibilities. One possibility was that the traits were forms and, consequently, can produce different traits. inherited as a unit. If one parent has round and yellow Today, we call different forms of one gene alleles. seeds and the other parent has wrinkled and green seeds, these pairs of traits might be passed together to Upon completing his monohybrid crosses, Mendel their offspring. The offspring would produce either proposed that each individual pea plant carried two round-yellow seeds or wrinkled-green seeds. The copies (two alleles) of each gene. Each allele is given a second possibility was that traits are inherited indepen- letter. The genetic makeup of an individual symbolized dently. If so, future generations would display a by letters is called its genotype. Uppercase or capital combination of the traits, such as round-green or letters indicate a dominant allele, and lowercase or wrinkled-yellow seeds. small letters indicate a recessive allele. An individual with two identical alleles is said to be homozygous. A Mendel investigated his hypothesis by crossing plants plant that is true-breeding for round seeds is homozy- that differed in two traits, called a dihybrid cross. He gous for the allele controlling round seed (RR), and a crossed plants that were homozygous true-breeding plant that is true-breeding for wrinkled seeds is strains for the dominant (RRYY) and recessive (rryy) homozygous for the allele controlling wrinkled seed alleles of both traits. The F1 generation seeds were all (rr). An individual that possesses two different alleles round and yellow. This agreed with results Mendel had of one gene is called heterozygous (Rr). The observable observed in earlier crosses where round was dominant characteristic (smooth, wrinkled) of an individual is to wrinkled and yellow was dominant to green. called its phenotype. Therefore, the genotype of the F1 generation had to be (RrYy). (See p. 11 to view the crosses made by Mendel.) Iowa State University Extension and ISU Office of Biotechnology 9 Educators Lesson Module I – Genetics According to Mendel The F1 plants were allowed to self-pollinate. Mendel combined in the square to produce the possible observed four different seed phenotypes from this cross: genotypes of the progeny (offspring.) round-yellow, round-green, wrinkled-yellow, and wrinkled-green. These phenotypes represent all the possible combinations of the two traits. From these and F2 Seeds from Selfed F1 Plants future experiments, Mendel concluded that seed shape and color were not inherited as a unit.