Bio 115 Ecology and Evolution MENDELIAN GENETICS Her
Bio 115 Ecology and Evolution MENDELIAN GENETICS
Her 103 Mendelian heredity: Ovists and Spermists are both Preformationists. Although people realized that traits were passed on from parents to children by way of reproduction, the underlying mechanisms remained largely unknown. The preformationists believed that all life was ‘preformed’ at the moment of creation and that successive generations of individuals were encased, one inside the other, in increasingly small versions of their adult selves. Thus, children started as miniaturized adults whose development was triggered by the act of mating. The ovists (Paracelsus, Jan Swammerdam, and Reinier de Graaf, accepted that the offspring was contained in a preformed state in the egg cell and that its development was triggered by the juices of the sperm cell. In contrast, the spermists claimed that the offspring sprung from the sperm cell and that the female only provided a nurturing womb for the new life to develop. Spermism had to await the discovery of sperm cells by the first microscopists (Anthony van Leeuwenhoek and Nicolas Hartsoeker) who discovered the homunculus or ‘little man’ hidden within the sperm cell. Jan Swammerdam Anthony van Leeuwenhoiek Nicolas Hartsoeker Bio 161 Lesson 01 Reinier de Graaf
Her 104 Mendelian heredity: Pangenesis and Blending Pangenesis states that traits are inherited directly. Elements or information of each of the parents’ body parts are transmitted to the offspring directly, and independently of the information of other parts. The child is formed after this hereditary material from all parts of the parents’ bodies has come together. Hippocrates called this reproductive material ‘gonos’ or seed. Darwin (1868) suggested that all cells and tissues excrete this material as microscopic granules or ‘gemmules’. The pangenesis theory assumes that both male and female contribute to the offspring and that the traits blend in the offspring. Conclusion: Distinction between soma and germ line. In pangenesis, information from the somatic cells (gemmules) is collected in the germ cells and passed on to the offspring. Bio 161 Lesson 01 The two classical assumptions lead to a paradox. Indeed, if variation and heredity occur within the boundaries of the species (no variation enters from outside the species) and the traits of the parents are blended in the offspring, than sooner or later all members of the same species should have the same appearance. Which is not the case. In addition, early experimental work of Josef Kolreuter (1760) and TA Knight (1790) casted doubt on the classical assumptions.
Her 105 Mendelian heredity: classic assumptions fail Observations cast doubt on the classic assumptions: Josef Kolreuter (1760) crossed different strains of tobacco. The resulting hybrids differed in appearance from both parent strains. When individuals of the hybrid generation were crossed, their offspring were highly variable: some resembling the hybrid generation (their parents) others resembling the original strains (their grandparents). T. A. Knight (1790) crossed a strain of peas with purple flowers with one with white flowers. All hybrid offspring had purple flowers. Among the offspring of these hybrids, however, were some plants with purple and some with white flowers. Josef kolreuter Bio 161 Lesson 01 Because traits could be masked in one generation to reappear in the next, inheritance could not occur by direct transmission of traits (How could a trait that is transmitted directly disappear and reappear? ). Similarly, blending theory can not explain differences between offspring of the same parents.
Her 106 Mendelian heredity: lamarckism. Lamarckism: inheritance of acquired traits: Aristole (384 -322 BC) proposed that the physical, intellectual, and personality characteristics acquired by people during their life time were passed on to their offspring. According to this theory, musical talent, cycling performance, or soccer aptitute can be passed on from the parents to their children if the parents dedicate their life to the pursuit of these activities because their genetic material is affected by their fervent dedication. Jean-Baptiste Lamarck (1744 -1829) used the idea to explain the mechanism of heredity and suggested that by simply using or not using certain organs, organs and body parts may be highly developed or atrophioed and the offspring can then inherit these acquired traits. Lamarckism rejected Bio 161 Lesson 01
Her 106 a Mendelian heredity: lamarckism rejected Lamarckism rejected; Lamarckism, or theory of inheritance of acquired traits, influenced scientific thinking for over 200 years. However, there are obvious problems to this idea. Children with normal limbs are begot from deformed parents. Also, think of the jewish practice of circumcisuin. August Weismann (1834 -1914) nipped the idea of inheriting acquired characteristics in the bud so to speak when he cut the tails off of rats and mated them, showing that the offspring still had tails. Bio 161 Lesson 01
Her 201 Who is Mendel? Gregor Johann Mendel (born on 22 July 1822 in Heizendorf, Austria [Hyncice, Czech Republic] and died on January 6, 1884 in Brno [Brunn], Austria) was the only son of a farmer and attended local schools and the Philosophic Institute at Olomouc. In 1843, he entered the Augustinian Order at st. Thomas Monastery in Brunn and began his theological studies. He was ordained in 1847. From 1851 to 1853 he studied zoology, botany, chemistry, and physics at the University of Vienna. Two professors were especially influentiual: the physicist Christian Doppler (who instilled a sense for experimental work and mathematical modeling) and the botanist Franz Unger (who aroused interest in the causes of variation in plants). Mendel returned to the monastery in 1854 to teach part-time at a school where other teachers shared his enthusiasm for sciences. Moreover, St. Thomas Monastery was a center of creative interest in the sciences and culture. There well-known philosophers, a musicologist, mathematicians, mineralogists, and botanists who were heavily engaged in scientific research and teaching. Most important, the monks had a longstanding interest in the breeding of plants. Mendel completed his experimental work with the garden peas between 1854 and 1865. Mendel tried to determine whether it was possible to obtain new variants of peas by cross-breeding. He presented his work in two lectures before the Society for the Study of the Natural Sciences in Brunn in 1865. His paper, Versuche uber Pflanzen-Hybriden (Experiments in plant hybridization) was published in the Society’s Proceedings in 1866. His work, however, was largely ignored. In the spring of 1900, three botanists (Hugo de Vries, Karl Correns, and E. von Tschermak) reported independent verifications of Mendel’s work. Bio 161 Lesson 02
Her 202 Why Mendel chose the garden pea: Earlier investigators had produced hybrids by crossing different strains. A large number of true-breeding varieties were available. Pea plants are small, easy to grow, and have a relative short generation time. It is easy to control self fertilization and cross-fertilization. Self-fertilization: Both male and female sexual organs are enclosed in the same flower and the male and female gametes produced can fuse to form viable offspring. Selffertilization takes place automatically within an individual flower if it is not disturbed. Bio 161 Lesson 02 Cross-fertilization: Cross-fertilization is the result of cross-pollination. The latter occurs when pollen of a different flower is introduced to the female stigma of a flower after first removal of the petals and male anthers of the flower.
Her 203 Mendel’s experimental design: Mendel selected pea varieties that differed in 7 traits from each other. He conducted his crosses in three stages: (1) He allowed pea plants of a given variety to produce progeny by self-fertilization for several generations. Mendel confirmed that the traits of the variants he studied were true (pure)-breeding (or homozygous). All white flowered plants produced white-flowered offspring regardless of the number of generations. He called these plants the Parental generation or P generation. (2) Mendel than performed crosses between varieties showing alternative forms of the trait in which he was interested. E. g. , cross a white flowered female with a purple flowered male. (He also carried out the reciprocal cross!) He called the offspring of the P generation the first Filial or F 1 generation. (3) Finally, Mendel permitted the F 1 hybrids to self -fertilize for several generations. He carefully counted and recorded the numbers of offspring exhibiting each trait in each succeeding generation. Bio 161 Lesson 02
Her 204 What Mendel found: purple- x white-flowers. What Mendel found: In a cross of white-flowered plants with purpleflowered plants, the F 1 offspring all had purple flowers. Of the F 2 offspring 25% had the recessive white flowers and 75% were purple-flowered. All recessive, white-flowered offspring were truebreeding (homozygous). In contrast only 1/3 of the dominant, purple-flowered offspring were truebreeding, while 2/3 were not. In general: Bio 161 Lesson 02
Her 204 a What Mendel found: in general What Mendel found: In a cross between two contrasting varieties of peas the F 1 offspring did not show properties intermediate of those of the parents, as the blending theory predicted. In every case, the offspring resembled one of the parents. The form of the trait expressed in the F 1 is said to be dominant; the alternative form of the trait is said to be recessive. Some human traits. ¾ (75%) of the F 2 exhibited the dominant trait, and ¼ (25%) of the F 2 offspring displayed the recessive trait. The dominant: recessive ratio among the F 2 plants was always 3: 1. The 3: 1 ratio of the F 2 generation was really a disguised 1: 2: 1 ratio with ¼ true-breeding dominant, 2/4 non-true-breeding dominant, and ¼ true-breeding recessive individuals. Results of Mendel’s other experiments. Bio 161 Lesson 02
Her 204 a 1 Results of Mendel’s other experiments. Bio 161 Lesson 02
Her 205 Mendel’s Model of Heredity (part 1) To explain his results/ observations, Mendel proposed a simple model for inheritance of traits. Parents do not transmit physiological traits directly to their offspring. Rather, they transmit discrete packages of information (Mendel’s factors, or genes) that encode the trait. Mendel expressed the factors by a set of symbols. The dominant purple color is represented by P (upper case) and the recessive white color by p (lower case). Each individual receives two factors of the same trait, one from the father and one from the mother. Individuals are said to be diploid. There alternative forms or alleles of a factor. An individual with thwo identical alleles of a trait is homozygous; an individual with two different alleles of a trait is heterozygous. The genotype of an individual that is true-breeding white (pp) and purple (PP) are homozygous. Heterozygous individuals have the genotype Pp. (continued) Bio 161 Lesson 02
Her 205 a Mendel’s Model of Heredity (part 1) Apply to the Purple x White cross. To explain his results/ observations, Mendel proposed a simple model for inheritance of traits. (continued) Apply to the Yellow x Green cross. The two alleles, one contributed by the male gamete and one by the female gamete, remain discrete units in the cells that develop into the new individual. Thus, when the individual matures and produces its own gametes, the alleles for each trait segregate randomly into these gametes. In heterozygous individuals, only one allele (the dominant) is expressed, while the other (the recessive) allele is present but not expressed. The presence of a particular allele does not ensure that the alternative form of the trait encoded by it will be expressed in an individual carrying that allele. The genotype is the totality of the alleles of an individual and the phenotype is the actual physical appearance of the individual. Examples of human traits. Bio 161 Lesson 02
Her 205 a 01 Mendel’s cross of Purple x white flowered peas. Note. The cross between two individuals each heterozygous for a single trait is called a monohybrid cross. Bio 161 Lesson 02
Her 205 a 02 Mendel’s cross of Green x yellow peas. A cross of true-breeding green and yellow pea pods yields all green offspring. The F 2 generation yields 75% green and 25% yellow pea-pods. Bio 161 Lesson 02
Her 205 a 03 Human dominant and recessive traits. Many traits in humans also exhibit simple dominance or recessive inheritance, similar to the traits Mendel studied in peas. Bio 161 Lesson 02
Her 206 Mendel’s interpretation of the crosses: the Punnett square (P cross). A. Parental cross (P) and F 1 generation: Cross of true-breeding purple-flowered female and true-breeding white-flowered male. (female genotype) (female gametes) P PP P p (male genotype) P P p Pp Pp pp p (male gametes) Place the different possible types of female gametes along the top of the square (because the female is homozygous all gametes are P, purple) and place all possible different types of male gametes (all are white, p) along the side of the square. Bio 161 Lesson 02 Each potential zygote can be represented by combining a female (column) and male (row) gamete. Mendel’s model, analyzed by way of a Punnett square, clearly predicts that all F 1 offspring will be heterozugous and purple.
Her 206 a Mendel’s interpretation of the crosses: the Punnett square (F 1 cross) B. The F 1 cross and F 2 generation: Cross (or self-fertilization) of heterozygous, purple, male and female F 1 plants. (female genotype) (female gametes) P Pp p P (male genotype) P p P PP Pp pp Pp p (male gametes) Place the different possible types of female gametes (P and p) along the top of the square and place all possible different types of male gametes (P and p) along the side of the square. Bio 161 Lesson 02 Each potential zygote can be represented by combining a female (column) and male (row) gamete.
Her 207 1 st Law of Heredity or Law of Segregation. Mendel’s 1 st lLaw of Heredity or the Law of Segregation. Mendel’s model thus accounts perfectly for the phenotypic ratio of offspring observed in each cross, and thereby, explains how traits are inherited, that is, the process of heredity. His model is based on the fact that alleles of a trait segregate from each other and are randomly distributed over the gametes during gametogenesis. Law of Segregation: Alleles segregate randomly during gametogenesis. Bio 161 Lesson 02 As we will study later (Bio 185), the segregation behavior of the alternative alleles is rooted in the alignment and separation of homologues chromosomes during meiosis I.
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