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Transcript
Introduction to Genetics 1 The Work of Mendel • Objectives: • Describe how Mendel studied inheritance in peas • Summarize Mendel’s conclusion about inheritance • Explain the principle of dominance • Describe what happens during segregation 2 Genetics: • the scientific study of heredity. Heredity is the passing of characteristics from parent to offspring through the genes. 3 Gregor Mendel • An Austrian monk born in 1822, instrumental in developing ideas about heredity. He worked at a monastery teaching school and managing the monastery garden. While at the monastery he experimented with pea plants and developed his ideas. 4 Some quick facts about the reproduction of peas: Plants reproduce sexually. The reproductive organ of a plant is the flower. Flowers can have male and female parts. • Male parts: The stamen, which contains the filament and the anther. The anther contains the male sex cell or gamete called pollen. 5 Female parts: • The carpel, which is composed of the stigma, the style and the ovary. The ovary contains the female sex cell or gamete called the ovule or egg. • When the pollen fertilizes the egg, a seed is formed. The seed will then give rise to a new plant. 6 Mendel’s Peas • Mendel worked with true breeding plants. True breeding plants are plants which have been allowed to self pollinate, thus producing offspring which are genetically identical to the parent. • By doing this he developed a number of separate stocks, each representing one of 7 different traits. 7 Plant Anatomy 8 Mendel • In order to avoid unwanted pollination of his plants, Mendel removed the stamens from all of the flowers (ouch), collected their pollen and pollinated them himself using a paint brush. 9 Why peas?? Mendel chose peas as an experimental subject for a number of reasons: • They have contrasting traits that usually come in only two forms. • They reproduce sexually which allows for diversity. • Their reproductive cycle is short allowing for production of a number of generations in a short time. 10 Why peas?, cont. • • • Their crosses (breeding) can be controlled. They produce a large number of offspring. They are easy to care for in the lab. 11 12 Mendel’s Experiments 1. Principle of dominance – some alleles (different forms of a gene) are dominant and others are recessive. 13 The Experiment • Mendel crossed true breeding plants with contrasting traits. (For example a true breeding tall plant to a true breeding short plant) • These plants were referred to as the P or parental generation. • The offspring of these plants are referred to as the F1 or first filial generation. 14 The Experiment, cont. • Because the offspring inherited traits from both parents they were called hybrids. • Instead of coming out half tall and half short or all medium, the F1 generation came out ALL TALL. 15 Conclusions • Mendel came up with two conclusions as a result of this experiment. Biological inheritance is determined by factors that are passed on from parent to offspring. We now call those factors genes. Different forms of a gene are called alleles. 16 Conclusions a. He came up with the principle of dominance. An organism that has the dominant allele for a trait will always have that trait. The only time the recessive form will show up is if two recessive alleles are inherited for that trait. 17 • The dominant allele is represented by a capital letter (usually the first letter of the dominant trait). The recessive allele is represented by a lower case letter (also the first letter of the dominant trait). Ex. T = tall, t = short 18 2. Principle of segregation: • • When gametes (sex cells) are formed the two alleles of a pair are segregated from one another so that each gamete carries only a single copy of the gene. The question: Had the recessive alleles in the F1 generation disappeared or were they still present? 19 The Experiment • Mendel allowed the F1 plants to self- pollinate, resulting in an F2 or second filial generation. • Mendel expected tall plants to result in tall offspring but this is not what happened. • 3/4 of the offspring were tall, but ¼ of them were short! 20 The Conclusions • The allele for shortness had separated from the allele for tallness. But when? • He believed that the alleles separated during the formation of gametes (pollen and egg). 21 Some terms to be familiar with: • Homozygous: Organisms that have identical alleles for a trait. Ex. TT or tt • Heterozygous: Organisms that have two different alleles for a trait. Ex. Tt 22 New Terms, cont. • Phenotype: The physical characteristic of a trait. Ex. Tall, Short • Genotype: The genetic make-up of a trait (the letters). Ex. TT, Tt, tt 23 Independent Assortment • Questions: Does the segregation of one pair of alleles affect the segregation of another pair of alleles? (Ex. Does the gene that determines hair color have anything to do with height?) 24 The Experiment • Mendel crossed a plant that was homozygous for both round seeds and yellow color (RRYY) with one that was homozygous for both wrinkled seeds and green color (rryy). • The gametes produced by the RRYY parent were all RY, and the gametes produced by the rryy parent were all ry. 25 The Experiment, cont • The offspring that resulted from this cross all had a genotype of RrYy and were heterozygous round and yellow. • Mendel wanted to know if the R’s would always stay with the Y’s and the r’s with the y’s so he allowed the RrYy offspring to self-pollinate. 26 • The gametes produced by these plants were RY, Ry, rY, and ry. When crossed the following phenotypes were produced: • 9/16 round and yellow • 3/16 round and green • 3/16 wrinkled and yellow • 1/16 wrinkled and green 27 28 The Conclusions • Because some offspring have trait combinations not found in either parent Mendel concluded that alleles for seed shape segregate independently from alleles for seed color. 29 The Conclusions, cont • The principle of independent assortment: genes for different traits can segregate independently from each other during the formation of gametes. 30 31