Genetics Lesson 1: The Work of Gregor Mendel PDF

Unit 3: Genetics Lesson 1: The Work of Gregor Mendel  OBJECTIVES: 1. Describe how an organism gets its unique characteristics. 2. Explain how different forms of a gene are distributed to offspring.  MENDEL’S EXPERIMENTS Every living thing has a set...

Unit 3: Genetics Lesson 1: The Work of Gregor Mendel  OBJECTIVES: 1. Describe how an organism gets its unique characteristics. 2. Explain how different forms of a gene are distributed to offspring.  MENDEL’S EXPERIMENTS Every living thing has a set of characteristics inherited from its parent or parents. Genetics: scientific study of biological inheritance. Who is Gregor Mendel? ○ Austrian scientist and priest who was born in 1822. ○ Mendel studied science and math at the University of Vienna and spent the next 14 years working in a monastery and teaching. ○ At the monastery, he was in charge of the garden, which allowed him to do the work that changed biology forever. ○ Mendel carried out his work with ordinary garden peas because peas are small, easy to grow, and can produce hundreds of offspring--- peas were Mendel’s “model system”.  MENDEL’S EXPERIMENTS Role of Fertilization ○ Mendel knew that part of each flower produces pollen grains containing male reproductive cells or sperm. ○ The female portion of each flower produces reproductive cells called eggs. ○ Fertilization: joining of male and female reproductive cells to produce a new cell. In peas, the new cell develops into a tiny embryo encased within a seed.  MENDEL’S EXPERIMENTS Role of Fertilization (continued) ○ Pea flowers are mostly self-pollinating, so egg cells are fertilized by sperm within the same flower. A plant grown from a single parent via self-pollination inherits all of the characteristics of the parent. “True breeding” plants: means that they were self-pollinating and produced offspring with traits identical to themselves. Trait: specific characteristic, like seed color or plant height One stock of Mendel’s seeds produced only tall plants, while another produced only short ones. One produced only green seeds, another produced only yellow seeds.  MENDEL’S EXPERIMENTS Role of Fertilization (continued) ○ Mendel decided to “cross” his stocks of “true-breeding” plants to learn how those traits were determined. “Crossing” meant he caused one plant to reproduce with another plant--- he prevented self-pollination by cutting away the pollen- bearing male parts of a flower and dusted pollen from a different plant onto the female part of that flower (cross-pollination). This allowed Mendel to cross both plants with different traits and study the results.  MENDEL’S EXPERIMENTS Role of Fertilization (continued) ○ Mendel studied seven different traits of pea plants. Hybrids: offspring of crosses between parents with different contrasting characteristics.  MENDEL’S EXPERIMENTS Genes and Alleles ○ P1= parental generation--- original pair of plants ○ F1= first filial generation--- offspring of original parents Filius and filia are Latin for “son” and “daughter” ○ In Mendel’s cross, the F1 hybrid plants had the characteristics of only one of its parents. In each cross, the traits of the other parents seemed to have disappeared. ○ Mendel drew two conclusions: Conclusion 1: An individual’s characteristics are determined by factors that are passed from one parental generation to the next--- these factors are called genes.  MENDEL’S EXPERIMENTS Genes and Alleles (continued) ○ Mendel’s conclusions (continued) Each of the traits Mendel studied were controlled by a single gene in two contrasting varieties, producing different forms of each trait--- these are called alleles. Dominant & Recessive Alleles ○ Conclusion 2: some alleles are dominant and others are recessive; An organism with both a dominant allele and a recessive allele for a particular trait will exhibit the dominant characteristic--- this is the Principle of Dominance  SEGREGATION Mendel had another question: Had the recessive alleles simply disappeared, or were they still present in the new plants? ○ To find out, he allowed all 7 kinds of F1 hybrids to self- pollinate. ○ F2 generation (second filial): cross of the F1 generation. ○ Mendel discovered that the traits produced by the recessive alleles reappeared in the second generation. Roughly ¼ of the F2 plant showed the trait controlled by the recessive allele. Why did the recessive alleles seem to disappear in the F1 generation, only to reappear in the F2 generation? SEGREGATION  SEGREGATION Explaining the F1 Cross ○ The reappearance of the recessive allele indicated that at some point, the allele for yellow pods had separated from the allele for green pods--- called segregation. ○ How did this separation, or segregation, occur? Mendel suggested that the alleles for green pods and yellow pods in the F1 plants must have segregated from each other during the formation of gametes (reproductive cells). The Formation of Gametes ○ During gamete formation, the alleles for each gene segregate from each other so that each gamete carries only one allele for each gene.  SEGREGATION The Formation of Gametes (continued) ○ Each F1 plant produces two kinds of gametes--- those with green pod allele and those with yellow pod allele. A capital letter represents a dominant allele. A lowercase letter represents recessive alleles. Whenever a gamete that carried the g allele paired with another gamete with the g allele to produce an F2 plant, that plant had yellow pods. Every time one or both gametes of the pairing carried the G allele, a plant with green pods was produced. The genes had been reshuffled to produce new combinations of alleles.