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BIOLOGY SEMESTER ONE UNIT 14 CHECKLIST UNIT 14: MENDEL AND THE GENE IDEA In this module you will investigate how genes are inherited or passed along to offspring. Mendel’s model of inheritance is based on the idea that genes are inherited in discrete packages form parents and were not “blended” as previously thought. Mendel crossed white flower and purple flower plants, and discovered the result was not a blend, but instead all purple flowered. He then considered the idea of heredity units, which he called "factors,” one of which is a recessive characteristic and the other dominant. Mendel said that factors (we now call them genes) normally occur in pairs in ordinary body cells, yet segregate during the formation of sex cells. Each member of the pair becomes part of the separate sex cell. The dominant gene, such as the purple flower in Mendel's plants, will hide the recessive gene, the white flower. After, he correctly theorized that genes can be paired in three different ways for each trait: AA, aa, and Aa. The capital "A" represents the dominant factor and lowercase "a" represents the recessive. Mendel stated that each individual has two factors (genes) for each trait, one from each parent. The two genes may or may not contain the same information. If the two genes for a trait are identical, the individual is called homozygous for that trait. If the two genes have different information, the individual is called heterozygous with regards to that trait. The different possible forms of a gene are called alleles. The genotype of an individual is made up of the many alleles it possesses. An individual's physical appearance, or phenotype, is determined by its alleles as well as by its environment. An individual possesses two alleles for each trait; one allele is given by the female parent and the other by the male parent. They are passed on when an individual matures and produces gametes: egg and sperm. When gametes form through meiosis, the paired alleles separate randomly so that each gamete receives a copy of one of the two alleles. The presence of an allele doesn't promise that the trait will be expressed in the individual that possesses it. In heterozygous individuals the only allele that is expressed is the dominant. The recessive allele is present but its expression is hidden. Mendel summarized his findings in two laws; the Law of Segregation and the Law of Independent Assortment. Mendel's findings allowed other scientists to predict the expression of traits on the basis of mathematical probabilities. In this module you will be introduced to Punnett Squares, which will aid you in finding probabilities for genetic expression when performing a test cross. Creative Commons Attribution 3.0 Unported License 1 BIOLOGY SEMESTER ONE UNIT 14 LEARNING OBJECTIVES At the end of this module you should be able to do the following: 1. Understand and use the terminology related to Mendelian genetics. 2. Identify and explain the four components of Mendel’s segregation model of genetic inheritance. 3. Create and use Punnett squares to predict the outcomes of various crosses. 4. Understand and apply the laws of probability (multiplication & addition rules) to Mendelian inheritance. 5. Discuss Mendel’s laws of segregation and independent assortment and relate them to meiosis. 6. Discuss various forms of genetic dominance (eg: complete / incomplete / codominant / recessive) and how they may apply to phenotype. 7. Explain why dominant alleles are not necessarily more common in a population. 8. Discuss gene interaction (eg: epistasis) and environmental influence on phenotype. 9. Discuss how alleles arise and are maintained in populations, even when they appear to be non-beneficial or even harmful. CHECK LIST Read Chapter 14 of Campbell and Reece’s Biology, 9th Ed. As you are reading, address each of the learning objectives listed above. We recommend that you make flash cards for the terminology list provided. This will be beneficial for studying for the midterm and final exams later in the semester. You may be able to review the PowerPoint Lecture and other resources for this unit. Refer to your instructor’s notes for more details. Complete the “Mendel Worksheet”. The Mastering Biology website has some extra tutorials on working through Punnett Squares and probabilities. They can be found in the student area by highlighting Chapter 14 in the top drop down menu and clicking “go.” Here you will find activities on monohybrid and dihybrid crosses, as well as the Gregor’s Garden activity. These may be helpful practice. Watch the video clip “Novelty Gene.” Post a response to the discussion board discussing which you feel contributes most strongly to a specific behaviour of your own: genetics or the environment? Give specific evidence to back up your claim. Read and comment on 2 of your fellow classmates’ posts. For extra practice try the Self Quiz or Practice Test on the Mastering Biology Website. To log onto the website, use the access code provided in your textbook. You will also Creative Commons Attribution 3.0 Unported License 2 BIOLOGY SEMESTER ONE UNIT 14 find other resources, such as downloadable MP3 tutorials for each chapter, a glossary, and an electronic copy of your text—you can catch up on your reading anywhere! KEY TERMS carrier character codominance complete dominance dihybrid dominant allele epistasis F1 generation F2 generation ROOT WORDS TO KNOW heterozygous homozygous hybridization incomplete dominance law of independent assortment law of segregation monohybrid P generation pedigree phenotype pleiotropy polygenic inheritance Punnett square recessive allele test cross trait true-breeding 1 -centesis = a puncture (amniocentesis: a technique for determining genetic abnormalities in a fetus by the presence of certain chemicals or defective fetal cells in the amniotic fluid, obtained by aspiration from a needle inserted into the uterus) co- = together (codominance: phenotype in which both dominant alleles are expressed in the heterozygote) di- = two (dihybrid cross: a breeding experiment in which offspring of a cross of parental varieties differing in two traits are mated) epi- = beside; -stasis = standing (epistasis: a phenomenon in which one gene alters the expression of another gene that is independently inherited) geno- = offspring (genotype: the genetic makeup of an organism) hetero- = different (heterozygous: having two different alleles for a trait) homo- = alike (homozygous: having two identical alleles for a trait) mono- = one (monohybrid cross: a breeding experiment that crosses offspring of a cross of parental varieties differing in a single character) 1 (Pearson Education, 2010) Creative Commons Attribution 3.0 Unported License 3 BIOLOGY SEMESTER ONE UNIT 14 pedi- = a child (pedigree: a family tree describing the occurrence of heritable characters in parents and offspring across as many generations as possible) pheno- = appear (phenotype: the physical and physiological traits of an organism) pleio- = more (pleiotropy: when a single gene impacts more than one characteristic) poly- = many; gene- = produce (polygenic: an additive effect of two or more gene loci on a single phenotypic character) SOURCES Campbell, N. A. (2008). Biology, Eighth Edition. San Francisco: Pearson, Benjamin Cummings. Pearson Education. (2010). Retrieved 2010, from Mastering Biology : http://session.masteringbiology.com NANSLO Biology Core Units and Laboratory Experiments by the North American Network of Science Labs Online, a collaboration between WICHE, CCCS, and BCcampus is licensed under a Creative Commons Attribution 3.0 Unported License; based on a work at rwsl.nic.bc.ca. Funded by a grant from EDUCAUSE through the Next Generation Learning Challenges. Creative Commons Attribution 3.0 Unported License 4