The Chromosomal Basis of Inheritance

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Chapter 15 The Chromosomal Basis of Inheritance Lecture Outline Overview • Today we know that genes—Gregor Mendel’s “hereditary factors”—are located on chromosomes. • A century ago, the relationship of genes and chromosomes was not so obvious. • Many biologists were skeptical about Mendel’s laws of segregation and independent assortment until evidence mounted that they had a physical basis in the behavior of chromosomes. Student Misconceptions 1. Many students have great difficulty understanding how the laws of Mendelian inheritance can be explained by the behavior of chromosomes during meiosis. Many cannot correctly describe the relationship between a pair of alleles and a homologous pair of chromosomes during meiosis, and do not recognize that meiosis is the mechanism for segregation of alleles. 2. To help students recognize the relationship between alleles of a gene and genes on a chromosome, expect your students to talk about and draw |chromosomes wherever possible. Incorporate drawings of chromosomes into your instruction, and use such drawings to illustrate lectures on cell division, cell cycle, fertilization, and life cycles. Include diagrammatic representations of chromosomes carrying alleles in Punnett squares, and expect your students to do the same. 3. Students are taught about dominant and recessive alleles when they learn about Mendel’s laws of inheritance. As a result, they may think that |dominance is the norm and that incomplete dominance and codominance are rare exceptions. In fact, complete dominance is exceptional and incomplete dominance is the norm. Less than one-third of clinical human genetic conditions are explained by a model of one gene and two alleles, with complete dominance. The conventional examples of human traits that display complete dominance are rather trivial: tongue curling, PTC tasting,
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