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Chapter 15 Mendel’s “hereditary factors” = genes Genes are located on chromosomes Chromosome theory of inheritance: ◦ Mendelian genes have specific loci (positions) on chromosomes ◦ Chromosomes undergo segregation and independent assortment The behavior of chromosomes during meiosis partially acounts laws of segregation and independent assortment Morgan’s experiments provided evidence that chromosomes are the location of Mendel’s heritable factors. Wild type = normal, common phenotypes in the fly populations Morgan’s Fruit Flies Red eye = wild type White eye = mutant Traits alternative to the wild type are called mutant phenotypes Two varieties of sex chromosomes: a larger X chromosome and a smaller Y chromosome ◦ Only the ends of the Y chromosome have regions that are homologous with the X chromosome A gene located on either sex chromosome = sex-linked gene Sex-linked patterns of inheritance ◦ For a recessive sex-linked trait to be expressed A female needs two copies of the allele A male needs only one copy of the allele Sex-linked recessive disorders are much more common in males than in females Linkage map = genetic map of a chromosome based on recombination frequencies ◦ Distances between genes expressed as map units; one map unit, or centimorgan, = 1% recombination frequency ◦ Indicate relative distance and order, not precise locations of genes Genes far apart on the same chromosome can have a recombination frequency near 50% ◦ Physically linked, but genetically unlinked, and behave as if found on different chromosomes Results from the fertilization of gametes in which nondisjunction occurred = abnormal number of a particular chromosome A monosomic zygote has only one copy of a particular chromosome A trisomic zygote has three copies of a particular chromosome More than two complete sets of chromosomes ◦ Triploidy (3n) is three sets of chromosomes ◦ Tetraploidy (4n) is four sets of chromosomes Common in plants, but not animals More normal in appearance than aneuploids Extranuclear genes (or cytoplasmic genes) are genes found in organelles in the cytoplasm ◦ Mitochondria, chloroplasts, and other plant plastids carry small circular DNA molecules ◦ Extranuclear genes are inherited maternally because the zygote’s cytoplasm comes from the egg Some defects in mitochondrial genes prevent cells from making enough ATP and result in diseases that affect the muscular and nervous systems 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Explain the chromosomal theory of inheritance and its discovery Explain why sex-linked diseases are more common in human males than females Distinguish between sex-linked genes and linked genes Explain how meiosis accounts for recombinant phenotypes Explain how linkage maps are constructed Explain how nondisjunction can lead to aneuploidy Define trisomy, triploidy, and polyploidy Distinguish among deletions, duplications, inversions, and translocations Explain genomic imprinting Explain why extranuclear genes are not inherited in a Mendelian fashion