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Chapter 11

Chapter 11 notes (Chromosomes and Inheritance).docx

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Biology 1201A
Richard Gardiner

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Biology: Exploring the Diversity of Life Chapter 11: Chromosomes and Inheritance (PAGES 234-254) - Thomas Hunt Morgan o First to associate a specific gene with a specific chromosome in the early 20th century. o Morgan uses a fruit fly, Drosophila melanogaster, that eats fungi on fruit  Small and versatile  Short life cycle (2 weeks)  A female fertilizes hundreds of fertilized eggs during her brief life span. The resulting large populations make statistical analysis easy and reliable.  Large (“polytene”) chromosomes o He spent years looking for variant individuals  Discovered a male with white eyes instead of the usual red  Wild type: An individual having the normal phenotype; that is, the phenotype generally found in a natural population of organisms.  Mutant: An individual having a phenotype that differs from the normal phenotype. o His findings:  When Morgan crossed his white-eyed male with a red- eyed female, all the F1 offspring had red eyes,  The red allele appeared dominant to the white allele.  Crosses between the F1 offspring produced the classic 3:1 phenotypic ratio in the F2 offspring.  Surprisingly, the white-eyed trait appeared only in males.  All the females and half the males had red eyes.  Morgan concluded that a fly’s eye color was linked to its sex. - Linkage o Linked genes tend to be inherited together because they are located on the same chromosome o Results of crosses with linked genes deviate from those expected according to independent assortment. o According to independent assortment, this should produce 4 phenotypes in a 1:1:1:1 ratio. o Surprisingly, Morgan observed a large number of wild-type (gray- normal) and double-mutant (black-vestigial) flies among the offspring. These phenotypes correspond to those of the parents. o Morgan observed this linkage and its deviations when he followed the inheritance of characters for body color and wing size.  The wild-type body color is gray (b+) and the mutant black (b).  The wild-type wing size is normal (vg+)and the mutant has vestigial wings (vg). - Independent assortment of chromosomes and crossing over produce genetic recombinants: o http://www.youtube.com/watch?v=xs0qolrQ-4Q o The production of offspring with new combinations of traits inherited from two parents is genetic recombination. o Genetic recombination can result from independent assortment of genes located on nonhomologous chromosomes or from crossing over of genes located on homologous chromosomes.  Can ‘un-link’ genes  http://www.youtube.com/watch?v=2pcpZiKHbM8 - Mendel’s dihybrid cross experiments produced some offspring that had a combination of traits that did not match either parent in the P generation o If the P generation consists of a yellow-round parent (YYRR) crossed with a green-wrinkled seed parent (yyrr), all F1 plants have yellow-round seeds (YyRr). o Half are be parental types, with phenotypes that match the original P parents, either with yellow-round seeds or green- wrinkled seeds. o Half are recombinants, new combination of parental traits, with yellow-wrinkled or green-round seeds. o In contrast, linked genes, genes located on the same chromosome, tend to move together through meiosis and fertilization. o Under normal Mendelian genetic rules, we would not expect linked genes to recombine into assortments of alleles
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