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Biology Lecture - Inheritance - October 3, 2012.pdf

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Western University
Biology 1201A
Richard Gardiner

Biology Lecture October 3, 2012 Chromosomes and Inheritance Introduction • It was not until 1900 that biology finally caught up with Gregor Mendel. •Independently, Karl Correns, Erich von Tschermak, and Hugo de Vries all found that Mendel had explained the same results 35 years before. •Still, resistance remained about Mendelʼs laws of segregation and independent assortment until evidence had mounted that they had a physical basis in the behavior of chromosomes. •Mendelʼs hereditary factors are the genes located on chromosomes. Mendelian inheritance has its physical basis in the behavior of chromosomes during sexual life cycles: • Around 1900, cytologists and geneticists began to see parallels between the behavior of chromosomes and the behavior of Mendelʼs factors. - Chromosomes and genes are both present in pairs in diploid cells. - Homologous chromosomes separate and alleles segregate during meiosis. - Fertilization restores the paired condition for both chromosomes and genes. Thomas Hunt Morgan • first to associate a specific gene with a specific chromosome in the early 20th century. • Like Mendel, Morgan made an insightful choice as an experimental animal, Drosophila melanogaster, a fruit fly species that eats fungi on fruit. Drosophila melanogaster • small and easily reared in the laboratory. • short life cycle. A new generation every two weeks. • a female lays hundreds of fertilized eggs during her brief life span. The resulting large populations make statistical analysis easy and reliable. •giant ("polytene") chromosomes -salivary glands of the mature larvae. - Morgan spent a year looking for variant individuals among the flies he was breeding - He discovered a single male fly with white eyes instead of the usual red. - The normal character phenotype is the wild type. - Alternative traits are mutant phenotypes. - When Morgan crossed his white-eyed male with a red-eyed female, all of 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 ration in the F2 offspring. - Surprisingly, the white-eyed trait only appeared in males. - All of the females and half the males had red eyes. - Morgan concluded that a flyʼs eye color was linked to its sex. Linkage: - Linkage genes tend to be inherited together because they are located on the same chromosome. - Each chromosome has hundreds or thousands of genes. - Genes located on the same chromosome, linked genes, tend to be inherited together because the chromosome is passed along as a unit. - Results of crosses with linked genes deviate from those expected according too independent assortment. - Morgan observed this linkage and its deviations when he followed the inheritance of characters for body colour and wing size - The wild-type body colour is gray (b+) and the mutant black is (b) - The wild-type wing size is normal (vg+) and the mutant has vestigial wings (vg) - According to independent assortment, this should produce 4 phenotypes in a 1:1:1:1 ration. - Surprisingly, Morgan observed a large number of wild-type and double mutant flies among the offspring. - These phenotypes correspond to those of the parents. - Independent assortment of chromosomes and crossing over produce genetic recombinants. - The production of offspring with new combinations of traits inherited from two parents is genetic recombination. - Genetic recombination can result from independent assortment of genes located on non-homologous chromosomes or from crossing over of genes located on homologous chromosomes. - Mendelʼs dihybrid cross experiments produced some offspring that had a combination of traits that did not match either parent in the P generation. - If the P generation consists of a yellow-round parent (YYRR) crossed with a green wrinkled seed parent (yyrr), all plants in the F1 have yellow rounded seeds (YyRr) - A cross between an F1 plants and a homozygous recessive plant produces four phenotypes. - Half are be parental types, with phenotypes that match the original P parents, either with yellow-round seeds or with green-wrinkled seeds. - Half are recombinant, new combination of parental traits, with yellow-wrinkled or green-round seeds. - A 50% frequency of recombination is observed for any two genes located on different (non-homologous) chromosomes. - The physical basis of recombination between unlinked genes is the random orientation of homologous chromosomes at metaphase 1. - In contrast, linked genes, genes located on the same chromosome tend to move together through meiosis and fertilization. - Under normal Mendelian genetic rules, we would not expect linked genes to recombine into assortments of alleles not found in the parents. - If the seed colour and seed coat genes were linked, we would expect the F1, offspring to produce only two types of gametes, YR and yr when the tetrads separate. - One homologous chromosome from a P generation parent carries the Y and R alleles on the same chromosome and the other homologous chromosome from the other P parent carries the y and r alleles. - The results of Morganʼs test cross for body colour and wing shape did not conform to either independent assortment or complete linkage. - Under independent assortment, the test cross should produce a 1:1:1:1 phenotypic ratio. - If completely linked we should expect to see a 1:1:0:0 ratio with only parental phenotypes among offspring. - Most of the offspring had parental phenotypes, suggesting linkage between the genes. - However, 10% of the flies were recombinants suggesting incomplete linkage. - Geneticists can use recombination data to map a chromosomes genetic loci (location) - One of Morganʼs students, Alfred Sturtevant, used crossing over of linked genes to develop a method for constructing a chromosome map. - This map is an ordered list of the genetic loci along a particular chromosome. - Sturtevant hypothesized that the frequency of recombinant offspring reflected the distances between genes on a chromosome. - The farther apart the two genes are, the higher the probability that a crossover will occur between them and therefore, a higher recombination frequency. - The greater the distance between two genes, the more point between them where crossing over can occur. - Sturtevant used recombination frequencies from fruit fly crosses to map the relative position of genes along chromosomes, a linkage map. - Sturtevan
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