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Download 1902: Sutton (American) and Boveri (German) Mendel`s genes
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1902: Sutton (American) and Boveri (German) Mendel’s genes during gamete formation behave like chromosomes during meiosis. Genes are in pairs; so are chromosomes; Alleles of genes segregate equally into gametes: so do homologous chromosomes; Different genes assort independently; so do different chromosome pairs. Theory: Genes are located on chromosomes. Occurs only in germ cells (precursors to gametes). One round of replication followed by two cell divisions. diploid (2n) Meiosis I (homologous chromosomes separate) Meiosis II (sister chromatids separate). 4 products of meiosis each (potentially) a gamete haploid (n) Reduce the number of chromosomes present in each gamete so that sexual reproduction does not lead to an increase in the # of chromosomes Meiosis allows for independent assortment of alleles (Mixes up the genes) to create a genetically distinct individual. Genetic diversity allows for adaptation = evolution. Major stages Meiosis I Prophase I Metaphase I Anaphase I Telophase I Meiosis II (like mitosis) Prophase II Metaphase II Anaphase II Telophase II Prophase I • Condensation of chromosomes chromosomes can be visualized, sister chromatids are joined at centromeres • Homologous chromosomes pair up • Recombination takes place. • Nuclear membrane breakdown Don’t worry about the different stages of Prophase, Leptotene-Diakinesis Metaphase I • Nuclear splindle forms • Paired chromosomes (4 DNA copies) line up on equatorial plane Anaphase I • Homologous chromosomes separate Telophase I • Nuclear membrane reforms • Chromosomes decondense (in some organisms) • Cytokinesis generates two daughter cells Metaphase II Prophase II • Condensation of chromosomes chromosomes can be visualized, sister chromatids are joined at centromeres • Nuclear membrane breakdown • Nuclear spindle fibers form • Chromosomes line up at equatorial plane Anaphase II • Sister chromatids are separated Telophase II • Nuclear membrane reforms • Chromosomes decondense • Cytokinesis generates four daughter cells Meiosis The number of homologous chromosomes (or the number of centromeres) is designated “n”. The amount (or concentration) of DNA is indicated by “c” (1c is the amount in the haploid genome). C is like the number of chromatids per homologous chromosome. A b B B A A A A B a b b B a a b b B a B a B a a a b B A A b b 4 different types of gametes can be produced in equal numbers. AB Ab aB ab A B A A b a B b a B egg AABB A AABb AaBB AaBb A b a B b a 9 A-B-: 3 aaB-: 3 A-bb: 1 aabb) sperm Validation of the Chromosome Theory Chapter 4 continued, pp 98-106. To prove the chromosome theory of inheritance, needed to show that specific traits were transmitted with specific chromosomes. Already knew that sex was correlated with the inheritance of the X or Y, what about other traits. Drosophila melanogaster Short life cycle - 12 days Many progeny Easy to grow in laboratories Nomenclature differs from peas Genes are named after the first mutant allele, not the wild type allele a - recessive allele a+ - wild type allele (dominant to mutant) A - dominant allele A+- wild type (recessive to mutant) Sex Chromosomes Discovered between 1891-1909 Males: mismatched pair of flies have X and Y just like humans chromosomes (XY) Females: matched pair (XX) X and Y are homologs and pair and segregate during meiosis, even though they are very different. Genes in differential region are hemizygous in males (only 1 copy), males are neither homozygous or heterozygous. w+ Differential region Pairing region Genes on sex chromosomes are not necessarily involved in sex functions. { X Y } Y has very few genes. The Fly Room Thomas H. Morgan’s laboratory Columbia University 1910 Calvin Bridges, an undergraduate dishwasher, discovered a white-eyed fly (normal flies have red eyes) in one of the fly bottles he was about to wash P White X Xw/Y (males are hemizygous) Xw+/Xw+ OK, red dominant. X F1 all daughters had red, but 1/2 of the sons had white F2 Xw+/Xw+ Xw+/Xw Xw+/Y Xw/Y this is what you would expect if eye color gene is on the X chromosome. In males, what you see is what you get, genotype = phenotype. Mutant phenotype (white eye) was inherited in the same manner as the X chromosome. This provided a partial proof of the chromosome theory. Definitive proof was done by Calvin Bridges (now a Ph.D student) in Morgan’s laboratory. white Reciprocal Cross P X Xw/Xw F1 X Xw+/Xw Xw Y Xw+ Xw+ Xw Xw+ Y Xw Xw+/Y Xw/Y white F2 Xw Xw Xw Y Mendel’s reciprocal crosses gave similar results, but not these. Bridges’ confusing exception: P X XwXw Xw+Y Expected F1s: Red-eyed females and White-eyed males. However, a white-eyed female or red-eyed male appeared 1/2000 times. Bridge’s exceptions to Mendel’s Law of Segregation were demonstrated cytologically to be exceptions to the regular segregation of mother’s X chromosomes. XwXwY white Normal Separation (disjoining) of Chromosomes Xw+O red Non-disjunction of chromosomes a double negative White females got two X’s from mother instead of 1, and red males got no X’s from female and just the X from father. Calvin Bridges proposed: Sex is determined by the number of X chromosomes, not by the presence of the Y chromosome as was always believed before. 2 X - female 1 X - male 3 or 0 X - lethal (never saw cases of these) P X XwXw XwXw Xw+ Xw+Y Xw+ Y Y Xw Xw Xw+ Xw Xw Xw Xw Y o Xw+ Y XwXwY white Xw+O red Non disjunction can also happen in male, but here phenotypes no different. Sex Chromosomes in Humans Also can have XYY males (XYY syndrome, very minor symptoms). Non-disjunction in humans, can occur at all of the chromosomes at a low frequency. Most trisomies (three chromosomes) and monsomies (one chromosome) die in utero, but with the smallest chromosome 21, they can live. Trisomy 21; Down syndrome. 47 chromosomes are present instead of 46 from duplication of chromosome #21. Extra level of gene product causes phenotype. Trisomy 21 appears to be related to maternal age, amniocentesis suggested. 1:100 Frequency 1:1500 35 Females gametes form shortly after birth, but complete the final stages of meiotic segregation during ovulation. Do XX females have twice the amount of gene products encoded by the X chromosome as males? Mammals = Humans One of X’s is inactivated early in development in females. Human females are mosaics, each cell makes a random choice of which X gets inactivated. Drosophila Both X’s expressed in females, single X has elevated expression in males. Fly females are not mosaics. Calico cats are almost always female. X linked coat color gene O - orange o - black Are there any male Calico cats? They are very rare and sterile (and are XXY). black orange