Occurs in cells that give rise to gametes•Produces cells with the haploid (1n) chromosome number•Mitosis occurs in somatic cells and produces diploid (2n) daughter cells•MeiosisStages of MeiosisSpindle formation○Breakdown of nuclear envelope○Nucleoli disappear○Chromosomes condense○Each chromosome consists of two chromatids from S phase of Interphase○Homologous chromosome pairs lie next to one another○Called chiasma (chiasmata - plural)□Hold homologous chromosomes together□Recombinant chromosomes result®Increases gamete diversity®Can result in a physical exchange of genetic material between homologs □Crossing overCrossing over of chromatids between pairs (chiasma formation)○Prophase 1•Chromosome pairs align on equator of spindle○Kinetochores of sister chromatids are attached to fibers going to the same pole○Metaphase 1•Homologous chromosome pairs separate○(sister chromatids remain together)○Anaphase 1•Depending on the species:○Nuclear membrane may reform○Cytokinesis may occur○Telophase 1•Meiosis 1Prophase 2•Meiosis 2Spermatogenesis in diploid reproductive tissues•Spermatocytes undergo meiotic division and produce spermatids•Spermatids mature into spermatazoa•Gamete formation in human malesBefore birth•Oogonia (2n) gives rise to primary oocytes (2n) stopped in prophase 1•At ovulation meiosis continues with polar body (1n) formation and a large egg (1n)•Inheritance of mitochondria from the mother•Gamete formation in human femalesMeiosisThursday, March 13, 20143:57 PM Notes Page 1Born in 1822•Monk at monastery st. Thomas•1866 published "experiments with Plant Hybrids"•1900 mendel's work was rediscovered•"particulate" rather than" blending" inheritance•English in 1824 worked with same plants before □Discovered the same thing mendel did but was not quantitative, so he doesn’t get any credit for it □John gossThere was a long history of work on peas○Self pollinatesStigma and anthers are completely enclosed by petals until after fertilizationNo cross contamination, complete control of reproductionPerfect organism for the peas○Simple single gene inheritanceMendel picked the "right" traits to follow○He followed the crosses for several generations. ○He was quantitative in his observations○He had a little help from his friends○Mendel worked with edible pea plants•Gregor MendelDiscrete unit of hereditary information consisting of a specific sequence of DNA○Often codes for a polypeptide chain○Gene•A particular position along a specific chromosome where a given gene is located○Gene locus•Alternative form of a gene (A, a or B, b)○Allele•Having 2 identical alleles for a trait (AA or BB)○Homozygous•Having 2 different genes (alleles) for a trait (Aa, Bb)○Heterozygous•The genetic make up of an individual○Genotype•How it appearsThe physical manifestation of a trait○Determined by the genotype○Phenotype•Allele that is fully expressed in the phenotype of a heterozygote○Only a single copy is required to express the trait○Represented by a capital letter (A)○Dominant allele•In a heterozygous individuals the allele that is completely masked (not expressed) in the phenotype○Recessive allele•DefinitionsGeneticsThursday, March 13, 20143:46 PM Notes Page 2phenotypeRepresented by lower case letter (a)○Codes for an inactive or defective enzyme○Parental○P•Filial one - offspring of the parental generation - hybrids○F1•Filial two - offspring of cross between F1 (hybrid) individuals○F2•GenerationsThe two members of a gene pair segregate (separate) from each other into the gametes•Each gamete carries one member of the gene pair•The homologous chromosomes separate during anaphase 1 of meiosis •Alleles segregate into different gametes•Mendel's First LawIndividual displaying a dominant trait may be homozygous or heterozygous•Individual displaying a recessive trait must be homozygous for the recessive allele•Genotype - phenotypeHow do you determine whether an individual is homozygous dominant, or heterozygous?•Cross a P? with a homozygous recessive pp, if any pp offspring occur, P? is Pp. If all offspring have the dominant phenotype, then the genotype of test plant is PP•Test crossHomozygous individuals are the same as in complete dominance, •Heterozygous individuals are a mix•Incomplete dominanceHeterozygous individuals express both traits. •A is codominant with B○A is dominant to O○B is dominant to O○Ex. Human ABO blood groups •CodominanceForeign macromolecule that is targeted by the immune system•AntigenProtective proteins targeted against foregn (non-self) materials•Binds to an antigen•AntibodiesBlood type A: anti-B antibody•Blood type A individuals have antibodies recognizing "non-self" type B polysaccharides•Blood type B will have anti-A antibodies•Blood type AB will have no antibodies•Blood type O: anti-A and anti-B antibody•Antibodies against type A and type B •Serum antibodiesTwo genes•Dihybrid Crosses Notes Page 3Two genes•Independent assortment or linkage•Mendel's Second Law: independent assortment•Physically located on the same chromosome are linked 2 genes○The alleles linked on the chromosome will be transmitted into the same gamete○Different expectation for the phenotypes of offspring○Linked genes•Ex. Skin color, eye color○Multiple genes contribute to a trait •Polygenic inheritanceAutosomal versus sex-linked genes•Sex-linked genes are carried on the sex chromosomes•XX(homogametic sex), XY(heterogametic sex)•XX, XO are female•Determines the sex of the child○Father•Allele occurs on one of the sex chromosomes○The X chromosome is the influential one in humans○Sex- or X-linked traits •Determines X-linked traits of the son○Mother•Always passed from father to son○Ex hairy ear rims○78 identified genes○y-linked trait•1098 genes identified○99 encoded proteins expressed in testis and in various tumor types ○168 Mendelian diseases have been explained by mutations in 113 x linked genes○X chromosome•Transmitted by mother to sons○Primarily males are affected ○Ex color blindness, hemophilia○X-linked traits•Inactivation of one X chromosome○Random○Heteroghromatin - tightly coiled inactive DNAResults in Barr Body○Cells of woman are a mosaic for genes on the X chromosome○Dosage compensation•Sex-linkageRh factor is an
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