Meiosis Thursday March 13 2014 3 57 PM Meiosis Stages of Meiosis Meiosis 1 Prophase 1 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 Spindle 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 Crossing over of chromatids between pairs chiasma formation Crossing over Called chiasma chiasmata plural Hold homologous chromosomes together Can result in a physical exchange of genetic material between homologs Recombinant chromosomes result Increases gamete diversity Metaphase 1 Anaphase 1 Chromosome pairs align on equator of spindle Kinetochores of sister chromatids are attached to fibers going to the same pole Homologous chromosome pairs separate sister chromatids remain together Telophase 1 Meiosis 2 Prophase 2 Depending on the species Nuclear membrane may reform Cytokinesis may occur Gamete formation in human males Spermatogenesis in diploid reproductive tissues Spermatocytes undergo meiotic division and produce spermatids Spermatids mature into spermatazoa Gamete formation in human females Before 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 Notes Page 1 Genetics Thursday March 13 2014 3 46 PM Gregor Mendel Born in 1822 Monk at monastery st Thomas 1866 published experiments with Plant Hybrids 1900 mendel s work was rediscovered particulate rather than blending inheritance Mendel worked with edible pea plants There was a long history of work on peas John goss 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 Perfect organism for the peas Self pollinates Stigma and anthers are completely enclosed by petals until after fertilization No cross contamination complete control of reproduction Mendel picked the right traits to follow Simple single gene inheritance He followed the crosses for several generations He was quantitative in his observations He had a little help from his friends Definitions Gene Gene locus Allele Homozygous Heterozygous Genotype Phenotype Discrete unit of hereditary information consisting of a specific sequence of DNA Often codes for a polypeptide chain A particular position along a specific chromosome where a given gene is located Alternative form of a gene A a or B b Having 2 identical alleles for a trait AA or BB Having 2 different genes alleles for a trait Aa Bb The genetic make up of an individual The physical manifestation of a trait How it appears Determined by the genotype Dominant allele Recessive allele 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 In a heterozygous individuals the allele that is completely masked not expressed in the phenotype Notes Page 2 phenotype Represented by lower case letter a Codes for an inactive or defective enzyme Generations P Parental F1 F2 Mendel s First Law Filial one offspring of the parental generation hybrids Filial two offspring of cross between F1 hybrid individuals The 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 Genotype phenotype Test cross Individual displaying a dominant trait may be homozygous or heterozygous Individual displaying a recessive trait must be homozygous for the recessive allele How 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 Incomplete dominance Codominance Homozygous individuals are the same as in complete dominance Heterozygous individuals are a mix Heterozygous individuals express both traits Ex Human ABO blood groups A is codominant with B A is dominant to O B is dominant to O Antigen Antibodies Serum antibodies Foreign macromolecule that is targeted by the immune system Protective proteins targeted against foregn non self materials Binds to an antigen Blood 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 Dihybrid Crosses Two genes Notes Page 3 Two genes Independent assortment or linkage Mendel s Second Law independent assortment Linked genes 2 genes Physically located on the same chromosome are linked The alleles linked on the chromosome will be transmitted into the same gamete Different expectation for the phenotypes of offspring Polygenic inheritance Multiple genes contribute to a trait Ex Skin color eye color Sex linkage Autosomal 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 Sex or X linked traits Allele occurs on one of the sex chromosomes The X chromosome is the influential one in humans Father Mother Determines X linked traits of the son y linked trait Always passed from father to son Ex hairy ear rims 78 identified genes X chromosome 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 linked traits Transmitted by mother to sons Primarily males are affected Ex color blindness hemophilia Dosage compensation Inactivation of one X chromosome Random Results in Barr Body Rh Factor Genetics Rh factor is an antigen on RBCs D is dominant d is recessive Genotypes DD or Dd Heteroghromatin tightly coiled inactive DNA Cells of woman are a mosaic for genes on the X chromosome Notes Page 4 Genotypes DD or Dd Phenotype Rh Genotype dd Phenotype Rh Lack Rh antigen Eye Color Polygenic ingeritance Sickle Cell Anemia Homozygous recessive autosomal Hemoglobin Protein consisting of 4 polypeptide chains 2 alpha and 2 beta Sickle cell hemoglobin Differs in one amino acid in the 6th
View Full Document
Unlocking...