Chapter15.pdf BIOLOGY 108 - 0001
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AP Biology 20122013 Unit 6 Chapter 15 The Chromosomal Basis of Inheritance 151 Physical Basis of Chromosomal Behavior Chromosome Theory of Inheritance genes have specific loci positions along chromosomes it is chromosomes that undergo independent assortment and segregation Law of Independent Assortment random lining up of homologous pairs during Metaphase I Law of Segregation alleles separate and go to different gametes Morgan39s Drosophila Discovery Through crossing wild type red eyed ies with mutant white eyed ies it was observed that only male offspring would inherit the mutant trait Conclusion since males have XY sex chromosomes and females have XX the mutant gene must be on the X chromosome meaning the males needed only one mutant gene to show the mutant phenotype while the female needed two mutant genes which was impossible with the tested parents Proves Chromosome Theory of Inheritance 152 SexLinked Genes a XY System females XX males XY Humans and most mammals Sex determined by sperm Y chromosome much smaller than X chromosome b X0 System females XX males X Grasshoppers cockroaches and some other insects c ZW System females ZW males ZZ Birds some fish and some insects Sex depends on chromosomes present in egg d HaploDiploid System females diploid males haploid Bees and ants Females develop from fertilized eggs and males develop from unfertilized eggs SexLinked Gene a gene located on either sex chromosome YLinked Genes genes linked to Y chromosome mainly function as sexdetermining genes X Linked Genes genes linked to X chromosome fathers pass xlinked alleles to all daughters but no sons mothers pass xlinked genes to both daughters and sons a girl will only express a gene if she is homozygous for an xlinked gene a son will express a gene if he is heterozygous or homozygous making xlinked disorders more common in males Duchenne Muscular Dystrophy Xlinked disorder that causes progressive weakening of muscles and loss of coordination Hemophilia Xlinked disorder which causes the inability to clot ones blood Inactivation of X Chromosome One of the two X chromosomes in most female mammals will become inactive Barr Body condensed object made out of inactivated X chromosome lies along inside of nuclear envelope clump visible in nucleus reactivated in cells that give rise to ovaries Somatic cells of males and females effectively have the same number of Xlinked genes Females have two types of cells those with active X and those with inactive X resulting in quotmosaicquot phenotypes Ex Calico cat coloring Human sweat gland patches 153 Gene Linkage Linked Genes genes close together on the same chromosome that tend to be inherited together Parental Types offspring exhibiting the phenotype that matches either parent Recombinant Types offspring with combinations of phenotypes not matching either parent result of crossing over variation happens in Prophase I lt50 Recombinant Frequency Linked Genes gt50 Recombinant Frequency genes are far apart on same chromosome crossed over OR genes are on separate chromosomes When genes are linked a great portion of offspring will exactly resemble the parents39 genotypes Genetic Map ordered list of genetic loci along a particular chromosome Recombinant frequency depends on distance between genes on a chromosome Farther apart genes are more likely to crossover Farther apart more space between in which crossing over can occur Map Units distances between genes 1 map unit 1 recombination frequency disclaimer map units do not correspond to actual physical distance Cytogenetic Maps locate genes with respect to chromosomal features ex stained bands 154 Alterations in Chromosome Number Nondisjunction homologous chromosomes do not move apart properly during Meiosis I or sister chromatids fail to separate during Meiosis 11 one gamete receives two of the same chromosome and another receives no copy Aneuploidy condition resulting from an irregular number of a particular chromosome Monosomic cell is missing a copy of a particular chromosome Trisomic cell has an extra copy of a particular chromosome Down Syndrome resulting disorder from abnormal chromosome number extra chromosome 21 trisomic increased rate of leukemia and Alzheimer39s disease decreased rate of high blood pressure and stroke all males and half of females are sexually underdeveloped and sterile frequency of syndrome increases With mother39s age Polyploidy condition in which an organism has more than two complete chromosome sets ex Triploid 3n occurs When a trisomic cell is fertilized Tetraloid 4n occurs When a 2n zygote fails to divide after replicating chromosomes Common in eaten plans bananas 3n Wheat 6n strawberries 8n etc Aneuploidy disrupts genetic balance more than polyploidy Alterations of Chromosome Structure Deletion chromosomal fragment is lost loses certain genes Cri Du Chat Syndrome caused by deletion in chromosome 5 child intellectually disabled unusual facial features and cries like a distressed cat Duplication When a quotdeletedquot fragment attaches to another sister chromatid Inversion When a chromosomal fragment reattaches to original chromosome but in the reverse orientation Translocation broken fragment joins a nonhomologous chromosome Chronic Myelogenous Leukemia caused by translocaton in cells that Will become White blood cell Large deletions duplications and translocations tend to be harmful Reciprocal translocations and inversions are not abnormal but can alter phenotype nondisjunctions in sex chromosomes aren39t as big of a deal as in autosomes Klinefelter Syndrome 1500 men born With XXY chromosomes have male sex organs but testes are abnormally small and the man is sterile Turner Syndrome 12500 females born With X chromosome only sex organs do not mature female is sterile 155 Inheritance Pattern Exceptions Genomic Imprinting variation in phenotype depending on Whether an allele is inherited from the mother or the father mostly occurs on autosomes Methyl groups are added to one of the alleles either shutting it down or activating it Only one parental allele of an imprinted gene is expressed Extranuclear Genes genes that reside outside the nucleus do not display Mendelian inheritance patterns these genes are passed down only from the mother in most cases