Lecture 5 BIO 311D 1st EditionOutline of Last Lecture I. The Chromosome Theory of InheritanceII. Morgan’s Experimental EvidenceIII. The Chromosomal Basis of SexIV. X Inactivation in Female MammalsV. How Linkage Affects InheritanceVI. Genetic RecombinationOutline of Current Lecture I. Mapping the Distance Between Genes Using Recombination DataII. Alterations of Chromosome Number or Structure cause Some Genetic DisordersIII. Human Disorders Due to Chromosomal AlterationsIV. Exceptions to Mendelian GeneticsV. Genomic ImprintingVI. Inheritance of Organelle GenesCurrent LectureMapping the Distance Between Genes Using Recombination Data:- Alfred Sturterant (Morgan’s Student) constructed a genetic map- Genetic map is an ordered list of genetic loci along a particular chromosome- He predicted that the farther apart two genes are, the higher probability that a crossover will occur between them, therefore a higher recombination frequency- Linkage map: Genetic map of a chromosome based on recombination frequencies- Map Units: Distances between genes o One centimorgan represents a 1%recombination frequencyo Map units indicate relativedistance/order, NOT the preciselocation of genes- Cytogenetic maps indicate the positions ofgenes with respect to chromosomalfeaturesAlterations of Chromosome Number or Structure Cause some Genetic Disorders:- Large scale chromosomal alterations lead to spontaneous abortions, miscarriages, and developmental disorders- Plants tolerate such genetic changes better than humans- Nondisjunction: Pairs of homologous chromosomes do not separate during meiosis (can happen in meiosis I or meiosis II)- Nondisjunction leads to an extra chromosome in gamete- As a result, one gamete receives two of the same type of chromosome and another gamete receives no copy- Aneuplody results from the fertilization ofgametes in which nondisjunction occurred- Monosomic Zygote: Has only one copy ofa particular chromosome- Trisomic Zygote: Has three copies of aparticular chromosome- Polyploidy: A condition in which anorganism has more than two completesets of chromosomeso Triplody (3n) is three sets ofchromosomeso Tetraploidy (4n) is four sets ofchromosomeso This is most common in plants- Breakage of a chromosome can lead to four types of changes in chromosomes:(a) Deletion: Removes a chromosomal agent(b) Duplication: Repeats a segment(c) Inversion: Reverses the orientation of a segment(d) Translocation: Moves a segment from one chromosome to anotherHuman Disorders Due to Chromosomal Alterations:- Down syndrome is an aneuploid condition that results from three copies of chromosome21o The frequency of Down Syndrome increases with the age of the mothero Caused by nondisjunction of sex chromosomes- Klinefelter syndrome is a result of an extra chromosome in a male (producing XXY)- Monosomy X or the Turner syndrome produces X0 females- Cri Du Chat Syndrome results from a specific deletion in chromosome 5o They are mentally retarded and has a catlike cryo Usually die in infancy or early childhood- Chronic myelogenous leukemia (CML) is a cancer caused by translocations of chromosomesExceptions to Mendelian Genetics:- Two normal exceptions are:1. Genes located in the nucleus2. Genes located outside the nucleus- In both cases, the sex of the parentcontributing an allele is a factor in thepattern of inheritanceGenomic Imprinting:- The phenotype depends on which parentpassed along the alleles for those traits- Involves the silencing of certain genes thatare stamped with imprint during gameteproduction- It appears that imprinting is the result ofthe methylation (addition of —CH3) ofcysteine nucleotides- Thought to affect a small fraction of mammalian genes- Most imprinted genes are critical for embryotic developmentInheritance of Organelle Genes:• Extranuclear genes (or cytoplasmic genes) are found in organelles in the cytoplasm• Mitochondria, chloroplasts, and other plant plastids carry small circular DNA molecules• Extranuclear genes are inherited maternally because the zygote’s cytoplasm comes from the egg• The first evidence of extranuclear genes came from studies on the inheritance of yellow or white patches on leaves of an otherwise green plant• Some defects in mitochondrial genes prevent cells from making enough ATP and result indiseases that affect the muscular and nervous systems • For example, mitochondrial myopathy and Leber’s hereditary optic
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