Lecture 5 BIO 311D 1st Edition Outline of Last Lecture I The Chromosome Theory of Inheritance II Morgan s Experimental Evidence III The Chromosomal Basis of Sex IV X Inactivation in Female Mammals V How Linkage Affects Inheritance VI Genetic Recombination Outline of Current Lecture I Mapping the Distance Between Genes Using Recombination Data II Alterations of Chromosome Number or Structure cause Some Genetic Disorders III Human Disorders Due to Chromosomal Alterations IV Exceptions to Mendelian Genetics V Genomic Imprinting VI Inheritance of Organelle Genes Current Lecture Mapping 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 frequency o Map units indicate relative distance order NOT the precise location of genes Cytogenetic maps indicate the positions of genes with respect to chromosomal features Alterations 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 of gametes in which nondisjunction occurred Monosomic Zygote Has only one copy of a particular chromosome Trisomic Zygote Has three copies of a particular chromosome Polyploidy A condition in which an organism has more than two complete sets of chromosomes o Triplody 3n is three sets of chromosomes o Tetraploidy 4n is four sets of chromosomes o 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 another Human Disorders Due to Chromosomal Alterations Down syndrome is an aneuploid condition that results from three copies of chromosome 21 o The frequency of Down Syndrome increases with the age of the mother o 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 5 o They are mentally retarded and has a catlike cry o Usually die in infancy or early childhood Chronic myelogenous leukemia CML is a cancer caused by translocations of chromosomes Exceptions to Mendelian Genetics Two normal exceptions are 1 Genes located in the nucleus 2 Genes located outside the nucleus In both cases the sex of the parent contributing an allele is a factor in the pattern of inheritance Genomic Imprinting The phenotype depends on which parent passed along the alleles for those traits Involves the silencing of certain genes that are stamped with imprint during gamete production It appears that imprinting is the result of the methylation addition of CH3 of cysteine nucleotides Thought to affect a small fraction of mammalian genes Most imprinted genes are critical for embryotic development Inheritance 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 in diseases that affect the muscular and nervous systems For example mitochondrial myopathy and Leber s hereditary optic neuropathy
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