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Photosynthesis

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Chapter 12 Chromosomal Inheritance – PowerPoint PPT presentation

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Title: Photosynthesis


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Outline
  • X-Linked Alleles
  • Human X-Linked Disorders
  • Gene Linkage
  • Crossing-Over
  • Chromosome Map
  • Changes in Chromosome Number
  • Changes in Chromosome Structure
  • Human Syndromes

3
Sex Determination in Humans
  • Sex is determined in humans by allocation of
    chromosomes at fertilization
  • Both sperm and egg carry one of each of the 22
    autosomes
  • The egg always carries the X chromosome as number
    23
  • The sperm may carry either and X or Y
  • If the sperm donates an X in fertilization, the
    zygote will be female
  • If the sperm donates a Y in fertilization, the
    zygote will be male
  • Therefore, the sex of all humans is determined by
    the sperm donated by their father

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X-Linked Alleles
  • Genes carried on autosomes are said to be
    autosomally linked
  • Genes carried on the female sex chromosome (X)
    are said to be X-linked (or sex-linked)
  • X-linked genes have a different pattern of
    inheritance than autosomal genes have
  • The Y chromosome is blank for these genes
  • Recessive alleles on X chromosome
  • Follow familiar dominant/recessive rules in
    females (XX)
  • Are always expressed in males (XY), whether
    dominant or recessive
  • Males said to be monozygous for X-linked genes

5
Eye Color in Fruit Flies
  • Fruit flies (Drosophila melanogaster) are common
    subjects for genetics research
  • They normally (wild-type) have red eyes
  • A mutant recessive allele of a gene on the X
    chromosome can cause white eyes
  • Possible combinations of genotype and phenotype

Genotype Genotype Phenotype Phenotype
XRXR Homozygous Dominant Female Red-eyed
XRXr Heterozygous Female Red-eyed
XrXr Homozygous Recessive Female White-eyed
XRY Monozygous Dominant Male Red-eyed
XrY Monozygous Recessive Male White-eyed
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X-Linked Inheritance
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Human X-Linked DisordersRed-Green Color
Blindness
  • Color vision In humans
  • Depends three different classes of cone cells in
    the retina
  • Only one type of pigment is present in each class
    of cone cell
  • The gene for blue-sensitive is autosomal
  • The red-sensitive and green-sensitive genes are
    on the X chromosome
  • Mutations in X-linked genes cause RG color
    blindness
  • All males with mutation (XbY) are colorblind
  • Only homozygous mutant females (XbXb) are
    colorblind
  • Heterozygous females (XBXb) are asymptomatic
    carriers

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Red-Green Colorblindness Chart
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X-Linked Recessive Pedigree
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Human X-Linked DisordersMuscular Dystrophy
  • Muscle cells operate by release and rapid
    sequestering of calcium
  • Protein dystrophin required to keep calcium
    sequestered
  • Dystrophin production depends on X-linked gene
  • A defective allele (when unopposed) causes
    absence of dystrophin
  • Allows calcium to leak into muscle cells
  • Causes muscular dystrophy
  • All sufferers male
  • Defective gene always unopposed in males
  • Males die before fathering potentially homozygous
    recessive daughters

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Human X-Linked DisordersHemophilia
  • Bleeders Disease
  • Blood of affected person either refuses to clot
    or clots too slowly
  • Hemophilia A due to lack of clotting factor IX
  • Hemophilia B due to lack of clotting factor
    VIII
  • Most victims male, receiving the defective allele
    from carrier mother
  • Bleed to death from simple bruises, etc.
  • Factor VIII now available via biotechnology

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Hemophilia Pedigree
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Human X-Linked DisordersFragile X Syndrome
  • Due to base-triplet repeats in a gene on the X
    chromosome
  • CGG repeated many times
  • 6-50 repeats asymptomatic
  • 230-2,000 repeats growth distortions and mental
    retardation
  • Inheritance pattern is complex and unpredictable

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Gene Linkage
  • When several genes of interest exist on the same
    chromosome
  • Such genes form a linkage group
  • Tend to be inherited as a block
  • If all genes on same chromosome
  • Gametes of parent likely to have exact allele
    combination as gamete of either grandparent
  • Independent assortment does not apply
  • If all genes on separate chromosomes
  • Allele combinations of grandparent gametes will
    be shuffled in parental gametes
  • Independent assortment working

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Chromosome NumberPolyploidy
  • Polyploidy
  • Occurs when eukaryotes have more than 2n
    chromosomes
  • Named according to number of complete sets of
    chromosomes
  • Major method of speciation in plants
  • Diploid egg of one species joins with diploid
    pollen of another species
  • Result is new tetraploid species that is
    self-fertile but isolated from both parent
    species
  • Some estimate 47 of flowering plants are
    polyploids
  • Often lethal in higher animals

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Chromosome NumberAneuploidy
  • Monosomy (2n - 1)
  • Diploid individual has only one of a particular
    chromosome
  • Caused by failure of synapsed chromosomes to
    separate at Anaphase I (nondisjunction)
  • Trisomy (2n 1) occurs when an individual has
    three of a particular type of chromosome
  • Diploid individual has three of a particular
    chromosome
  • Also caused by nondisjunction
  • This usually produces one monosomic daughter cell
    and one trisomic daughter cell in meiosis I
  • Down syndrome is trisomy 21

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Nondisjunction
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Chromosome NumberAbnormal Sex Chromosome Number
  • Result of inheriting too many or too few X or Y
    chromosomes
  • Caused by nondisjunction during oogenesis or
    spermatogenesis
  • Turner Syndrome (XO)
  • Female with single X chromosome
  • Klinefelter Syndrome (XXY)
  • No matter how many X chromosomes, presence of Y
    renders individual male

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Chromosome NumberAbnormal Sex Chromosome Number
  • Ploy-X females
  • XXX simply taller thinner than usual
  • Some learning difficulties
  • Many menstruate regularly and are fertile
  • More than 3 Xs renders severe mental retardation
  • Jacobs syndrome (XYY)
  • Tall, persistent acne, speech reading problems

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Abnormal Chromosome Structure
  • Deletion
  • Missing segment of chromosome
  • Lost during breakage
  • Translocation
  • A segment from one chromosome moves to a
    non-homologous chromosome
  • Follows breakage of two nonhomologous chromosomes
    and improper re-assembly

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Deletion, Translocation,Duplication, and
Inversion
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Abnormal Chromosome Structure
  • Duplication
  • A segment of a chromosome is repeated in the same
    chromosome
  • Inversion
  • Occurs as a result of two breaks in a chromosome
  • The internal segment is reversed before
    re-insertion
  • Genes occur in reverse order in inverted segment

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Abnormal Chromosome Structure
  • Deletion Syndromes
  • Williams syndrome - Loss of segment of chromosome
    7
  • Cri du chat syndrome (cats cry) - Loss of
    segment of chromosome 5
  • Translocations
  • Alagille syndrome
  • Some cancers
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