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

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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

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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
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• Cri du chat syndrome (cats cry) - Loss of
  segment of chromosome 5
• Translocations
• Alagille syndrome
• Some cancers