THE CHROMOSOMAL BASIS OF INHERITANCEspeedwaybiology.weebly.com/uploads/1/3/2/4/... · •The unique...
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THE CHROMOSOMAL BASIS OF INHERITANCE
CHAPTER 15
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What you must know: • How the chromosome theory of inheritance
connects the physical movement of chromosomes in meiosis to Mendel’s laws of inheritance.
• The unique pattern of inheritance in sex-linked genes.
• How alteration of chromosome number or structurally altered chromosomes (deletions, duplications, etc.) can cause genetic disorders.
• How genetic imprinting and inheritance of mitochondrial DNA are exceptions to standard Mendelian inheritance.
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Chromosome theory of inheritance:
• Genes have specific locations (loci) on chromosomes
• Chromosomes segregate and assort independently
Chromosomes tagged to reveal a specific gene (yellow).
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Thomas Hunt Morgan
• Drosophila melanogaster – fruit fly
– Fast breeding, 4 prs. chromosomes (XX/XY)
• Sex-linked gene: located on X or Y chromosome
– Red-eyes = wild-type; white-eyes = mutant
– Specific gene carried on specific chromosome
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Sex determination varies between
animals
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Sex-linked genes
• Sex-linked gene on X or Y
• Females (XX), male (XY)
– Eggs = X, sperm = X or Y
• Fathers pass X-linked genes to daughters, but not sons
• Males express recessive trait on the only X (hemizygous)
• Females can be affected or carrier
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Transmission of sex-linked recessive traits
Sperm
Ova
Sperm
Ova
Sperm
Ova
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Sex-linked disorders
• Colorblindness
• Duchenne muscular dystrophy
• Hemophilia
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X-Inactivation Barr body = inactive X chromosome; regulate gene
dosage in females during embryonic development
Because of this only female cats can be
tortoiseshell or calico.
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Human development • Y chromosome required for development of testes
• Embryo gonads indifferent at 2 months
• SRY gene: sex-determining region of Y
• Codes for protein that regulates other genes
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Linked genes: located on same chromosome and tend to be inherited together during cell division
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Genetic Recombination: production of offspring with new combo of genes from parents
• Unlinked genes: follow law of independent assortment
–50% frequency of recombination observed for any 2 genes on different chromosomes
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Crossing over: explains why some linked genes get separated during meiosis
• the further apart 2 genes on same chromosome, the higher the probability of crossing over and the higher the recombination frequency
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Geneticists can use recombination data to map a
chromosome's genetic loci
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Linkage Map: genetic map that is based on % of cross-over events
• 1 map unit = 1% recombination frequency
• Express relative distances along chromosome
• 50% = far apart on same chromosome or on different chromosomes
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Nondisjunction: chromosomes fail to separate properly in Meiosis I or Meiosis II
• Aneuploidy: incorrect # chromosomes
– Monosomy (1x) or Trisomy (3x)
• Polyploidy: 2+ complete sets of chromosomes;
3n or 4n
– Rare in animals, frequent in plants
A tetraploid mammal. Scientists think this species may have arisen when an
ancestor doubled its chromosome # by errors in mitosis or meiosis.
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Nondisjunction
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The structure of an individual chromosome can be altered during DNA replication
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Karyotyping can detect nondisjunctions.
Down Syndrome = Trisomy 21
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Karyotyping can detect nondisjunctions.
Klinefelters Syndrome: 47XYY, 47XXY
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Karyotyping can detect nondisjunctions.
Turners Syndrome = 45XO
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Extranuclear Genes • Some genes located in
organelles
– Mitochondria, chloroplasts, plastids
– Contain small circular DNA
– Do not display Mendelian inheritance
• Mitochondria = maternal inheritance (eggs)
Variegated (striped or spotted) leaves result from mutations in
pigment genes in plastids, which generally are inherited from
the maternal parent.