chapter 9 – chromosomal variation. chromosome morphology metacentric –centromere is centrally...
TRANSCRIPT
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Chapter 9 – Chromosomal Variation
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Chromosome Morphology• Metacentric
– Centromere is centrally located; arms equal length• “p” and “q” – “p” is smaller when there is a size difference
• Submetacentric– Centromere is off center
• Acrocentric– Centromere is close to one end– p arm has satellites (knobs on stalks)
• Telocentric– Centromere is at one end– Not present in humans
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Karyotype
• Complete set of chromosomes arranged in homologous pairs
• Sample is from an actively dividing cell– Chemical inhibits spindle
assembly formation• Cell can not complete
mitosis
– Hypotonic solution swells cell
• Allows chromosomes to spread out
– Dropped on slide and stained
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Staining • G banding
– Giemsa stain; most common– Stains A-T rich regions
• C banding– Stains centromeric heterochromatin and portions of
chromosomes with large sections of heterochromatin• 1, 9, 16, Y
• R banding– Stains G-C rich regions– Gives opposite banding pattern of G banding
• Q banding– UV light is used– Same pattern as G banding
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Staining
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Types of chromosome mutations
• Chromosomal rearrangement– Structure is altered
• Aneuploidy– Abnormal number of chromosomes– Missing one or more/having one or more extra
• Polyploidy– 1 or more additional sets of chromosomes
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Chromosome rearrangements
• 4 types– Duplications
– Deletions
– Inversions
– Translocations
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Duplications• Section of chromosome is
doubled
• Tandem– Repeated segment is right
after the original
• Displaced– Repeated segment is
located elsewhere on chromosome, or on a different chromosome
• Reverse– Sequence is inverted from
the original sequence
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Duplications• Heterozygotes
– During paring of homologous chromosomes, duplicated region loops out
– Offspring receive two copies of involved genes from parent with duplication, and a third copy of the other parent
• Partial trisomy for all involved genes
• Alters gene dosage
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Gene dosage
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Deletions• Loss of a portion of
chromosome
• Large deletions can be seen cytogenetically; microdeletions by FISH
• If the deleted region includes the centromere, entire chromosome will be lost
• Usually lethal in homozygous form
• Heterozygotes – Normal chromosome must
loop out during pairing– Partial monosomy for all
involved genes
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Deletions - heterozygotes• Affects gene dosage
• Pseudodominance– Expression of mutant/recessive phenotype
due to loss of normal/dominant copy
• Haploinsufficiency – Both copies of the gene are needed to
manufacture adequate amount of gene product
• One gene doesn’t produce enough for a normal phenotype
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Inversions• Two breaks in chromosome, then flipped and
reinserted
• Paracentric inversion– Both breaks occur in one arm
• Pericentric inversion– Breaks on both arms; centromere is involved– Can change morphology by altering centromere position
• Effects– Disruption of a gene – no functional product– Position effect
• Change in gene position can affect gene expression
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Inversion loops• Chromosomes have to
loop when pairing
• Paracentric inversion loops
– If crossing over occurs within loop:
– Creates a dicentric chromosome and an acentric chromosome
• Acentric is lost• Dicentric forms a dicentric
bridge, and breaks• Nonviable recombinant
gametes
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Paracentric inversion loop
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Inversion loops
• Pericentric inversion loops
– Crossing over within loop creates recombinant chromosomes with duplications and deletions
• nonviable
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Pericentric inversion loops
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Translocations• Rearranges genetic material
to another part of the same chromosome; or nonhomologous chromosome
• Nonreciprocal– Segment moves from one
chromosome to another
• Reciprocal– Exchange between two
chromosomes
• Effects– Loss of gene function –
break– Position effect– Creation of a
fusion/abnormal protein
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Robertsonian translocation• Between two
acrocentric chromosomes– 13, 14, 15, 21, 22
• 2 q arms are joined at a common centromere– Forms a metacentric
chromosome if two chromosomes are same size
• Small fragment is usually lost– Tends to be acentric
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Translocations
• Translocated chromosome is named after the chromosome that is the origin of the centromere
• Heterozygotes have one normal copy of a chromosome, and one translocated one– During meiosis, all 4 chromosomes will
associate– Can segregate 1 of 3 ways
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Translocation segregation
• Alternate– Both normals go to one
pole; both translocated go to the other
• Balanced; viable
• Adjacent 1– Each pole gets one normal,
and the opposite translocated
– Partial monosomies/partial trisomies
• inviable
• Adjacent 2– Each pole gets both the
normal and translocated of the same chromosome
– Inviable; rare
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Translocation segregation
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Fragile sites
• Under certain conditions/culturing techniques, chromosomes develop breaks/restrictions at particular locations
• Now routinely tested for by FISH analysis
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Aneuploidy
• Abnormal number of chromosomes– Caused by:
• Loss of chromosome during cell division; random error or loss of centromere; nondisjunction
• Robertsonian translocation
• Types– Nullisomy 2n – 2 – missing both members of a
homologous pair– Monosomy 2n – 1 – missing one chromosome– Trisomy 2n + 1 – one extra chromosome– Tetrasomy – 2n + 2 – two extra chromosomes of the
same type/homologous
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Aneuploidy
• Alters phenotype dramatically– Often lethal if constitutional
• Can see elaborate abnormalities in tumor cells – X inactivation in mammals takes care of extra Xs, so
not as severe
• Down syndrome– Primary
• 3 free copies of #21– Familial
• Extra copy due to translocation– Can be involved in Robertsonian translocation– Parent can have 45 chromosomes, but have normal phenotype
since all genetic material is present
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Uniparental Disomy• Both chromosomes of a homologous pair
from the same parent
• Probably originated from a trisomy– One chromosome is lost early in development
• Recessive diseases– One carrier parent and one normal parent can
have an affected child
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Mosaicism
• Nondisjunction in later development can cause “patchiness” – normal cells and abnormal cells
• Approximately 50% of Turner syndrome can be mosaics– 45, XO/46, XX
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Polyploidy• Extra sets of chromosomes
– Triploid – 3n; tetraploid – 4n
• Common in plants – more tolerant of extra sets of chromosomes
• Autopolyploidy– Extra set is from same species
• Error in cell division
– Extra chromosome caused pairing problems; especially with odd numbers
• 3n usually sterile; produce small seeds– Bananas; “seedless” watermelon
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Polyploidy
• Allopolyploidy– Hybridization between two species– AABBCC x GGHHII– F1 generation ABCGHI – not homologous
• Gametes are inviable, but may be able to reproduce asexually
– Nondisjunction error can lead 2x, which could then reproduce sexually
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