general genetics ayesha m. khan spring 2013. chromosome morphology metacentric – centromere is...
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GENERAL GENETICS
Ayesha M. KhanSpring 2013
Chromosome Morphology• Metacentric
– Centromere is centrally located; arms equal length
• Submetacentric– Centromere is off center
• Acrocentric– Centromere is close to one
end– a long arm and a knob, or
satellite, at the other• Telocentric
– Centromere is at one end– Not present in humans
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
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• 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
Staining
(a) G bands (b) Q bands (c) C-bands (d) R-bands
6
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
7
Chromosome rearrangements
4 types– Duplications
– Deletions
– Inversions
– Translocations
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
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
Gene dosage
Deletions• Loss of a portion of
chromosome• 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
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
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
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
Paracentric inversion loop
Inversion loops
• Pericentric inversion loops– Crossing over within
loop creates recombinant chromosomes with duplications and deletions• nonviable
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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