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GENERAL GENETICS Ayesha M. Khan Spring 2013. Chromosome Morphology Metacentric – Centromere is centrally located; arms equal length Submetacentric – Centromere.

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Presentation on theme: "GENERAL GENETICS Ayesha M. Khan Spring 2013. Chromosome Morphology Metacentric – Centromere is centrally located; arms equal length Submetacentric – Centromere."— Presentation transcript:

1 GENERAL GENETICS Ayesha M. Khan Spring 2013

2 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

3 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

4 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

5 Staining (a) G bands (b) Q bands (c) C-bands (d) R-bands

6 Types of chromosome mutations 6 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 7

8 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

9 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

10 Gene dosage

11 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

12 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

13 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

14 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

15 Paracentric inversion loop

16 Inversion loops Pericentric inversion loops – Crossing over within loop creates recombinant chromosomes with duplications and deletions nonviable

17 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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