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
On human chromosomes, the short arm is designated by the letter p and the long arm by the letter q.

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
A picture of metaphase chromosomes lined up in descending order of their size

4. Staining G banding C banding R banding Q 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
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

8. Duplications Section of chromosome is doubled Tandem Displaced Reverse
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
abnormal gene dosage: imbalances in the amounts of gene products

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