Chromosome Mutations Flashcards
Change from normal chromosome structure and or number.
- Change in gene expression pattern if the organism is viable
- detectable through karyotyping
Chromosomal aberrations
Common occurrences of chromosome mutations:
- 6/1000 live births
- still births/miscarriages
- embryonic lethal
- if survive embryonic stage:
- mental and developmental disorders and infertility
Variations in chromosome number
Euploid
Aneuploid
True ploidy level; # of chromosomes in the individual’s cells is a multiple of the normal haploid set.
ex. humans- haploid: 23; diploid: 46
Euploid
Not a multiple of the normal haploid set; one or more extra or missing chromosomes.
ex. humans- haploid: 23; Turner Syndrome: 45,X
2n-1
Aneuploid
Homologous chromosomes during anaphase I or sister chromatids during anaphase II don’t segregate properly.
-fertilzation of aneuploid gametes leads to aneuploid progeny.
Nondisjunction
Aneuploid Conditions
Nullisomy
Monosomy
Trisomy
Tetrasomy
Both homologous chromosomes are absent
-embryonic lethal
Nullisomy
One chromosome is missing
-Turner Syndrome: 2n-1
Monosomy
One extra chromosome
-2n+1
Trisomy
Two extra chromosomes
-2n+2
Tetrasomy
Example of aneuploidy: Trisomy 21
Down Syndrome in humans
-consequences: problem with gene dosage
Trisomy 21 and Advance Maternal Age
Oocytes are arrested in prophase I
-each month, after puberty, meiosis continues in the one egg.
Ploidy levels of somatic cells
monoploidy
diploidy
polyploidy
One set
monoploidy
Two sets
Diploidy
At least two sets
Polyploidy
Somatic cell; result from meitotic division.
Monoploidy
Result from meiosis; germ-line cells which will fuse with other haploid cells
Haploidy
Different Types of Polyploidy
Autopolyploidy
Allopolyploidy
Endopolyploidy
Extra set of chromosomes is identical to the normal haploid set
Autopolyploidy
Chromosomes come from two different species to produce a hybrid
Allopolyploidy
When only certain cells within an organism are polyploidy.
Endopolyploidy
Changes in chromosome structure (4 major types)
- Deletion
- Duplication
- Inversion
- Translocation
How do changes in chromosomes occur?
- Breaks in the chromosome
- General errors in recombination
When part of a chromosome or a sequence of DNA is missing.
Deletion
If deleted piece contains recessive allele
Normal phenotype
If deletion of a dominant allele and unmasking of a recessive allele.
Pseudodominance
Deletion reduces gene dosage
Haplosufficiency
Deletion removes a centromere (nondisjunction–>aneuploidy)
Acentric chromosome
Cri-du-chat
gene encoding telomerase is missing
46, 5P
Deletion
when part of a chromosome is copied abnormally, resulting in extra genetic material from the duplicated segment.
-Slippage during S phase can also cause this to happen
Duplications
- Gene amplification; more gene product
- Unequal crossing over can induce a deletion
- Duplication loops
Consequences of duplication
Reduced number of eye facets due to extra negatively influencing 16A allele.
-incomplete dominance
Bar allele in Fruit Flies; example of duplication
Duplications might result in _______.
ex. hemoglobin subunits and rRNA genes
Multigene families
Multiple genes with related functions near each other on the same chromosome
Multigene family
Type of homolog, 2 or more genes in an organism that have similar or identical functions.
Paralogs
when a segment of a chromosome is reversed from end to end. (breakage and rearrangement within itself)
Inversion
Two types of inversions
- Paracentric
2. Pericentric
Inversion that doesn’t include the centromere
Paracentric
Inversion that does include the centromere
Pericentric
What causes breakage for an inversion to happen
Inversion loop
location of gene in chromosome can effect how much of that gene is transcribed
-over-expression of genes that are not meant to be silenced
Consequences of Inversions: Position effect
Rearrangement of parts between nonhomologous chromosomes.
Translocation
Occurs when the transfer of chromosome material is one way
Non-reciprocal translocation
Occurs when chromosomes swap parts
Reciprocal translocation
Consequences of Translocations
- position effect
- Non- wild type gene order
Philadelphia chromosome in chronic myelogenous leukemia (CML)
Example of a translocation
Happen because of trinucleotide repeat expansion
- Narrow sites of some chromosomes that are prone to breakage
- breakage would cause a deletion
Fragile sites
Fragile X
- Male offspring have a 50% chance of receiving fragile X from a carrier mom (can result in mental retardation or transmitting male–normal)
- Molecular basis: FMR-1 gene located at the fragile X site which contains 3 basepair (CGG) repeats in the 5’ UTR
- FMR-1 encodes FMRP (RNA binding protein) that is active at synapses in the brain.
