Chromosomal mutation Flashcards
types of chromosomal mutations
rearrangements
aneuploids
polyploids
types of rearrangements
duplication
deletion
inversion
translocation
is a duplication or deletion of the same region of a chromosome more harmful
deletion
intra chromosomal duplications vs inter chromosomal duplications and which is more common
same chromosome vs when the new copy goes onto a different chromosome
intra more common
tandem vs displaced duplication
duplication occurs next to each other vs new copy elsewhere in genome
reverse duplication
segment flipped over
duplications in meiosis-pairing
in an individual heterozygous for a duplication, the duplicated chromosome loops out during pairing in prophase 1
why do duplications alter phenotype
dosage effects-unbalanced gene dosage
position in genome also influences genotype
cause of duplication and deletion
unequal crossing over
unbalanced gene dosage
leads to developmental abnormalities
relative dosage of proteins/gene products that interact thought to be important
protein complexes effected
segmental duplications
duplications longer than 1000bp (less than 1000 is an indel)
detected by sequencing
deletions in prophase 1
heterozygote
loop forms for homologous sequences to align
cause of inversions
region between two breaks in a chromosome is flipped before the breaks are repaired
paracentric inversion vs pericentric inversion
centromere unaffected vs centromere affected
why do inversions affect fitness
affects positions of promoters and chromatin structure
eg inversion could move a gene near heterochromatin
breaks genes
relative order of genes influences how they are expressed (position effects)
disrupts meiosis is pericentric
variegation
variation in the phenotype that can be caused by somatic mutations or transposable elements
paracentric inversions in prophase 1for a heterozygote
inversion loop forms
single cross over within inverted region
unusual structure forms
one of the four chromatids now has two centromeres and the other lacks a centromere
anaphase 1 for paracentric inversion
centromeres separate, stretching dicentric chromatid (forms dicentric bridge) causing it to break. chromosome lacking a centromere is lost.
gamete result from paracentric inversion
2 gametes: wild type non recombinant chromosomes. one is normal, the other has the paracentric inversion
2 gametes: recombinant chromosomes missing some genes. Nonviable
prophase 1 pericentric inversions (heterozygote)
inversion loop forms
crossing over within inverted region
two chromatids have too many copies of some genes and no copies of others
anaphase 1 + 2 and gamete results for pericentric inversion
chromosomes separate
chromatids separate
1. normal non recombinant gamete
2. two non viable recombinant gametes as too many/too few of genes
3. non recombinant gamete with pericentric inversion
same length
translocation
movement of material between non homologous chromosomes
non reciprocal vs reciprocal translocation
unidirectional (unusual) vs both directions
robertstonian translocation
reciprocal
short arm of one acrocentric chromosome is exchanged with long arm of another
creates a metacentric chromosome and a fragment that fails to segregate so is lost
acrocentric chromosome
centromere towards end (long and short arm)
metacentric chromosome
centromere near middle
fitness effects of translocations
cut within genes-can find which genes cause diseases
position effects
associated with loss of genes, eg roberstonians
types of aneuploidy
nullisomy
monosomy
trisomy
tetrasomy
nullisomy
loss of both members of a homologous pair of chromosomes
2n-2
monosomy
loss of a single chromosome
2n-1
trisomy
gain of a single chromosome
2n+1
tetrasomy
gain of two homologous chromosomes
2n+2
causes of aneuploidy
deletion of centromere during mitosis and meiosis
roberstonian translocation
nondisjunction during meiosis and mitosis
normally in mitosis (somatic mutations)
non disjunction
failure of a pair of chromosome to separate during anaphase
more common in first meiotic division
gametes have extra or missing chromosomes
gamete result in first division nondisjunction vs second division nondisjunction
gametes have extra or missing chromosome vs gametes have extra chromosome, missing chromosome or normal number of chromosomes
increasing chromosome size…
aneuploidy more deleterious
fewer live births
primary down syndrome
3 copies of chromosome 21
mainly caused by spontaneous non disjunction, mainly in egg
incidence increases with maternal age
cohesin
forms a protein complex to keep chromosomes together
aids in proper alignment and segregation of sister chromatids
cross over keeps them together
reduced cohesin
destabilizes homologous chromosome pairing, leading to fewer or improperly formed crossovers.
errors in segregation-non disjunction can occur
how can translocation cause down syndrome
chromosome 21 translocated onto another chromosome
eg long arm of 21 attached to 14
the person with this karotype is a carrier but at increased risk of having children with down syndrome
polyploidy
organism has more than two complete sets of chromosomes
more common in plants
associated with increased cell size
autopolyploidy
all chromosomes a gamete receives are from a single species
allopolyploidy
chromosomes are from two species
how autopolyploidy arises
mitotic nondisjunction
failure of cytokinesis leads to autotetraploids
nondisjunction in germ line the gametes will be diploid and when they mate with normal haploid gamete a triploid forms
meiotic non disjunction
autotriploids
non viable
homologous chromosomes can pair or not pair in 3 ways
resulting gametes have extra chromosomes or lack chromosomes
how allopolyploids arise
hybridisation between two species, followed by chromosome doubling
hybrid produced by 2 different species has nonhomologous chromosomes that cant pair properly forming unbalanced, nonviable gametes
nondisjunction leads to doubling of the chromosomes=allotetraploid
chromosome pairing and segregation are normal and produce viable gametes
