Eukaryotic Chromosome Abnormalities: Changes to Chromosome Structure + Number Flashcards
Define aneuploidy
‘not euploid’
the addition or removal of a chromosome which alters the euploid number (n+1 or n-1 if one chromosome pair is affected)
caused by nondisjunction
Define polyploidy
the presence of three or more sets of chromosomes in the nucleus of an organism (ex. 3n, 5n, 6n, etc.)
common in plants
What is the difference between aneuploidy and polyploidy?
Aneuploidy is a change in the number of chromosomes in a single set
Polyploidy is a change in the number of sets of chromosomes
Define monosomic
an organism/fertilized egg with 2n-1 ploidy that results in an aneuploid gamete fusing with a normal haploid gametes at fertilization and giving the organism a single copy of one of the chromosomes rather than a homologous pair
Define trisomic
an organism/fertilized egg with 2n+1 ploidy that results in an aneuploid gamete fusing with a normal haploid gametes at fertilization which gives an organism 3 of one of the chromosomes rather than a homologous pair
Describe nondisjunction
the failure of chromosomes and sister chromatids to properly disjoin/detach during anaphase of meiosis which causes abnormalities in chromosome numbers
How does nondisjunction lead to monosomy?
Nondisjunction in meiosis causes aneuploidy (different number of chromosomes in a set) and when an aneuploid gamete fuses with a regular gamete, there’s only a single copy of one of the chromosomes = monosomy
How does nondisjunction lead to trisomy?
Nondisjunction in meiosis causes aneuploidy (different number of chromosomes in a set) and when an aneuploid gamete fuses with a regular gamete, there’s three copies of one of the chromosomes = trisomy
Describe where nondisjunction occurs during meiosis I
Nondisjunction in meiosis I causes the failure of homologous pairs to be pulled apart during anaphase I which results in both homologs moving to the same side of the pole = a different number of chromosomes for that set in both cells
Describe where nondisjunction occurs during meiosis II
Nondisjunction in meiosis II causes the failure of sister chromatids to be pulled apart during anaphase II
What gametes would be produced from nondisjunction in meiosis I and II in a heterozygous individual?
Either monosomic (2n-1) or trisomic (2n+1)
Why are half of a trisomic organism’s gametes (offspring) aneuploid?
Because half of the gametes produced during meiosis were effected by nondisjunction
Why are aneuploids often not as viable as polyploids?
the chromosomes are not distributed equally and there is missing genetic material
What does it mean to be heterozygous for a chromosomal rearrangement?
one chromosome of a pair has a mutation resulting from chromosome breakage such as translocation, deletion, inversion, or duplication and one of the chromosomes is normal
Describe a deletion mutation
A mutation that results from the loss of part of a chromosome
Can be caused by chromosome breakage
What are the two results of deletion mutations?
terminal
interstitial
Describe terminal deletion
Chromosome breakage that results in the detachment of all or part of one chromosome arm
Describe an interstitial deletion
chromosome breakage that leads to the loss of an internal portion of a chromosome
Why are some deletion mutations lethal when homozygous?
if both chromosomes in a homologous pair are missing pieces of the genetic material, essential genes may be deleted and will not be transcribed and translated which may lead to an unviable organism
Why are some deletion mutations lethal when heterozygous?
deletion can cause gene dosage problems and if the organism is highly sensitive to changes in gene dosage, having deletion mutations in one chromosome can lead to death
Why are recombination frequencies lower in organisms heterozygous for a deletion?
recombination cannot occur in the region of the chromosome that forms a deletion loop (where deletion has occurred
Why do deletion heterozygotes form loops when homologs pair in meiosis?
organisms that are heterozygous for deletion mutations have one normal homolog and one partially deleted homolog so there is a mismatch in genetic material when they pair
Describe a duplication mutation
An unequal crossover event results in some genetic material of one homolog being duplicated
Why are recombination frequencies higher in organisms that are heterozygous for a duplication?
the chromosome with the duplication has extra genetic material so there is more space available for crossover
Why do duplication heterozygotes form loops when homologs pair in meiosis?
organisms that are heterozygous for duplication mutations have one normal homolog and one partially duplicated homolog so there is a mismatch in genetic material when they pair
Describe an inversion mutation
A mutation that occurs when chromosome breakage leads to the reattachment of DNA strands at the wrong broken ends, causing the chromosome to be in the wrong orientation
Why are some inversion mutations lethal when homozygous?
if the inversion includes essential genes, a homozygous individual will not survive
Why do viable inversion homozygotes pair and recombine normally during meiosis?
Why do viable inversion homozygotes have different linkage maps than organisms with wild-type chromosomes?
the gene order will be reversed in the area of inversion so the map distances between genes will also differ when compared to a wild-type chromosome
Why are some inversion mutations lethal when heterozygous?
with pericentric inversion (crossover inside the inversion region), the recombinant gametes will have a duplicated and deleted region and may not survive due to abnormal dosages of some of the genes
with paracentric inversion (crossover outside the inversion region), recombinant gametes will have altered gene dosage and an acentric and dicentric centromere which cause losses and breakages of chromosomes –> lethal
Why do inversion heterozygotes form loops when homologs pair in meiosis?
When a normal chromosome pairs with its inverted homolog, the genetic material will be mismatched and will cause a loop
What is the result of crossover within the inversion loop in a heterozygous organism?
duplications and deletions in the recombinant chromosomes
Why does crossover within the inversion loop of an organism that is heterozygous for an inversion result in non-viable recombinants?
pericentric inversion results in nonviable offspring because it causes duplications and deletions in chromosomes of the gamete which changes the gene dosages and can be lethal
Why do inversion heterozygotes have reduced recombinant frequencies?
the inversion loop forms during meiosis to maximize pairing but if crossover occurs in the inversion loop, duplications and deletions of the chromosomes occur and this leads to nonviable recombinant chromosomes
crossovers do occur, but they produce nonviable recombinants that are not observed in progeny