An organism with monosomy for chromosome 5 dies early in development, while one with trisomy 21 survives to adulthood with significant but compatible abnormalities. Why is this pattern expected?
ATrisomy always produces less DNA damage than monosomy
BLosing 50% of a gene's output disrupts dosage-sensitive networks more severely than gaining 50%
CChromosome 21 contains fewer genes than chromosome 5
DMonosomy triggers apoptosis while trisomy does not
The key is dosage asymmetry: many genes are haploinsufficient, meaning a single copy cannot produce enough product for normal function. Monosomy eliminates this floor — losing 50% of expression from all genes on the chromosome. Trisomy creates a 50% excess, which is disruptive but often tolerable because cells can still maintain essential processes with extra product. The harm from trisomy comes from cumulative overexpression of hundreds of genes simultaneously, not from any single gene being present in excess.
Question 2 Multiple Choice
Turner syndrome females (45,X) are viable despite monosomy for the X chromosome, unlike virtually all autosomal monosomies. What best explains their viability?
AThe X chromosome contains fewer dosage-sensitive genes than typical autosomes
BTurner syndrome females compensate by upregulating the single X chromosome twofold
CX-inactivation normally silences one X in females, so having a single active X resembles the normal dosage state
DMonosomy X is only survivable when it occurs in meiosis II rather than meiosis I
Normal XX females already have one X inactivated in each cell, so the functional dosage from one X is the baseline. Turner syndrome (45,X) mirrors this — there is one active X, which is what cells are already calibrated to. Autosomal monosomies have no equivalent buffering mechanism: losing one copy of an autosome drops gene dosage to levels cells cannot tolerate. This is also why sex chromosome trisomies (XXY, XXX) are generally mild — extra X chromosomes are largely inactivated, limiting dosage imbalance.
Question 3 True / False
Trisomy produces a syndrome with many features rather than a single defect because hundreds of dosage-sensitive genes on the extra chromosome are all slightly overexpressed simultaneously.
TTrue
FFalse
Answer: True
This is exactly right. In trisomy 21, for example, ~300 genes are overexpressed by roughly 50%. No single gene is solely responsible for Down syndrome's features. Instead, transcription factors, adhesion molecules, and enzymes across multiple developmental pathways are all mildly dysregulated at once, and their cumulative interaction produces a wide-ranging syndrome. This distinguishes aneuploidy from single-gene disorders, where one gene's malfunction produces a more focused phenotype.
Question 4 True / False
The increased risk of aneuploidy with maternal age is primarily caused by the accumulation of DNA mutations in oocytes over decades of exposure to environmental mutagens.
TTrue
FFalse
Answer: False
The mechanism is degradation of cohesin proteins, not DNA mutation accumulation. Human oocytes begin meiosis I before birth and remain arrested in prophase I for decades. The cohesin complexes holding homologous chromosomes together deteriorate over this time, leading to premature separation of homologs during meiosis I. This increases the rate of nondisjunction specifically in older mothers. Sperm are produced continuously from spermatogonia and do not experience this decades-long arrest, which is why paternal age has a much smaller effect on aneuploidy rates.
Question 5 Short Answer
Why is aneuploidy harmful in a way that polyploidy (having extra complete chromosome sets) often is not?
Think about your answer, then reveal below.
Model answer: Polyploidy multiplies all chromosomes proportionally, preserving the ratio of gene products to one another. Aneuploidy adds or removes just one or a few chromosomes, disrupting the balance between the products of those chromosomes and the rest of the genome. Cells have evolved to function with precise ratios of gene products — particularly for proteins that participate in multi-subunit complexes or regulatory networks. Aneuploidy throws these ratios off, while polyploidy does not.
The critical concept is that it is the *ratio* of gene products, not the absolute amount, that matters most. A cell with 4 copies of every chromosome (tetraploid) may function near-normally because all gene products are equally doubled. But a cell with 3 copies of chromosome 21 and 2 copies of everything else has imbalanced expression specifically for the ~300 genes on chromosome 21, disrupting their interactions with products encoded elsewhere. This imbalance is the source of harm.