Subject C’s karyotype contains 47 chromosomes instead of the normal 46, with an extra copy of chromosome 13. This condition is known as trisomy 13, or Patau syndrome. In the widely used University of Arizona karyotyping lab, Subject C is described as having died shortly after birth with multiple abnormalities, including extra fingers (polydactyly) and a cleft lip.
Normal Karyotype vs. Subject C
A normal human karyotype contains 46 chromosomes arranged in 23 pairs. Twenty-two of those pairs are autosomes, which look the same in males and females. The 23rd pair consists of the sex chromosomes: two X chromosomes in females (46, XX) or one X and one Y in males (46, XY).
Subject C has all 22 autosomal pairs plus the sex chromosomes, but also carries a third copy of chromosome 13. That brings the total to 47. In standard karyotype notation, this would be written as 47, XY, +13 for a male or 47, XX, +13 for a female. The “+13” indicates the extra chromosome 13 specifically.
How the Extra Chromosome Gets There
The extra chromosome results from a cell division error called nondisjunction. During the formation of egg or sperm cells, chromosomes are supposed to separate evenly so each resulting cell gets exactly one copy. In nondisjunction, that separation fails. Two copies of a chromosome get pulled to the same side, producing a reproductive cell with 24 chromosomes instead of 23.
This error can happen at two different stages. If it occurs during the first round of division, all four resulting cells end up abnormal: two with an extra chromosome and two missing one. If it happens during the second round, only two of the four cells are affected, and the other two are normal. Either way, when an egg carrying 24 chromosomes is fertilized by a normal sperm (or vice versa), the embryo ends up with 47 chromosomes total.
Nondisjunction errors involving chromosome 13 occur more frequently in pregnancies where the mother is over 35, though they can happen at any age.
What Trisomy 13 Looks Like on a Karyotype
When you compare Subject C’s karyotype image to a normal one, the difference is visible in the group containing chromosome 13. Instead of a matched pair, there are three copies of chromosome 13 lined up together. Every other chromosome pair in the karyotype looks normal. The sex chromosomes are unaffected, so they still appear as either XX or XY.
This is what makes karyotyping useful as a diagnostic tool. Lab technicians photograph a cell’s chromosomes during division, arrange them by size and banding pattern into numbered pairs, and then look for any group that has more or fewer than two. In Subject C’s case, the only abnormality is that single extra chromosome in position 13.
Physical Effects of the Extra Chromosome
The third copy of chromosome 13 disrupts normal development because it produces excess copies of all the genes on that chromosome. In the lab scenario, Subject C presents with polydactyly (extra fingers or toes) and a cleft lip, both hallmark features of Patau syndrome. The condition typically causes severe problems with brain, heart, and organ development.
Trisomy 13 is rare, occurring in roughly 1 in every 20,000 live births. It is one of the most serious autosomal trisomies. Subject C in the lab exercise is described as having died shortly after birth, which reflects the clinical reality: most infants with full trisomy 13 do not survive beyond the first weeks of life.
How This Differs From Other Karyotype Abnormalities
The karyotyping lab typically includes several patients with different chromosomal conditions, so it helps to see how Subject C’s abnormality compares. Trisomy 13 is an autosomal trisomy, meaning the extra chromosome is one of the numbered, non-sex chromosomes. Trisomy 18 (Edwards syndrome) and trisomy 21 (Down syndrome) work the same way but involve different chromosomes and produce different physical features.
Sex chromosome abnormalities, by contrast, involve the 23rd pair. A karyotype reading 47, XXY, for instance, indicates an extra X chromosome rather than an extra autosome. That condition, Klinefelter syndrome, has a very different set of effects and a much longer life expectancy than trisomy 13. The key distinction on the karyotype image: in Subject C, the extra chromosome sits with autosome pair 13, not with the sex chromosomes at the end of the chart.
In all these cases, the total chromosome count is 47, but which chromosome is tripled determines the specific syndrome and its severity. Chromosome 13 carries genes critical to early development, which is why an extra copy causes such widespread and serious disruption compared to extra copies of some other chromosomes.