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In this age of genome sequencing, we can lose sight of the importance of how our genomes are distributed over 23 pairs of chromosomes. Rearrangements of the pairs are invisible to sequencing if the correct amount of genetic material is present.
A recent genetic counseling session reminded me of a chromosomal quirk that flies completely under the radar of genome sequencing, yet if it were to turn up in two copies in a bunch of people who have children together, could theoretically seed a second human species, one characterized by a chromosome number of 44, not 46.
The young couple I counseled had suffered several early pregnancy losses, and tests had revealed extra material from chromosome 22. Although it’s a tiny chromosome, it is gene-dense and the extra genetic material ends development just as an embryo is becoming a fetus.
The lab report profiled single nucleotide polymorphism (SNP) landmarks, and detected overrepresentation of pieces of chromosome 22. But this was on a genomewide basis using microarray technology — not the cut-and-paste size-ordered chromosome chart that is an old-fashioned karyotype. SNPs may be newer, but the distinction between an extra chromosome 22, which the lab report inferred, and the actuality of an extra tiny chromosome glommed onto one of the other chromosomes, is critical for predicting recurrence in a family — because such a piggybacked chromosome would explain the repeated losses. It is a case of not seeing the genetic forest for the trees. Chromosomes still count.
Read full, original post: Can a Quirky Chromosome Create a Second Human Species?
