Although the replication and transcription of DNA is highly standardized in eukaryotes, the same cannot be said for their karotypes, which are often highly variable. There may be variation between species in chromosome number and in detailed organization. In some cases there is significant variation within species. Often there is variation 1. between the two sexes. 2. between the germ-line and soma (between gametes and the rest of the body). 3. between members of a population, due to balanced genetic polymorphism. 4. geographical variation between races. 5. mosaics or otherwise abnormal individuals. Finally, variation in karyotype may occur during development from the fertilised egg.
The technique of determining the karyotype is usually called karyotyping. Cells can be locked part-way through division (in metaphase) in vitro (in a reaction vial) with colchicine. These cells are then stained, photographed and arranged into a karyogram, with the set of chromosomes arranged, autosomes in order of length, and sex chromosomes (here XY) at the end: Fig. 3.
Like many sexually reproducing species, humans have special gonosomes (sex chromosomes, in contrast to autosomes). These are XX in females and XY in males.
Historical note
Investigation into the human karyotype took many years to settle the most basic question: how many chromosomes does a normal diploid human cell contain? In 1912, Hans von Winiwarter reported 47 chromosomes in spermatogonia and 48 in oogonia, concluding an XX/XO sex determination mechanism.[37] Painter in 1922 was not certain whether the diploid number of man was 46 or 48, at first favouring 46.[38] He revised his opinion later from 46 to 48, and he correctly insisted on man having an XX/XY system.[39] Considering their techniques, these results were quite remarkable.
New techniques were needed to definitively solve the problem:
- 1. Using cells in culture
- 2. Pretreating cells in a hypotonic solution, which swells them and spreads the chromosomes
- 3. Arresting mitosis in metaphase by a solution of colchicine
- 4. Squashing the preparation on the slide forcing the chromosomes into a single plane
- 5. Cutting up a photomicrograph and arranging the result into an indisputable karyogram.
It took until the mid 1950s until it became generally accepted that the karyotype of man included only 46 chromosomes.[40][41] Rather interestingly, chimpanzees (our closest living relatives) have 48 chromosomes.
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