Karyotypes and Karyotypic EvolutionBy Branden Holmes (Surroundx) on November 27, 2010
|A karyotype is the number of, and the arrangement, of the chromosomes of an organism. Generally all individuals within a species (or sub-species) all have the same number of chromosomes. Thus we tend to speak of the karyotype of the species. However, within some species the number of chromosomes varies from individual to individual. The most familiar species which possesses this unusual characteristic is the red fox (Vulpes vulpes). Native to Britain, but introduced to many parts of the world where it has done considerable damage to the native fauna of those regions, their karyotypes vary from 34 to 38 chromosomes. The easiest way to understand this is to imagine that the â€œnormalâ€ karyotype of a red fox is 34 chromosomes. But some individuals have more chromosomes; what are called â€œsatelliteâ€ chromosomes. These â€œextraâ€ chromosomes contain genes which are not necessary to the development of the individual. Thus a red fox with only 34 chromosomes is no worse off fitness-wise than is an individual with 4 extra satellite chromosomes.|
We also see great variation in the karyotypes of closely related species. For example, wolves and their close cousins (encompassing several different genera) all have 78 chromosomes. Whereas the chromosome count varies even amongst species of the same genus of foxes, Vulpes for example (even if we exclude the red fox from the picture). This fissioning of chromosomes seems to be inconsistent; prevalent amongst some types of organisms but rare or non-existent in others. Thus it is one of the great evolutionary mysteries still to be solved. But at the opposite end of the spectrum there are also merges of two separate chromosomes into one. We ourselves are the beneficiary of such an event. All other apes have 48 chromosomes, but we only have 46 because what are the Chimpanzees’ chromosomes 2a and 2b merged head-to-head sometime since our split with their ancestors. The tell tale sign of this is the existence of two telomeres in the centre of our second chromosome. These telomeres are normally the very ends of the chromosomes, thus we know that our second chromosome was produced by a merging event. Genetics is as much about detective work as it is anything else.
We humans have a 2n number of 46. This is in reference to the total number of chromosomes, which are in pairs (23 of which you inherited from your mother, and 23 from your father). Somatic, or everyday, cells (skin, liver, heart, eye etc.) contain two sets of 23 (i.e. 46) chromosomes inside the nucleus of every somatic cell. Somatic cells are produced by mitosis, the normal type of cell division. However, meiosis is responsible for producing the sex cells which only contain one set of chromosomes each (23 in our case). This is because when two sex cells unite in fertilization, the resulting zygote (or fertilized egg) will have the full set of two pairs of 23 chromosomes. Otherwise chromosome numbers would double every generation since life began if every sperm and egg had the full number of chromosomes.
If you have ever seen a picture or illustration of a human karyotype it would have been in a special format called a karyogram (or idiogram): a white background with the chromosomes contrasting as black entities, roughly arranged in corresponding pairs from the largest chromosomes in the top left-hand corner, to the smallest in the bottom left-hand corner also. To make the chromosomes stand out so much geneticists use a process called staining, which simply means that a special dye was applied to darken them. When a karyogram has been created by staining and arranging the chromosomes, a geneticists will look for 6 specific characteristics of the karyotype: size of the chromosomes, the position of the centromeres, differences in size between two corresponding chromosomes, the banding pattern on the chromosomes, the shape of the chromosomes, and of course, the number of chromosomes present.
|I am an amateur evolutionist interested in the theoretical side of the subject.|
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