Chromosomes within a cell are arranged in sets. Each cell contains 46 chromosomes or 23 sets.
Such sets of chromosomes are classified according to an international nomenclature: from the largest to the smallest, followed by sex-chromosomes.
As you can see, we have:
two number 1 chromosomes,
two number 2 chromosomes,
two number 3 chromosomes,
and so on for all sets.
Chromosomal alteracions in number occurs when the inheritance received by the daughter cells is different between them as a result of an error in the distribution: some daughter cells receive more while others receive less chromosomes.
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Introduction - The origin of life - Cell specialization - Chromosomes - How do we acquire our inheritance?
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