What is a chromosome?

A chromosome is a large ''bundle'' of genetic material containing tens of thousands of DNA sequences. These sequences make up individual genes, and each chromosome has many, many genes within it. We all have 46 chromsomes including two pairs of sex chromosomes: X denoting female and Y denoting male. So XY is male and XX is female. There are chromosomal sydromes in which there are too few or too many chromosomes. For example, in Down''s syndrome, there is an extra chromosome 21 (trisomy 21), making 47 chromosomes in total. In Turner''s syndrom (45XO), there are 45, ie one X chromosome is missing.
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