Biology of DNA - Chromosomes
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Because cells are very small, the genomic DNA is condensed and arranged on 23 individual structures called chromosomes. Every cell in the human body (except red blood cells) contains 23 pairs of chromosomes. (a) Each chromosome is made up of a tightly coiled strand of DNA. (b) DNA's uncoiled state reveals its familiar double helix shape.
Human beings have 46 chromosomes (23 pairs) and are numbered from 1 to 22 with chromosome 1 being the largest and 22 the smallest, the sex chromosomes are named X and Y. A child inherits one set of chromosome from the father and the other set from the mother however, there are two special cases to this rule.
Boys inherit the Y chromosome from their father and then pass it on only to their male offspring. All children inherit the mitochondrial DNA exclusively from their mothers but only girls then pass it on to all their children.
There is 50% chance of a child inheriting one of the parent's chromosomes. Only sex chromosomes do not strictly obey this rule — girls have a 50% chance of inheriting a specific copy of the mother's X chromosome but 100% chance of inheriting the only father's X chromosome. The same is true for boys with the exception that they have the father's Y chromosome.
