Chromosomes are long pieces of DNA contained in the nucleus of cells.


Chromosomes come in pairs. In humans, the nucleus of each cell has 23 pairs of chromosomes (46 total chromosomes).

DNA is also found in another part of the cell called the mitochondrion. Mitochondria have their own DNA strand, sometimes called “the 47th chromosome.” All of your genes are contained within these 46 nuclear chromosomes and 1 mitochondrial chromosome.

Two of the chromosomes (the X and the Y chromosome) determine your gender and are called the SEX CHROMOSOMES:

  • Females have 2 X chromosomes.  
  • Males have 1 X and 1 Y chromosome.

The Y chromosome determines the male gender, but does little else.

The remaining chromosomes are called AUTOSOMAL CHROMOSOMES. For convenience, scientists have numbered these chromosome pairs 1 through 22.

Each parent contributes one half of each chromosome pair to their child - 22 autosomal chromosomes and 1 sex chromosome. The mother always contributes an X chromosome to the child. The father may contribute an X or a Y. Therefore, it is the father that determines the gender of the child.

The mitochondrial chromosome comes from the mother. Fathers make no contribution to the mitochondrial genes of their offspring.

Johns Hopkins patient information

Last revised: December 5, 2012
by Potos A. Aagen, M.D.

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