• Question: How many chromosomes do sperm and eggs have?

    Asked by caitlinlewis to Tristan, Sophie, Ravinder, Matt, Barbara on 7 Mar 2015.
    • Photo: Barbara Shih

      Barbara Shih answered on 7 Mar 2015:

      23 chromosomes (no pairs) :
      22 + 1 sex chromosome (x for eggs, x or y for sperms).

    • Photo: Matthew Moore

      Matthew Moore answered on 7 Mar 2015:

      As Barbara said, eggs and sperm alike have 23 chromosomes (in a regular cell it’s 23 pairs of chromosomes). The pairs of chromosomes in regular cells are acquired one from an egg and one from a sperm which at some point formed an embryo which became you!

      Chromosomes 1-22 are referred to as autosomes then the 23 determine gender. An X chromosome is for a female and a Y for a male, females are XX and males XY. That means that an egg always has chromosomes 1-22 then an X while a sperm contains chromosomes 1-22 then an X or a Y –this explains how it’s 50/50 to have a boy or a girl.

      The number in other animals than humans is different and birds can even have gender determination swapped around where it is the females that have ZW and males have ZZ (the equivalent of XX and XY).

      There are however some exceptions and chromosome abnormalities can occur where there are too many or too few chromosomes such as Down Syndrome.

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