Reduction Division

Each human cell (aside from red blood cells and gametes) contains a full set of 46 chromosomes. Clearly havoc would result if a sperm and egg cell each containing 46 human chromosomes were to fuse!  Not only would the resulting offspring have 98 chromosomes in each cell but the number would keep on doubling with each successive generation.  For this reason, a process other than mitosis which produces cells with a full complement of chromosomes is necessary to produce the sperm and egg cells.   This process is meiosis.

The result of meiosis is four cells, each having a single #1 chromosome, and one #2 and one #3 and so on up to one each of 23 chromosomes.  Thus each cell has one complete set of chromosomes and is ready to become either a sperm or egg cell.

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Animation by Steven Kuensting (Own work) [CC BY-SA 4.0 (], via Wikimedia Commons (modified for this website)

is the process by which the chromosome number is halved and chromosomes are sorted and packaged to be passed on to an organism’s offspring.  Each of the resulting reproductive cells, or gametes (sperm and egg), has only a single set of 22 autosomes plus a single sex chromosome, either an X (female) or a Y (male).  A cell with a single chromosome set is called a haploid cell. 

The process of producing a sperm or egg cell is call gametogenesis. Even more specifically, the process of producing an egg cell is called oogenesis and the process of creating sperm cells is called spermatogenesis. The little animation above shows the process of meiosis as a part of oogenesis.

There is notable difference between oogenesis and spermatogenesis:

All four of the eventual products of meiosis and cytogenesis in males become viable sperm cells able to fertilize an egg.

However, during the two cytokinesis events of oogenesis, the cytoplasm is divided unevenly among the daughter cell with one cell receiving most of the cell cytoplasm and machinery. That cell goes on to become the egg. The three other resulting polar bodies cannot be fertilized by sperm because they are deficient in both cytoplasm and cell organelles.

diagram of gametogenesis

By Elversberg (Own work) [CC BY-SA 3.0 ( or GFDL (], via Wikimedia Commons (modified for this website)

The chromosomes in each sperm or egg are a random mix from the maternal set and paternal set of the original cell. Because each of the 23 pairs of chromosomes sort independently of the other 22 pairs, one person can produce more than 8 million different kinds of eggs or sperm. 

By means of sexual intercourse, a sperm cell carrying one 23 chromosome set from the father reaches and fuses with an egg cell carrying a corresponding set of 23 chromosomes from the mother.  When fertilization occurs 70 trillion different zygotes are possible! The new combination of genes in the zygote are passed on with precision by the process of mitosis to produce a unique human individual with over 30 trillion cells.

For the genealogist, this means that any one of his or her chromosomes could have come from any one of eight great-grandparents or, going back say twenty generations, from any one of over a million potential ancestors. (In reality, many of these potential ancestors may be the same individuals as lines cross and intermarry over generations.)  In the same light, after many generations of shuffling and dividing, contributions from some ancestors may have been lost completely. 

Where Can I Go From Here?

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Where Can I Go From Here?
