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Review. Spermatogenesis, Oogenesis, Crossing Over, Imprinting---A Review. Primary germ cells migrate from the yolk sac to the ovaries or testes where they undergo mitosis and become oogonium or spermatogonium or remain germ cells. Spermatogenesis.

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  • Spermatogenesis, Oogenesis, Crossing Over, Imprinting---A Review

Primary germ cells migrate from the yolk sac to the ovaries or testes where they undergo mitosis and become oogonium or spermatogonium or remain germ cells.


  • Spermatogenesis, or sperm production, begins around puberty and continues for the remainder of a man's life

  • A young healthy man produces several hundred million sperm per day.



  • During embryonic development, diploid cells in the ovaries called oogonia divide by mitosis to produce primary oocytes.  The primary oocytes are diploid.

  • Primary oocytes start meiosis.  They complete interphase and prophase I.  They are “frozen” at the end of prophase I and remain this way until the female reaches puberty.

  • A female is born with about 2 million primary oocytes.  By the time she reaches puberty, about 400,000 are left.

Oogenesis cont….

  • After puberty, each month one of the primary oocytes is selected for ovulation (release from the ovary).  Just prior to ovulation, the primary oocyte is “unfrozen” and completes meiosis I, forms the first polar body, and is then frozen in meiosis II at metaphase II. 

  • After ovulation, if a sperm penetrates the secondary oocyte (egg) after it is released from the ovary, the secondary oocyte will be stimulated to complete meiosis II forming one more polar body and a mature egg.  The first polar body formed from meiosis one may also complete meiosis II to form another polar body.

Prophase I

Fertilization takes place in the fallopian tubes

Crossing Over –Genetic Variation

Crossing over

Actual photograph of crossing over

No crossing over during gametogenesis

Crossing Over



1. When does this happen in the formation of the egg and sperm?

2. Show the gametes formed using the letters “A” and “B” and “a” and “b”.




  • Imprinting


  • Uniparental embryos

  • Uniparental disomy

  • Differences in maternal and paternal genefunction

Epigenetics and Disease:

Genomic imprinting

Parent specific expression or repression of genes

or chromosomes in offspring.

So… even though two copies of a given gene are inherited,

one from each parent, only the maternal or paternal allele

is expressed.

The non-expressed allele is said to be “imprinted.”

Life Cycle of an Imprint

  • Wilms Tumor

  • -Childhood Tumor of the kidney (nephroblastoma)

  • Accounts of 7% of all childhood cancers

  • -Caused by a defect in imprinting of the Insulin-like

  • Growth Factor 2 (IGF2) gene

  • IGF2 is usually only expressed from the paternal locus, ie

  • maternally imprinted

  • -Defects in imprinting that cause expression of the maternal

  • locus lead to cancer

Genomic imprinting and disease:

Several diseases are associated with genomic imprinting


Beckwith–Wiedemann syndrome

Prader–Willi syndrome

Angelman syndrome

Wilms Tumor

Fragile X syndrome

Myotonic dystrophy (congenital)

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