A R E D U C T I O N D I V I S I O N T O P R O D U C E G A M E T E S W H I C H U L T I M A T E L Y M A I N T A I N S A C O N S I S T E N T C H R O M O S O M E N U M B E R I N T H E

S P E C I E S

P A R T 2

T H I S W O R K I S L I C E N S E D U N D E R A C R E A T I V E C O M M O N S A T T R I B U T I O N -

N O N C O M M E R C I A L - S H A R E A L I K E 4 . 0 I N T E R N A T I O N A L L I C E N S E .

Meiosis

Prophase in Meiosis

http://biowiki.ucdavis.edu/Genetics/Unit_II%3A_Replication,_Maintenance_and_Alteration_of_the_Genetic_Material/8._Recombination_of_DNA/Meiotic_recombination https://creativecommons.org/licenses/by-nc-sa/3.0/us/ No changes have been made.

Leptotene Stage of Prophase I in Meiosis

Nuclear Membrane

Maternal Sister Chromatids

Paternal Sister Chromatids

Homologous Chromosome Pair

Cell Membrane

Drawing: Lynda Jones, MS, ONPRC

Centromere

Centromere

http://commons.wikimedia.org/wiki/File:Tetrad.png http://creativecommons.org/licenses/by-sa/2.5/deed.en No changes were made

Zygotene Stage of Prophase I of Meiosis Synapsis

Synapsis

https://www.boundless.com/biology/textbooks/boundless-biology-textbook/meiosis-and-sexual-reproduction-11/the-process-of-meiosis-92/meiosis-i-404-11631/images/fig-ch11-01-01/ http://creativecommons.org/licenses/by-sa/4.0/ No changes were made.

Formation of the Sex Body in the Late Zygotene-Early Pachytene Stage of Prophase I of Meiosis in the Testes

Centromeres

Paternal Y Sister Chromatids

Maternal X Sister Chromatids

SRY Gene Pseudoautosomal Region (PAR) where the X and Y chromosomes synapse or forming a fused X and Y tetrad known as the sex body

Drawing: Lynda Jones, MS, ONPRC

SRY is the Sex-determining Region Y gene (also known as the TDF or Testis-Determining Factor gene).

Other PAR regions

Crossing Over - Pachytene Stage of Prophase I of Meiosis

https://upload.wikimedia.org/wikipedia/commons/0/0e/Morgan_crossover_1.jpg Public Domain

http://en.wikipedia.org/wiki/File:Frans_Janssens.jpg Public Domain

Frans Alfons Janssens (1865 – 1924) Discovered and described crossing- over in chromosomes during meiosis which he called ‘chiasmatypie’ - 1909

http://upload.wikimedia.org/wikipedia/commons/8/8f/Thomas_Hunt_Morgan.jpg Public Domain

Thomas Hunt Morgan (1866-1945)

• Discovered sex-linked genes and did further work on crossing over based on Janssens’ work and provided the first true genetic interpretation of meiosis

• Won Nobel Prize in Physiology or

Medicine in 1933 “for his discoveries concerning the role played by the chromosome in heredity” http://www.nobelprize.org/nobel_prizes/medicine/laureates/1933/

http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@9.85:57/Biology http://creativecommons.org No changes were made.

Maternal Sister Chromatids

Paternal Sister Chromatids

Chiasmata Centromeres

Homologous Chromosomes in the Diplotene Stage of Prophase I of Meiosis

Drawing: Lynda Jones, MS, ONPRC

https://en.wikibooks.org/wiki/Medical_Physiology/Cellular_Physiology/DNA_and_Reproduction#/media/File:Meiosis.png By Boumphreyfr (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) No changes were made.

The Origin of Aneuploidy Gametes by Nondisjunction in Meiosis

http://upload.wikimedia.org/wikipedia/commons/3/31/Mitotic_nondisjunction.png http://creativecommons.org/licenses/by-sa/3.0/deed.en No changes have been made.

The Origin of Aneuploidy Gametes by Nondisjunction at the First or Second Meiotic Division

http://upload.wikimedia.org/wikipedia/commons/b/ba/Trisomy_due_to_nondisjunction_in_maternal_meiosis_1.png http://creativecommons.org/licenses/by-sa/3.0/deed.en No changes have been made.