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What is Crossing Over?. A Chromosome from your MOM. A Chromosome from your DAD. The Chromosomes that end up EGG or SPERM cells. Homologous Chromosomes Crossing Over. Ref: http://gnn.tigr.org/whats_a_genome/Chp3_2.shtml. It is the exchange or transfer of genes between HOMOLOGOUS CHROMOSOMES.

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What is Crossing Over?

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A Chromosome from your MOM

A Chromosome from your DAD

The Chromosomes that end up EGG or SPERM cells.

Homologous Chromosomes Crossing Over

Ref: http://gnn.tigr.org/whats_a_genome/Chp3_2.shtml

It is the exchange or transfer of genes between HOMOLOGOUS CHROMOSOMES

when does it happen

When Does it Happen?

Meiosis creates reproductive cells needed

for sexual reproduction which can

increase genetic diversity in a species.

Mitosis creates genetically identical cells

by asexual binary fission.

During Prophase I of Meiosis

Remember the differences between Mitosis and Meiosis?


An actual microscope image of

homologous chromosomes crossing over

From: http://www.ultranet.com/~jkimball/BiologyPages/M/Meiosis.html#crossing_over



46 chromosomes: DIPLOID (23 pairs)

Chromosomes get copied during Interphase



46 chromosomes AND their copies: still DIPLOID

After MEIOSIS I there are 23 chromosomes PLUS THEIR MODIFIED COPIES in each: now HAPLOID



After MEIOSIS II there are 23 SINGLE chromosomes in each of these 4 cells: still HAPLOID

Human Meiosis and Crossing Over

  • Before meiosis begins, there are homologous pairs (MOM & DAD) ; only one pair shown, but really 23.
  • The MOM and DAD chromosomes are NOT IDENTICAL, but they do code for the same traits. For example, assume that the “A” and “a” are genes that code for hair color. A=brown and a=blonde. Since “A” is dominant, the person that this cell belongs to must have brown hair.
  • Now, during Interphase, DNA is COPIED!!
  • During Prophase of Meiosis I, Crossing OverMIXESmoms and dads genes. Because of the mixing, the copies are not exact copies anymore; you might call them “modified” copies.
  • During Anaphase of Meiosis I, the HOMOLOGOUS PAIRS get separated, but still have their modified COPIES WITH THEM.
  • During Anaphase of Meiosis II, the modified COPIES get SEPARATED.
  • QUESTION: The 4 cells at the bottom are SPERM cells. If each of them fertilize an egg, which of them MUST create a person with brown hair?