What is Crossing Over?

Ref: Homologous Chromosomes Crossing Over A Chromosome from your MOM A Chromosome from your DAD The Chromosomes that end up EGG or SPERM cells. It is the exchange or transfer of genes between HOMOLOGOUS CHROMOSOMES

When Does it Happen? During Prophase I of Meiosis Remember the differences between Mitosis and Meiosis? 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.

An actual microscope image of homologous chromosomes crossing over From:

It’s Why You and Your Brother or Sister Don’t Look Identical! (or like anyone else either) BOTTOM LINE

Human Meiosis and Crossing Over The Basics

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 Over MIXES moms 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? Human Meiosis and Crossing Over 46 chromosomes: DIPLOID (23 pairs) 1 Chromosomes get copied during Interphase 2 46 chromosomes AND their copies: still DIPLOID 3 CROSSING OVER 4 After MEIOSIS I there are 23 chromosomes PLUS THEIR MODIFIED COPIES in each: now HAPLOID 5 After MEIOSIS II there are 23 SINGLE chromosomes in each of these 4 cells: still HAPLOID 6

END