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Topic 5 ‐ Crossing over and independent assortment Why aren’t offspring identical to each other if they all get half chromosomes from dad and half from mom?
http://highered.mcgraw‐hill.com/sites/dl/free/0072835125/126997/animation5.html (on next page)
­ In meiosis, the new cells have different combinations of genetic material than the parent cell
n As opposed to mitosis in which the daughter and parent cell have identical genetic material
­ Meiosis produces genetically different cells through two different processes
n Independent assortment
n Crossing over Crossing over
­ When homologous chromosomes pair up during prophase I (synapsis), they may exchange pieces of chromosome
­ This is known as crossing over ­ As a result, individual chromosomes contain some genes of maternal origin and some genes of paternal origin.
­ Gene – a specific unit of DNA that expresses a certain trait
­ Allele – different forms of the same gene
‐ Expressed on homologous chromosomes
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Independent assortment http://highered.mcgraw‐hill.com/sites/0072495855/student_view0/chapter3/animation__random_orientation_of_chromosomes_during_meiosis.html ­ During metaphase I, homologous chromosomes align along the equatorial plate
­ In each pair, the chromosome of maternal origin is orientated towards one pole, and the paternal chromosome is orientated toward the other pole.
­ Therefore, some maternal and some paternal chromosomes face the same pole of the cell
­ This results in new cells that have mixtures of the paternal and maternal chromosomes
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