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Chapter 10.4 How Meiosis Introduces Variations in Traits AP Biology Fall 2010 Objectives • Describe the various processes that contribute to genetic variation Crossing Over in Prophase I • Homologous chromosomes pair up • Non-sister chromatids exchange segments in a process called crossing over Crossing Over in Prophase I • Because alleles for the same trait can vary between homologues, new combinations of genes in each chromosome can result – This is one source of genetic variation • Crossing over leads to genetic recombination in gametes Metaphase I Alignments • During metaphase I, homologous chromosomes randomly line up at the spindle equator • During anaphase I, homologous chromosomes (still duplicated) separate into two haploid cells, each of which has a random mix of maternal and paternal chromosomes Metaphase I Alignments • Every time a human sperm or egg forms, the possible combinations of chromosomes equal 223 (8,388,608) • Figure 10.7 Crossing Over in Prophase I • Gene swapping isn’t pointless – Genes vary and come in slightly different forms • Alleles – A number of the alleles on one chromosome will not be identical to their partner alleles on homologous chromosomes Review 1. Be able to describe crossing over Answers 1. Homologous duplicated chromosomes pair up. Intimate contact encourages crossovers at various intervals along length of non-sister chromatids. Non-sister chromatids exchange segments at cross over site. Crossing over breaks up old combinations of alleles and puts new ones together in homologous chromosomes, mixes up maternal and paternal information about traits.