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Genetic Variation

Genetic Variation. SBI3U @ RHSA. The Law of Independent Assortment. Alleles segregate independently when gametes are formed during meiosis. Every individual has two alleles for each gene. Each gamete contains only one allele for a given gene.

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Genetic Variation

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  1. Genetic Variation • SBI3U @ RHSA

  2. The Law of Independent Assortment • Alleles segregate independently when gametes are formed during meiosis. • Every individual has two alleles for each gene. • Each gamete contains only one allele for a given gene. • This process is beneficial to sexually reproducing organisms because it increases genetic variation in gametes.

  3. Independent Assortment

  4. Meiosis-Prophase I Crossing-over probably occurs here. Crossing-over is when chromosomes overlap and exchange portions of their chromatids.

  5. Crossing-over During Meiosis  • During the early stages of cell division in meiosis, two chromosomes of a homologous pair may exchange segments of DNA. • Crossing over increases genetic variation again.

  6. Genetic Variation In Humans n =23 chromosomes in haploid 2n = 46 chromosomes in diploid 2n = 223 = ~ 8 million possible combinations!

  7. Nondisjunction: members of a pair of homologous chromosomes do not separate properly during meiosis I or sister chromatids fail to separate during meiosis II Monosomy - missing chromosome Trisomy - extra chromosome (Down’s syndrome) Polyploidy - extra sets of chromosomes, occurs primarily in plants Chromosomal Errors 1: Non-Disjunction

  8. Down's Syndrome

  9. Deletion: removal of a chromosomal segment Duplication: repeats a chromosomal segment Inversion: segment reversal in a chromosome Translocation: movement of a chromosomal segment to a non-homologous chromosome Chromosomal Errors 2: Changes in Structure

  10. Work: • Read p 171 – 174, do p174 # 4,7,9 • Read p 226 – 230, do p 230 # 3

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