biology 12.4

• Chromosome number:

  • fruit fly cells have 8 chromosomes

  Number (4 from each parent)

  • two sets of chromosomes are called

  homologous chromosomes because each

  of 4 chromosomes from father has a

  corresponding chromosome from mother

  • human cells have 46 chromosomes

  (23 from each parent)
• Diploid cells contain two sets of homologous chromosomes, they also contain 2 complete sets of chromosomes and 2 complete sets of genes

  • symbol “2N” represents diploid number

  of chromosomes

  • 2N=8 in fruit flies and 2N=46 in humans
• Haploid cells contain a single set of chromosomes, & therefore only a single set of genes, gametes of sexually reproducing

  organisms are haploid

  • symbol “N” represents haploid number of

  chromosomes

  • N=4 in fruit flies and N=23 in humans
• Meiosis is the process in which number of chromosomes per cell is cut in half

  • homologous chromosomes of a diploid cell are separated to produce haploid cells

  • 1 diploid cell divides twice to produce 4 haploid cells (gametes)

  • chromosomes are replicated prior to meiosis (during interphase, as in mitosis)

  • divided into meiosis I and meiosis II
• Crossing over is the process during meiosis I in which bits

  and pieces of homologous chromosomes are exchanged, it also produces new combinations of alleles on each chromosome

  • sorting and recombination of genes helps

  to increase genetic variation in offspring
• Animal meiosis is when male animals produce 4 functional

  haploid gametes (sperm) and female animals produce 1 functional

  haploid gamete (egg) that receives most of the cytoplasm – other 3 cells called polar bodies are discarded
• Comparison is when mitosis produces 2 genetically identical diploid cells and meiosis produces 4 genetically different haploid cells

  • mitosis involves 1 round of division while

  meiosis involves 2 rounds of division

  • mitosis does not change number of

  chromosomes while meiosis cuts number

  of chromosomes in half
• Gene Linkage is when alleles of different genes tend to be

  “linked” and inherited together when those genes are located on the same chromosome