Chapter 13

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Chapter 13

  1. 1. Chapter 13 notes Meiosis and sexual life cycle
  2. 2. An Introduction to Heredity Heredity: the transition of traits from one generation to the next Along w/ similarities, there is also variation- offspring differ somewhat in appearance from parents to offspring Genetics: the study of heredity and hereditary variation
  3. 3. An Introduction to Heredity Parents pass information to offspring through coded hereditary units called genes. - 30 to 40 thousand genes in humans - genes are segments of DNA
  4. 4. Concept 13.1 Two types of reproduction: Asexual reproduction: single parent passes copies of all its genes to offspring; “like begets like.” Sexual reproduction: two parents give rise to offspring; results in greater variation
  5. 5. Concept 13.2 Life cycle: generation-to-generation sequence of stages in the reproductive history of an organism The human life cycle - each somatic cell has 46 chromosomes (23 pair); 2 copies of each pair are called homologous chromosomes
  6. 6. Concept 13.2 - chromosomes can be displayed through a karyotype - pairs 1-22 are called autosomes - pair 23 are called sex chromosomes - XX = female; XY = male - we inherit one chromosome of each pair from each parent
  7. 7. Concept 13.2
  8. 8. Concept 13.2
  9. 9. Concept 13.2 - gametes, or sex cells, contain 22 autosomes and 1 sex chromosome; a cell with a single chromosome set is called a haploid cell. - haploid cells are abbreviated 1n (n = 23) - the combining of gametes forms a zygote; becomes a diploid cell (2n)
  10. 10. Concept 13.2 - The only cells of the human body not produced by mitosis are gametes - this process is a form of cellular division called meiosis; meiosis reduces the chromosome number in half
  11. 11. Concept 13.2 The variety of sexual life cycles Animal cells - after meiosis, the gametes undergoes no division before fertilization Fungi - after meiosis, mitosis occurs and produces a 1n multicellular organism; forms 1n gametes that then fertilize
  12. 12. Concept 13.2 Plants (alternation of generations) - has both diploid (sporophyte) and haploid (gametophyte) multicellular stages;
  13. 13. Concept 13.2
  14. 14. Concept 13.3 Meiosis, like mitosis, is preceded by the replication of chromosomes; however, the single replication is followed by 2 consecutive divisions (meiosis I and meiosis II)
  15. 15. Concept 13.3
  16. 16. Concept 13.3 Meiosis I - during prophase I, chromosomes pair up in synapsis; 4 chromatids form a tetrad - during metaphase I, homologous pairs line up on equator - during anaphase I, chromosomes, not chromatids, separate to poles
  17. 17. Concept 13.3 Meiosis II - goes through the same steps as meiosis I, but does not replicate DNA Meiosis outcome is 4 1n gametes from a single cell
  18. 18. Concept 13.4 3 mechanisms contribute to the genetic variation arising from sexual reproduction Independent assortment of chromosomes -each homologous pair of chromosomes is positioned independently of the others; variation is 223 or about 8 million
  19. 19. Concept 13.4
  20. 20. Concept 13.4 Crossing over - when chromosomes line up along the equator, parts of chromatids can combine genes from parents Random fertilization - even w/out considering crossing over, any two parents will produce a zygote with any of 70 trillion combination (223 x 223 )
  21. 21. Concept 13.4

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