Download Document

Ch. 9 Meiosis Notes
I.
Variation in traits is the foundation for evolutionary change
A. Sexual reproduction:
i.
meiosis forms gametes which, when united, become fertilized
ii.
the newly formed single-celled organism has homologous
chromosomes paired from each parent
B. Homologous: chromosomes that occur in pairs (one from mom, one
from dad) with different information for the same trait
i.
Humans have 23 homologous pairs
ii.
44 somatic chromosomes
iii. 2 sex chromosomes (XX female, XY male)
C. Variation in humans is from sexual reproduction when new
combinations of alleles (molecular forms of a gene) are inherited
II.
How Meiosis Halves the Chromosome #
A. Meiosis: chromosomes proceed through two consecutive divisions
which leading to the formation of 4 haploid nuclei
B. Gametes: Sperm originate in the testis while eggs originate in the ovary
C. Chromosome # :
i.
Diploid (2N): somatic (body) cells that contain a double set
of chromosomes, one from each parent
ii.
Haploid (N): gametes (sex cells) that contain a single set of
chromosomes
D. Meiosis I: duplicated chromosomes line up with its paired homologue
E. Meiosis II: sister chromatids split forming 4 nuclei