Campbell and Reece Chapter 13
Sexual reproduction and meiosis give rise to biological variability, which
is very fundamental in evolution since new varieties may be more adapted
and hence survive (to reproduction) better.
Sexual reproduction probably evolved about 1.5 billion years ago, about
when eukaryotic cells evolved, and probably gave an evolutionary advantage.
TRANSPARENCY (Fig. 13.4) human life cycle haploid (n), diploid (2n), gametes,
ova, sperm, fertilization, zygote. Note that while meiosis creates haploid
gametes in humans, there are many organisms where meiosis creates a gamete-forming
organism (alternation of generations, many examples next semester).
TRANSPARENCY (Fig. 13.6) Meiosis. Note that there are two homologous chromosomes,
23 pairs for the human diploid number, 46. Theoretically, one could go to
each gamete with one meiotic division. Instead, they align, duplicate, and
TRANSPARENCY (from AHSturtevant and GWBeadle, An introduction to genetics,
Dover, New York, 1939 and 1962) details two divisions, tetrad, crossing
TRANSPARENCY (Fig. 13.7 A and B) Your book's version of the first division.
Awesome variability TRANSPARENCY (Fig. 13.9) for 2 pairs of chromosomes,
there are 4 possible outcomes. For 46 chromosomes, there are 2 to 23 power
= 8.4 million.
Crossing over increases this beyond measure.(shown for one pair of chromosomes
TRANSPARENCY (Fig. 13.10))
story of grains of wheat on chess board
Unequal crossing over and gene duplication as major means of
protein family evolution
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