1. Genetic Variation
Propose a model to illustrate your ideas about how this is possible. Think about what you have learned about DNA and how it is passed from generation to generation.

2. Meiosis Review ~ Mitosis vs. Meiosis ~ Meiosis Animation

3. Paramount in successful development of an embryo
**The key lies in Meiosis I - #chromosomes is halved – 2n → 1n
**Each cell now has half the original amount!!!
Paramount in successful development of an embryo

4. Spermatogenesis

5. Oogenesis

6. A comparison

7. Predict…
There are over 6 billion people on the planet. We’re all human – so how come we don’t all look alike? What causes all of the variety? Think about what you know about the process of gamete production (meiosis). If you had to pinpoint one specific part of the process most responsible for this variation, which would it be? Why?

8. The Meiotic Shuffle
During Prophase I homologous chromosome pairs line up
Crossing over can occur
section of a chromosome switches spots with the matching section of homologous chromosome

9. 2 variations
4 variations
Meiosis Animation

10. crossing over can happen on all homologous chromosomes
The more crossing over, the more variation
What do you think will happen to variation as you increase the number of chromosomes???

11. The more chromosomes – more possible variations
Example:
Humans – 23 chromosome pairs
Dogs – 39 chromosome pairs
Fruit Flies – 4 chromosome pairs

12. Randomosity of crossing…
Crossing is completely random
Usually 2-3 per chromosome
Example: 23 chromosome pairs – Let’s say 2 genes per pair cross over
223 (Mom) x 223 (Dad) ~> more than 70 trillion different combinations

13. What if something goes wrong
What if something goes wrong? What do you think would happen if meiosis didn’t happen correctly???

14. Sometimes chromosomes do not segregate the way they are supposed to – this is called nondisjunction
** Extra chromosomes often means survival**
**Missing chromosomes often means DEATH**

15. Monosomy
One less chromosome because one gamete has a chromosome missing Ex: Turner Syndrome

16. Gamete has an extra chromosome Example: Trisomy 21 (Down Syndrome)

17. Tri/Polyploidy Complete EXTRA sets of chromosomes
almost ALWAYS lethal to animals
plants can be healthier & larger

18. Putting it all together…
meiosis  fertilization  mitosis + development
gametes 46 23 46 23 46 46 46 46 46 23
meiosis
egg 46 46 46 46 23
zygote
fertilization
mitosis
sperm
development