Bell ringer

1
Bell ringer

• How can you tell in a pedigree whether a trait is
  dominant or recessive?
• If two unaffected parents have a child with a
  trait, what must be true about the parents?

2
What is the great advantage of sex in the living
world?

• Variation!

3
How variation occurs

• I. Independent assortment

4
How many possibilities?

• 223 8,388,608 possible sperm or egg
• (223)2 70,368,744,000,000 possible zygotes

5
How variation occurs

• II. Crossing over

6
Crossing over and gene linkage
Crossing over is not such a random
process Crossing over is more likely to occur
when genes are farther apart. So genes that are
said to be linked tend to occur together most
of the time.
7
How much crossing over is there?

• 2-3 crossing over events per chromosome
• So the 70 trillion possible zygotes based on
  independent assortment is increased even more!
• No two individuals (except identical twins) will
  be alike!

8
The ultimate source of variation

• Mutation!
• Independent assortment and crossing over account
  for difference among individuals within one
  species
• Mutationfundamental changes in genesaccounts
  for all the different species that ever have been
  and will be.

9
New frontiers

• Epigenetics
• How environment influences whether and how genes
  are expressedturns genes on and off
• So in reality, even identical twins are not
  identical their epigenome will be different.

10

• http//www.pbs.org/wgbh/nova/body/epigenetics.html