1. Introduction to Genetic Variation
Or, lame photo montage thinly disguised as illustration of genetic variation

2. Meiosis Key Haploid (n) Diploid (2n) Haploid gametes (n = 23) Egg (n)
Sperm (n)
MEIOSIS
FERTILIZATION
Ovary
Testis
Diploid
zygote
(2n = 46)
Mitosis and
development
Multicellular diploid
adults (2n = 46)

3. Contributors to Genetic Variation
Independent assortment
Which chromosome does a gamete get?
Crossover events (“recombination”)
Chimeric alleles (remember chiasma formation?)
Random fertilization
Any sperm can fertilize any egg

4. Independent assortment
Whose chromosome did I get in Meiosis I?
50-50 shot at maternal or paternal per gamete
Independence of pairs
Each homologous pair is sorted independently from the others
For humans (n = 23) there are about 8 million possible combinations of chromosomes!
Maternal
Paternal
n=2 chromosomes
M1/M2
P1/P2
M1/P2
P1/M2

5. Separation of Homologs
Example: individual who is heterozygous at two genes
Allele that
contributes
to red hued hair
Allele that
contributes
to dark hair
Allele that
contributes
to green
eyes
Allele that
contributes
to blue eyes
Hair color gene
Eye color gene
During meiosis I, tetrads can line up two different
ways before the homologs separate. OR
Green eyes
Red hues
Blue eyes
Dark hair
Green eyes
Dark hair
Blue eyes
Red hues

6. Crossing Over- Genetic Recombination
Recombinant chromosomes
combine genes from each parent.
Prophase I
Chromosomes pair up gene by gene
Chiasma
Homologous portions of two nonsister chromatids traded
In Humans
two to three times per chromosome pair.
New combinations of alleles
combinations that did not exist in each parent.
Independent assortment builds on this variability

7. Key Events in Prophase of Meiosis I
Centromere
Sister chromatids
Chromosomes
One homolog
Synaptonemal
complex
Second homolog
Prophase I
2 pairs of sister chromatids are held tightly together
Crossing over can occur at many locations
Swapping of segments between maternal and paternal chromosomes.
Non-sister
chromatids
Protein complex

8. Recombinant chromosomes
Fig
Prophase I
of meiosis
Nonsister
chromatids
held together
during synapsis
Pair of
homologs
Chiasma
Centromere
TEM
Anaphase I
Figure The results of crossing over during meiosis
Anaphase II
Daughter
cells
Recombinant chromosomes

9. Random Fertilization Any sperm can fuse with any egg. Humans (n=23)
Each ovum is one of 8 million possible chromosome combinations
Successful sperm is one of 8 million different possibilities
Zygote (diploid offspring) is 1 of 70 trillion possible combinations of chrms
Amazing how similar siblings/offspring
can look!
Recombination adds even more variation to this.
Independent assortment builds on recombination
Mutations- ultimately create a population’s genetic diversity
…or not!

10. Gregor Mendel (1822-1884) Lots of training Monastery garden
Augustine monk
Beekeeper
Physics teacher
Meteorologist
Monastery garden
Pea plants

11. “Experiments on Plant Hybridization”
Published in 1866
Before 20th century, cited 3 times
NOT cited in “The Origin of Species” (1859)
Rediscovered
Hugo de Vries
Better publicity

12. Mendel and the Gene Idea
What he knew:
Heritable variations exist
Traits are transmitted from parents to offspring
Two main theories existed
Blending (mixing of traits)
Particulate inheritance (direct passage of one trait over another)
Where he started:
documented particulate inheritance with garden peas (Pisum sativum).

13. Why Peas are Awesome Genetic Models for 1860s
Trait
Phenotypes
Seed shape
Seed color
Pod shape
Pod color
Flower color
Flower and   pod position
Stem length
Tall
Dwarf
Terminal (at tip)
Axial (on stem)
Purple
White
Yellow
Green
Inflated
Constricted
Round
Wrinkled
Lots of visible traits (“phenotypes”)
flower color, seed shape, pod shape, etc.
Controlled mating
Hermaphroditic
sperm-producing organs (stamens) and egg-producing organs (carpels)
Cross-pollination (fertilization between different plants) can be done intentionally

14. Mendel Focused on Particulate Inheritance
True-breeding varieties
Offspring of the same variety when they self-pollinate
Hybridization
mate two contrasting, true-breeding varieties
True-breeding parents P generation
Hybrid offspring of the P generation are called the F1 generation
F1 individuals self-pollinate, the F2 generation is produced

15. How was this Technically Done?
TECHNIQUE 1
Peas normally self-fertilize
This is a problem…
Cut the stamen
Removes male gametes
Prevents selfing
Manually add pollen
Carpels fertilized by non-self plants
Forced outcrossing 2
Parental
generation
(P)
Stamens
Carpel 3 4
RESULTS
First
filial
gener-
ation
offspring
(F1) 5

16. Cross-Pollination (“Forced outcrossing”)
Control over matings
Allows observations and predictions
Great approach for genetics at large
Self-pollination
Female organ
(receives pollen)
Eggs
Male organs
(produce pollen
grains, which
produce male
gametes)
Cross-pollination
CROSS-
POLLINATION
3. Transfer pollen to the
female organs of the individual
whose male organs have
been removed.
2. Collect pollen from a
different individual.
1. Remove male organs
from one individual.
SELF-

17. Particulate Inheritance: Dominant and Recessive Traits
Mendel’s outcrossed plants
Seed shapes were either round or wrinkled
No “chimeric” version
NOT 50-50; round seeds were more common
Dominant trait
Round seeds
Recessive trait
Wrinkled seeds
Writing convention for alleles: R vs. r
Capital letter = dominant allele; lowercase = recessive allele
Individuals with two copies of the same allele (RR or rr) are homozygous, and those with two different alleles (Rr) are heterozygous.
RR or Rr
If Dominant gene is present, offspring WILL have the trait without exception
always rr