Introduction to Genetic Variation
Or, lame photo montage thinly disguised as
illustration of genetic variation
Meiosis
Key
Haploid gametes (n = 23)
Haploid (n)
Egg (n)
Diploid (2n)
Sperm (n)
MEIOSIS
Ovary
FERTILIZATION
Testis
Diploid
zygote
(2n = 46)
Mitosis and
development
Multicellular diploid
adults (2n = 46)
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
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
n=2 chromosomes
Maternal
Paternal
M1/M2
P1/P2
M1/P2
P1/M2
• For humans (n = 23) there are about 8 million
possible combinations of chromosomes!
Separation of Homologs
Example: individual who is heterozygous at two genes
Allele that
contributes
to red hued
hair
Allele that
Allele that
contributes
contributes
to green
to blue eyes
eyes
Eye color gene
Allele that
contributes
to dark hair
Hair 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
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
Key Events in Prophase of Meiosis I
Centromere
• Prophase I
– 2 pairs of sister
chromatids are held
tightly together
• Crossing over can occur
at many locations
Sister chromatids
Chromosomes
One homolog
Synaptonemal
complex
Second homolog
Non-sister
chromatids
• Swapping of segments
between maternal and
paternal chromosomes.
Protein complex
Fig. 13-12-5
Prophase I
of meiosis
Pair of
homologs
Nonsister
chromatids
held together
during synapsis
Chiasma
Centromere
TEM
Anaphase I
Anaphase II
Daughter
cells
Recombinant chromosomes
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!
…or not!
• Recombination adds even more variation to this.
• Independent assortment builds on recombination
• Mutations- ultimately create a population’s genetic diversity
Gregor Mendel (1822-1884)
• Lots of training
– Augustine monk
– Beekeeper
– Physics teacher
– Meteorologist
• Monastery garden
– Pea plants
“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
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).
Why Peas are Awesome
Genetic Models for 1860s
Trait
Phenotypes
Seed shape
Round
Wrinkled
Yellow
Green
Inflated
Constricted
Green
Yellow
Purple
White
Axial (on stem)
Terminal (at tip)
Tall
Dwarf
Seed color
• 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
Pod shape
Pod color
Flower color
Flower
and pod
position
Stem length
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
How was this
Technically Done?
TECHNIQUE
1
• Peas normally self-fertilize
– This is a problem…
• Cut the stamen
2
Parental
generation
(P)
– Removes male gametes
– Prevents selfing
Stamens
Carpel
3
4
• Manually add pollen
– Carpels fertilized by non-self
plants
• Forced outcrossing
RESULTS
First
filial
generation
offspring
(F1)
5
Cross-Pollination (“Forced outcrossing”)
• Control over matings
– Allows observations and predictions
– Great approach for genetics at large
Self-pollination
Female organ
(receives pollen)
SELFPOLLINATION
Eggs
Cross-pollination
1. Remove male organs
from one individual.
Male organs
(produce pollen
grains, which
produce male
gametes)
2. Collect pollen from a
different individual.
CROSSPOLLINATION
3. Transfer pollen to the
female organs of the individual
whose male organs have
been removed.
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
If Dominant gene is
present, offspring WILL
have the trait without
exception
RR or Rr
always rr
• 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.