Review
Mendel’s “rules of the game”
1) Genes occur in pairs - Genetic characteristics are
controlled by genes that exist in pairs called alleles.
2) Dominance/Recessiveness- When two unlike alleles responsible
for a single character are present in a single individual, one is
dominant (expressed) to the other which is said to be
recessive (silent).
3) Segregation- during formation of the gametes, the paired
alleles separate or segregate randomly.
Question
Yellow seeded plants in the F2 are predicted
to have either GG or Gg genotypes.
Is there a way to distinguish the genotype?
Test Cross
The organism of dominant phenotype but
unknown genotype is crossed to a homozygous
recessive individual.
Test Cross
o
+
? Gg
GG
X
o
Possible sperm cells
gg
G
Possible
egg cells
g
g
g
Gg
Gg
gg
gg
If the offspring show 1:1 ratio of dominant:recessive phenotypes,
the parent in question must have been heterozygous (Gg).
Test Cross
o
+
? Gg
GG
X
Possible sperm cells
o
gg
G
Possible
egg cells
G
g
g
Gg
Gg
Gg
Gg
If the offspring show only the dominant phenotype, the parent in
question must have been a homozygous dominant (GG) individual.
Discovering genes via Mutant
analysis
• Generating mutants
– Chemical mutagenesis (EMS)
– base transition, point mutation
– Radiation
– deletions
– Transposons/ T-DNA tags
– insertion/deletions (indels)
Forward Genetics
- from phenotype to gene
ID the phenotype
ID the physiological, developmental,
molecular differences
ID the gene (DNA sequence)
And
observing segregation ratios
• Planned crosses and Punnett squares,
• Pedigree analysis
– Mutant or polymorphism analysis
• Autosomal dominant/recessive
• Sex-linked genes
If A represents a gene that displays
autosomal dominant/recessive
inheritance.
The genotype of I-1 must be:
a.
b.
c.
d.
AA
Aa
Aa
Insufficient
datatototell
tell
Insufficient data
1
2
3
R
r
If the DNA seen on gel 1 is from a pea plant heterozygous for “R”
If the DNA seen on gel 1 is from a pea plant heterozygous for “R”
and
most likely
likely from
from a
and“r”
“r”atatthe
theSBE1
SBE1locus,
locus,the
theDNA
DNAin
inlane
gel 22 isis most
apea
peaplant
plant
a.
a.
b.b.
c.c.
d.d.
Homozygous for the “R” allele
Homozygous for the “R” allele
Homozygous
allele
Homozygousfor
forthe
the“r”
“r”
allele
Unable
Unableto
tomake
makefunctional
functionalstarch
starchbranching
branchingenzyme
enzyme11
That
Thatisismaking
makingtwice
twicethe
thenormal
normalamount
amountof
ofstarch
starch
branching
branchingenzyme
enzyme11
Reverse Genetics
- from gene to phenotype
• ID a gene
• Mutate, knockout, over-express the gene
• Analyze the morphological, physiology,
developmental effects (the phenotype).
Independent Assortment
Chapter 3
Dihybrid cross
- crosses between individuals that
differ in two traits.
Mendel’s dihybrid crosses
P1 cross
P1 cross
X
X
yellow, round
green, wrinkled
green, round
yellow, wrinkled
F1
yellow, round
Self-pollination of the F1
X
F1
yellow, round
yellow, round
9/16 yellow, round
F2
3/16 yellow, wrinkled
3/16 green, round
1/16 green, wrinkled
X
yellow (Gg), round (Ww) yellow (Gg), round (Ww)
o
+
GW
o
GW
Gw
gW
gw
F2
GGWW
GGWw
GgWW
GgWw
GGWw
GGww
GgWw
Ggww
GgWW
GgWw
ggWW
ggWw
GgWw
Ggww
ggWw
ggww
Gw
gW
gw
G
e
n
e
r
a
t
i
o
n
Mendel’s dihybrid ratio
9:3:3:1
Mendel’s Second Principle of Inheritance
Independent Assortment - during gamete
formation, segregating pairs of unit factors
assort independently of each other.
But, what if
X
yellow (Gg), round (Ww) yellow (Gg), round (Ww)
X
yellow (Gg), round (Ww) yellow (Gg), round (Ww)
o
+
GW
o
GW
GW
gw
gw
F2
GGWW
GGWW
GgWw
GgWw
GGWW
GGWW
GgWw
GgWw
GgWw
ggww
ggww
GgWw
ggww
ggww
GW
gw
GgWw
gw
GgWw
G
e
n
e
r
a
t
i
o
n
Phenotypic Ratio
X
yellow (Gg), round (Ww) yellow (Gg), round (Ww)
Expected F2
9
Yellow, round
3
Yellow, wrinkle
3
green, round
1
green, wrinkle
0
Yellow, wrinkle
0
green, round
4
green, wrinkle
Resulting F2
12
Yellow, round
Chromosomal Basis of
Inheritance
Meiosis
(the prelude to
sexual
reproduction)
For sexual reproduction to occur, chromosomes must be
duplicated and divided between the gametes.
Meiosis I
Meiosis II
Independent Assortment
- during gamete formation, segregating pairs
of chromosomes (not genes) assort
independently of each other.
Mitosis takes
takes place
place in
in
Mitosis
a.
a.
b.
b.
c.
c.
d.
d.
e.
e.
Haploid cells
cells only
only
Haploid
Diploid cells
cells only
only
Diploid
Haploid or
Haploid
ordiploid
diploidcells
cells
Bacterial cells
cells
Bacterial
None of
of the
the above
above
None
Meiosis takes place in
a.
b.
c.
d.
e.
Haploid cells only
Diploid
Diploid cells
cells only
only
Haploid or diploid cells
Somatic cells
None of the above
Polygenic Traits
(Quantitative trait loci, QTLs)
Observation: Wheat kernel color is a continuum from
white
dark red
Experiment 1
o
+
X
o
P1
F1 - all light red
Experiment 2
X
o
+
F1
o
F2
1
6
15
20
15
6
1
Frequency Diagram
Wheat Kernel Color
3 loci (polygenic), 6 different alleles
Dark
red
white
F2
seed color
# of dominant
alleles
6
5
4
3
2
1
0
Continuous variation is
determined by two or more genes.
These are polygenic or quantitative traits.
Cytoplasmic Segregation
(Non-Mendelian Genetics)
• Chloroplast DNA (cpDNA)
• Mitochondrial DNA (mt DNA)
Cytoplasmic Segregation
Test Cross ?
• Reciprocal crosses
– Who’s the pollen donor?
humandisease
diseaseassociated
associatedwith
withdysfunction
dysfunctionof
ofmitochondria,
mitochondria,
AAhuman
whichresults
resultsfrom
fromaamutation
mutationininaasingle
singleautosomal
autosomalgene
genelocus
locusinin
which
nuclearDNA,
DNA,isismost
mostlikely
likelyto:
to:
nuclear
Beinherited
inheritedfrom
fromthe
themother,
mother,because
becausemitochondria
mitochondriaare
are
a.a. Be
notinherited
inheritedfrom
fromthe
thefather
father
not
Showheteroplasmy
heteroplasmy(ainmix
the progeny
of mutant
affected
b.b. Show
of WT and
individuals, depending
on chance
eventsindividuals,
during meiosis
mitochondria)
in the progeny
of affected
c. depending
Show a non-Mendelian
inheritance
pattern, because the
on chance events
during meiosis
number
of mitochondriainheritance
varies from
cell tobecause
cell
c. Show
a non-Mendelian
pattern,
the
d. number
Show a of
Mendelian
inheritance
pattern
mitochondria
varies from
cell to cell
e. Show
None aofMendelian
the above inheritance pattern
d.
e. None of the above