Transcript Document

Announcements
 Discussion sections meet next week (T, W, TH) -check
the web page for the room (some have changed)
 HW1 is due Monday in lecture (know who your TA is)
 Any questions about logistics??
Review of basic Mendelian inheritance
Scott
Miller
1. Given that I do not have
a cleft chin, which trait
is dominant?
2. What is Caleb’s
genotype?
Caleb
Fishman
Miller
3. What information would
we need to calculate the
probability that the new
baby has a cleft chin?
cleft
chins
Review of basic Mendelian inheritance
1. Given that I do not have a cleft chin, which trait is
dominant? This is a bit of a trick question. It is
most likely (and true) that cleft is dominant, but
from this information you can’t be totally sure.
2. What is Caleb’s genotype? Cc, if cleft allele is C.
3. What information would we need to calculate the
probability that the new baby has a cleft chin?
What is Scott’s genotype??
How would we figure that out?
Review of basic Mendelian inheritance
4. Assuming that Scott is a heterozygote and cleft is
dominant, what is the probability that the new baby
will have a cleft chin?
Use a Punnett square or simple math...
Learning Goals
• Understand Mendel’s 3rd principle
 Independent assortment
• Use Punnett square and forked-line diagram
to predict the inheritance of trait
combinations
• Understand a test cross
TRAIT
VARIANTS
Purple
White
Axial
Terminal
Seed color
Yellow
Green
Seed shape
Round
Wrinkled
Pod shape
Inflated
Constricted
Pod color
Green
Flower color
Flower position
Height
Tall
Yellow
Dwarf
Experimental Protocol
(1) Develop parental lines (P) by self-breeding
individuals until they are true breeding.
(2) Create first-generation progeny (F1) by mating
parental lines with alternative phenotypes
(e.g., tall x dwarf).
(3) Create second-generation progeny (F2) by
self-fertilizing F1’s.
PHENOTYPE
GENOTYPE
TT
P generation
tt
Cross-fertilization
F1 generation
100%
Tt
(tall)
100% tall progeny (hybrids)
Self-fertilization
25%
TT
F2 generation
75% tall
25% dwarf
50%
Tt
(tall)
25%
tt
(dwarf)
F 2 progeny
Rat io
787:277
2.84:1
round seeds X wr inkled seeds
5474:1850
2.96:1
yellow seeds x gr een seeds
6022:2001
3.01:1
vi olet flowers X whi te flowers
705:224
3.15:1
inflated pods X constr icted pods
882:299
2.95:1
g reen pods X yel low pods
428:152
2.82:1
axial flowers X terminal flowers
651:207
3.14:1
Parental Strains
tall x dwarf
Mendel’s Principles, so far
1. Dominance: In a heterozygote, one allele may
conceal another.
2. Segregation: In a heterozygote, two different
alleles segregate from each other with equal
probability during the formation of gametes.
What happens if we consider two traits
simultaneously?
TRAIT
VARIANTS
Purple
White
Axial
Terminal
Seed color
Yellow
Green
Seed shape
Round
Wrinkled
Pod shape
Inflated
Constricted
Pod color
Green
Flower color
Flower position
Height
Tall
Yellow
Dwarf
R
r
RR
Rr
Round
Round
Rr
rr
Round
Wrinkled
R
r
3 round : 1 wrinkled
Y
y
YY
Yy
Yellow
Yellow
Yy
yy
Yellow
Green
Y
y
3 yellow : 1 green
Yellow X Green
YY
yy
Yellow (Yy)
Yellow (YY, Yy) 6,022
Green (yy) 2,000
Round X Wrinkled
RR
rr
Round (Rr)
Round (RR, Rr) 5,474
Wrinkled (rr) 1,850
Dihybrid cross - seed color and seed texture
yyrr
YYRR
YR
F1 hybrid seed
gametes?
yr
YyRr
???
2 possiblities: complete linkage or independent assortment
round, yellow
RRYY
wrinkled, green
rryy
round, yellow
RrYy
Gametes
RY
ry
Complete linkage: only parental combinations
round, yellow
RRYY
wrinkled, green
rryy
round, yellow
RrYy
Gametes
RY
Ry
rY
ry
Independent assortment: all combinations possible
Fork Diagram
RY
25%
y
Ry
25%
Y
rY
25%
y
ry
25%
Y
R
R
RrYy
r
r
Independent assortment predictions: Dihybrid F2 phenotypes
RrYy
RY
Ry
x
RrYy
rY
RY RRYY RRYy RrYY
Ry RRYy RRyy
RrYy
ry
RrYy
Rryy
9 round, yellow
3 round, green
3 wrinkled, yellow
rY RrYY
RrYy
rrYY
rrYy
ry RrYy
Rryy
rrYy
rryy
1 wrinkled, green
Mendel’s Dataround, yellow
wrinkled, green
rryy
RRYY
round, yellow
RrYy (self-fertilize)
round, yellow
wrinkled, yellow
round, green
315
101
108
9:3:3:1
green, wrinkled
32
Mendel’s Principles
1. Dominance: In a heterozygote, one allele may
conceal another.
2. Segregation: In a heterozygote, two different
alleles segregate from each other during the
formation of gametes.
3. Independent Assortment: The alleles of
different genes (or loci) segregate (or assort)
independently of each other.
TtYyRr x TtYyRr
The forked-line method can be used to predict the
outcome of an intercross involving three
independently assorting genes in peas.
Use of a test cross
Tall
Dwarf
TT or Tt
tt
TT or Tt
How would you determine if the genotype of
this phenotypically tall plant is TT or Tt?
Test cross
TT or Tt
100% tall (Tt)
X
tt
Tall (Tt) or dwarf (tt)
50%
50%
Dihybrid test cross
ttyy
x
TtYy
TY
Ty
tY
ty
TtYy
Ttyy
ttYy
ttyy
Dwarf,
yellow
Dwarf,
green
25%
25%
ty
Tall, yellow Tall, green
25%
25%
Mendelian laws of segregation: mechanism
1: Alleles at a single gene segregate into the gametes
at random (1:1 ratio)
2: Alleles at unlinked genes assort independently, so
all combinations are equally likely.
Both of these laws result from how homologous
chromosomes line up in metaphase of Meiosis I.
Why is independent assortment important??
Why is independent assortment important??
How many chromosomally unique gametes can one
person make? LOTS!!!!
Next: Video clip for HW 1
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Independent assortment of chromosomes
Meiosis I
metaphase
Meiosis II
metaphase
gametes
A
A
’
A
’
B
B
’
B
A
A
’
B
B
’
AB
parental
A’B
’
A
B
’
A’B
AB’
A’B
AB’
Independent assortment of chromosomes
Meiosis I
metaphase
Meiosis II
metaphase
gametes
A
A
’
A
’
B
B
’
B
A
A
’
B
B
’
AB
parental
A’B
’
A
B
’
A’B
AB’
A’B
AB’
nonparental