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http://history.nih.gov/exhibits/nirenberg/popup_htm/01_mendel.htm
Gregor Mendel
monkgardenercareful observerexperimenter
Mendel
at that time (1850’s-1860’s) it was thought that the traits passed on by the parents were blended in the offspring
red + white = ?pink
purple + white = purple or white
Mendel
parents contributed to offspring factorsfactors remained unchanged
set out to trace their movements
looked at phenotypeappearancelooked at ratio’s
Mendel
started with true-breeding plants
round round round
wrinkled wrinkled wrinkled
parent 1 parent 1 offspring
x
x
Mendel
seed shape round vs. wrinkled
round x wrinkled
wrinkled x round
round
round
Ppollen Pflower F1
Mendel
seed shape round vs. wrinkled
round x wrinkled
wrinkled x round
round
round
Ppollen Pflower F1
hybridsdominant vs. recessive
© 2006 Jones & Bartlett Publishers
Fig. 2.3. Traits studied in peas by Mendel
dom
inan
tdo
min
ant
rece
ssiv
ere
cess
ive
F1 cross
roundhybrid
roundhybrid
X
each hybrid has two parents
therefore
each hybrid has two “factors”
F1 cross
roundhybrid
roundhybrid
X
To solve:
define terms (use consistently)
determine parent genotypedetermine gamete genotypePunnett square
To solve:
R = round
? = wrinkled
capital letter=dominant
lower case letter-recessive
r
define terms
R and r are alleles:alternative forms of a gene
To solve:
original cross:
round vs. wrinkled(pure-breeding)
RR
determine parent genotypes
parents: rr
(homozygous)
To solve:
original cross:
round vs. wrinkled(pure-breeding)
RR rr
determine gamete genotypes
R or R r or r
parents:
gametes:
Mendel’s first law……The Law of Segregation
The two “factors” in the adult separate from each other during the production of the gametes.
homologous pairs of chromosomes separate during meiosis I.
#2
A problem:
Suppose you have a plant with purple flowers, but unknown ancestry. What is its’ genotype?
Is it homozygous dominantor
heterozygous?
PP
Pp
2 P’s or not…..
A solution:
if PP x pp
if Pp x pp
all offspring would be Pp(heterozygous)(purple flowers)
some offspring would be Pp
some offspring would be pp(purple)
(white)
Mendel also looked at two traits at once
round vs wrinkledyellow vs green
RR, rrYY, yy
pure-breeding round and yellow
pure-breeding green and wrinkled
pure-breeding round and yellow
pure-breeding green and wrinkled
RRYY rryyparentalgenotype
gametesgenotype
? ?RY ry
all F1 were round, yellow hybrids RrYy(dihybrids)
RrYy RrYyparentalgenotype
gametesgenotype
? ?
Mendel then did a dihybrid cross (F1 cross)
ryRY
ry
RY RRYY
RrYy
RrYy
rryy
rYRy
rY
Ry RRyy
RrYy
RrYy
rrYY
3 round, yellow1 wrinkled, green
1 round, green2 round, yellow1 wrinkled, yellow
round, yellowround, greenwrinkled, yellowwrinkled, green
315108101 32556
9331
Mendel’s results:
#4
(dihybrid cross)
Mendel’s second law……The Law of Independent Assortment (in modern language)
How a pair of homologous chromosomes align at Metaphase I is independent of how all the other pairs of chromosomes align at metaphase I
#5
Yy x Yyyellow yellow
2001 green6022 yellow
2001 (6022 + 2001)
= 1 4.01
Y y
YY Yy
Yy yy
Y
yWhat is the probability of gettingplants with green seed ?
Yy x Yyyellow yellow
What is the probability of getting plants with yellow peas?
YYYy
Y y
YY Yy
Yy yy
Y
y
1/42/4
prob [YY or Yy] = prob [YY] + prob [Yy]
= 1/4 + 2/4 = 3/4
prob [YY] + prob [Yy]
#6
RrYy x RrYy
What is the probability of getting plants with round, yellow peas?
Y y
YY Yy
Yy yy
Y
y
prob [round and yellow] = prob [round] x prob [yellow]
= 3/4 x 3/4 = 9/16
R r
RR Rr
Rr rr
R
r
9/16prob [round] x prob [yellow]
#7
YY or Yyand
RR or Rr
3/4
3/4
© 2006 Jones & Bartlett Publishers
Fig. 2.20. Three attributes of phenotype affected by Mendel's alleles W and w, which determine round versus wrinkled seeds.
dominance is not necessarily all or none
RR and Rr same phenotype in peas, but really different at the molecular level
Are there some cases wherehomozygous dominant (RR)
looks different thanheterozygous (Rr) ?
Incomplete dominance
When the heterozygote is intermediate between the homozygous phenotypes
Seen with traits that are quantitative(can be measured on a continuous scale)
as opposed to a discrete trait(appears to be all or none)
Blood typing (ABO)
IA
IB
i
three alleles (multiple alleles)
make “A” carbohydratemake “B” carbohydratemake neither carbohydrate
Blood typing (ABO)
Human blood types:
AB
ABO
phenotype: genotype:
IAIA, IAiIBIB, IBiIAIB
iicodominant
Blood typing (ABO)
Our immune system makes proteins
called antibodiesto attack foreign molecules
called antigens.
Blood typing (ABO)
For someone with type A blood (someone with the IA allele):
A carbohydrate “self”
B carbohydrate “non-self” or foreign antigen
Blood typing (ABO)
For someone with type B blood (someone with the IB allele):
A carbohydrate “non-self” or foreign antigen
B carbohydrate “self”
Blood typing (ABO)
For someone with type O blood (someone with ii alleles):
A carbohydrate “non-self” or foreign antigen
B carbohydrate “non-self” or foreign antigen
Blood typing (ABO)
For someone with type AB blood (someone with IA and IB alleles):
A carbohydrate “self”
B carbohydrate “self”
Blood typing (ABO)
What kind of antibodies would they make?
TypeABABO
- B antibodies- A antibodies- neither A nor B antibodies- A and B antibodies
© 2006 Jones & Bartlett Publishers
Table 2.3. Genetic control of the ABO blood groups
AB universal recipientO universal donor
© 2006 Jones & Bartlett Publishers
Fig. 2.23. Antibody against type-A antigen will agglutinate red blood cells carrying the type-A antigen.
Other reasons why Mendel rules aren’t always observed
incomplete dominancemultiple alleles
variable expressivitysame mutation-different results
penetrancecomplete 100%incomplete < 100%
polygenictraits ?
(phenotype = expected)
lung cancer
example
2.7 Epistasis
e.g.,When the expected 9:3:3:1 ratioof a dihybrid cross is altered
non-allelic genes interacting to affect the same trait
Merriam Webster: suppression of the effectof a gene by a nonallelic gene
2.7 Epistasis
C purple flowersc white flowers
plants with C- and P- genotypes havepurple flowers (wt)
plants with cc or pp have ? flowers
Given:
P purple flowersp white flowers
Two different genes C and P for flower color
white flowers
2.7 Epistasis aside:
How could that happen?(hypothetically)
A B C D purple pigment
mutation#1
mutation#3
E1 E2 E3 E4
2.7 Epistasis
CC pp x cc PP
phenotype
gametegenotype
F1 genotypeF1 phenotype
white flowers
Cp cP
CcPpall purple
© 2006 Jones & Bartlett Publishers
Fig. 2.24. A cross showing epistasis in the determination of flower color in peas.
F1 crosswhat gametes ?
CP Cp cP cp
CP
Cp
cP
cp
9:7 ratiopurple to white
epistasis
When the expected 9:3:3:1 ratio of a dihybrid cross is altered
CCPP CCPp CcPP CcPp
CCPc CCpp CcPp Ccpp
CcPP CcPp ccPP ccPp
CcPp Ccpp ccPp ccpp
© 2006 Jones & Bartlett Publishers
Fig. 2.26. Complementation reveals whether two recessive mutations are alleles of different genes.
2.8 Complementation
means that mutations affect different genes
means that mutations affect the same gene
Lack of complementation
© 2006 Jones & Bartlett Publishers
Fig. 2.27. Results of complementation tests among six mutant strains of peas.