Download Mendelian genetics

Mendel and Heredity
Heredity: The transmission of characteristics
from one generation to the next.
Genetics: The study of heredity
-what characteristics get passed on, and how are
they passed on?
Origins of Genetic Science
Gregor Mendel (1822-1884)
-Augustinian Monk, designed
experiments to see if environment
or “something else” influenced the
traits of organisms.
Mendel published his work on genetics
in 1866, but it was largely ignored until
1900.
Why was Mendel’s work ignored?
His work was finally recognized as an
important part of genetic research.
Why was his work finally recognized?
Mendel worked with large numbers of
pea plants.
Benefits of using pea plants:
-many traits expressed on 1 plant
-easy to grow in large numbers
-peas reproduce sexually,
recombining genes
Generations
Parental Generation (P)
First Filial Generation (F1)
Second Filial Generation (F2)
Mendel’s Early Experiments
-crossed peas with smooth seeds and
peas with wrinkled seeds (PxP cross)
-the result was plants, all with smooth
seeds (F1 generation)
P
rr
RR
Smooth Peas
F1
Wrinkled Peas
Rr
Smooth Peas
Mendel then crossed F1 plants with
F1 plants
Rr x Rr
The result was a ratio of 3 plants that
produced smooth peas to 1 plant that
produced wrinkled peas .
Mendel’s Hypothesis:
-For every trait, there are 2 factors,
one from the male, one from the
female.
The combination of these factors
determines the trait that is
expressed.
-(we now call the “factors” genes.)
Mendel represented dominant genes
with a CAPITAL letter, and recessive
genes with a lower case letter.
Ex:
R for Round, smooth peas
r for wrinkled peas
Terms:
Allele: One of two or more
alternate forms of a gene.
Example: R - round seeds
r - wrinkled seeds
Genotype: The combination of alleles
that an organism has.
Example: in peas, for seed shape, the
possible genotypes are RR, Rr, and rr.
Phenotype: The physical or visible
features determined by the organisms
genotype:
Example: round peas, blue eyes, tallness
Homozygous: When an organism has
2 identical alleles for a trait
Example: RR or rr
Homozygous Dominant (RR)
Homozygous Recessive (rr)
Heterozygous: When an organism has 2
different alleles for a trait
Example Rr
Mendel’s 2 Laws:
1) Law of DominanceOne form of a hereditary
trait, the dominant trait,
dominates or prevents the
expression of the recessive
trait.
2) The Law of
Segregation: During
gamete formation, the pair
of genes responsible for
each trait separates so that
each gamete receives only
1 gene for each trait.
Punnett Squares
-allow us to predict the outcome of
the combination of possible gametes
Gametes
A
A
A
AA
AA
a
Aa
Aa
Incomplete Dominance
-neither allele is dominant to
the other
-three phenotypes result, with
the third being intermediate to
the other two
Example: In some flowers, the
alleles for flower color show
incomplete dominance. The allele for
RED is R, the allele for WHITE is W.
The resulting cross of 2 homozygous
individuals would be:
RR x WW
RR x WW (PxP cross)
RR= Red Flowers
WW= White Flowers
R
W
R
RW RW
W RW RW
RW= Pink Flowers
F1
RW x RW(F1xF1 cross)
RR= Red Flowers
WW= White Flowers
R
R
W
RR RW
W RW WW
RW= Pink Flowers
F2
Crosses with 2 traits
EX: Tall/Red x short/white
T= Tall
R=Red
t = short
r= white
Parent Genotypes: TTRR x ttrr
Both are homozygous
Things to remember:
1) alleles for traits always sort
independently.
2) gametes MUST have one
allele for each trait.
TTRR x ttrr
Possible
Gametes:
TR
TR
(P x P cross)
Possible
Gametes:
tr
tr
Possible Gametes
TR
tr
TR
TtRr TtRr
tr TtRr TtRr
F1 cross (TtRr x TtRr)
TtRr
TtRr
Possible
Gametes:
TR
Tr
tR
tr
Possible
Gametes:
TR
Tr
tR
tr
TR
Tr
tR
tr
TR TTRR
TTRr
TtRR
TtRr
Tr
TTRr
TTrr
TtRr
Ttrr
tR
TtRR
TtRr
ttRR
ttRr
tr
TtRr
Ttrr
ttRr
ttrr
Codominance
-Codominance is similar to incomplete
dominance, because neither allele is
dominant to the other.
-But it is different, because instead
of an intermediate trait, being shown,
both traits are exhibited.
Example: Blood types
Type A
Type B
Type AB
Type O
There are 3 alleles for blood
type.
A,B,and O
A and B are codominant
O is recessive to both A and B
Phenotype
A
B
AB
O
Genotype
AA , AO
BB , BO
AB
OO
AO x
AB
A
O
A
AA
AO
B
AB
BO
AO x
BO
A
O
B
AB
BO
O
AO
OO