Monohybrid Crosses in Genetics
GENETICS AND INHERITANCE
MONOHYBRID CROSSES
E
X
A
M
I
N
A
T
I
O
N
G
U
I
D
E
L
I
N
E
S
MONOHYBRID
CROSSES
T
E
M
P
L
A
T
E
F
O
R
A
G
E
N
E
T
I
C
C
R
O
S
S
P1
phenotype
genotype
meiosis
gametes
fertilization
F1
genotype
phenotype
U
S
I
N
G
A
P
U
N
N
E
T
S
Q
U
A
R
E
I
N
A
G
E
N
E
T
I
C
C
R
O
S
S
P2
phenotype
genotype
meiosis
gametes
fertilization
F2
genotype
TT, Tt, Tt, tt
phenotype
3 Tall: 1 dwarf
M
E
N
D
E
L
’
S
P
R
I
N
C
I
P
L
E
O
F
S
E
G
R
E
G
A
T
I
O
N
•
States that the two alleles for a
characteristic
separate
or
segregate
during gamete formation (meiosis) so
that each gamete contains only one
allele for that characteristic.
M
E
N
D
E
L
’
S
L
A
W
O
F
D
O
M
I
N
A
N
C
E
•
States that if homozygous
individuals
with
contrasting characteristics
are
interbred
, the
F1 generation will show
the dominant characteristic
A
C
T
I
V
I
T
Y
1
•
In pea plants round
(R)
seed shape is dominant
over wrinkled
(r)
seed
shape. Use a genetic cross
to determine the
genotypic ratio of the
offspring that result from
crossing two
heterozygous pea plants.
T
E
M
P
L
A
T
E
F
O
R
A
G
E
N
E
T
I
C
C
R
O
S
S
P1
phenotype
genotype
meiosis
gametes
fertilization
F1
genotype
genotypic
ratio
phenotype
1RR: 2 Rr: 1 rr
ACTIVITY 2 – Q2.4 P2 NOV 2019
QUESTIONS
Spotted back
Spotted frogs produced offspring without spots
The spotted offspring were three times
more than offspring without spots/ ratio of spotted offspring to offspring without spots is 3:1
T
E
M
P
L
A
T
E
F
O
R
A
G
E
N
E
T
I
C
C
R
O
S
S
Can do a
cross using
Punnet
square if
you prefer
T
Y
P
E
S
O
F
D
O
M
I
N
A
N
C
E
:
C
O
M
P
L
E
T
E
D
O
M
I
N
A
N
C
E
•
A genetic cross where the
dominant allele masks
(blocks) the expression of a
recessive allele in the
heterozygous condition
•
E.g. In Pea plants tall is
dominant (T), dwarf is
recessive (t)
I
N
C
O
M
P
L
E
T
E
D
O
M
I
N
A
N
C
E
A genetic cross between
two
phenotypically
different
parents
produces
offspring
different from both
parents but with an
intermediate phenotype
•
None of the two alleles of
a gene is dominant
•
Both alleles are
represented with a capital
letter but different letters
are used
X
X
Red flower
White flower
Pink flower
RR
WW
RW
C
O
-
D
O
M
I
N
A
N
C
E
•
A genetic cross in which
both alleles
are
equally
dominant
and are
expressed equally
in the
phenotype of offspring
in the heterozygous state
White camellia
Red camellia
Red and White camellia
https://biologywise.com/codominance-explained-with-
examples
WW
RR
RW
C
O
-
D
O
M
I
N
A
N
C
E
I
N
A
N
I
M
A
L
S
Other examples: Blood
groups in humans e.g. AB
blood group (SEE LATER)
A
C
T
I
V
I
T
Y
:
G
E
N
E
T
I
C
S
W
O
R
K
S
H
E
E
T
2
Explore Mendel's monohybrid crosses using pea plants to study inheritance of specific characteristics, such as plant height. Learn about genetic principles like segregation and dominance through practical examples and Punnett squares.
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Presentation Transcript
GENETICS AND INHERITANCE MONOHYBRID CROSSES
EXAMINATION GUIDELINES EXAMINATION GUIDELINES
Based on Mendels experiments conducted with pea plants MONOHYBRID CROSSES Pure (true) breeding pea plants were crossed to study the inheritance of one characteristic at a time e.g. tall pea plants were crossed with dwarf/short plants. QUESTION? STATE THE CHARACTERISTIC THAT WAS BEING INVESTIGATED IN THIS EXPERIMENT? ANSWER:HEIGHT All offspring in the first (F1) generation were tall , none were dwarf. He allowed the offspring of the F1 generation to self fertilise and the offspring of the second generation came out in the ratio 3 Tall plants : 1 short Plant
Tall TT X X Dwarf tt P1 phenotype genotype meiosis gametes fertilization T, T X t, t TEMPLATE FOR TEMPLATE FOR A GENETIC A GENETIC CROSS CROSS Tt Tt Tt Tt F1 genotype All Tall phenotype
P2 phenotype genotype meiosis gametes fertilization Tall Tt X X Tall Tt T, t X T, t USING A PUNNET USING A PUNNET SQUARE IN A SQUARE IN A GENETIC CROSS GENETIC CROSS T t T TT Tt Tt tt t F2 genotype TT, Tt, Tt, tt phenotype 3 Tall: 1 dwarf
MENDELS PRINCIPLE OF SEGREGATION MENDEL S PRINCIPLE OF SEGREGATION States that the two alleles for a characteristic separate or segregate during gamete formation (meiosis) so that each gamete contains only one allele for that characteristic.
MENDELS LAW OF DOMINANCE MENDEL S LAW OF DOMINANCE States that if homozygous individuals with contrasting characteristics are interbred, the F1 generation will show the dominant characteristic
In pea plants round (R) seed shape is dominant over wrinkled (r) seed shape. Use a genetic cross to determine the genotypic ratio of the offspring that result from crossing two heterozygous pea plants. ACTIVITY 1 ACTIVITY 1
P1 phenotype genotype meiosis gametes fertilization Round X Rr Round X Rr R, r X R, r TEMPLATE FOR TEMPLATE FOR A GENETIC A GENETIC CROSS CROSS genotypic ratio phenotype F1 genotype RR Rr Rr rr 1RR: 2 Rr: 1 rr 3 Round, 1 wrinkled
QUESTIONS Spotted back Spotted frogs produced offspring without spots / The spotted offspring were three times more than offspring without spots/ ratio of spotted offspring to offspring without spots is 3:1
Phenotype Spotted Without spots P1 x Dd dd Genotype x Can do a cross using Punnet square if you prefer Meiosis G/gametes D , d d, d x TEMPLATE FOR TEMPLATE FOR A GENETIC A GENETIC CROSS CROSS Fertilisation DD ; Dd ; dd ; dd * F1 Genotype Phenotype (2) spotted ; (2) without spots* P1 and F1 Meiosis and fertilisation 2 compulsory + any 4
TYPES OF DOMINANCE: TYPES OF DOMINANCE: COMPLETE DOMINANCE COMPLETE DOMINANCE A genetic cross where the dominant allele masks (blocks) the expression of a recessive allele in the heterozygous condition E.g. In Pea plants tall is dominant (T), dwarf is recessive (t) T x t Tt Tall
INCOMPLETE DOMINANCE INCOMPLETE DOMINANCE A genetic cross between two phenotypically differentparents produces offspring different from both parents but with an intermediate phenotype None of the two alleles of a gene is dominant Both alleles are represented with a capital letter but different letters are used Red flower White flower Pink flower X RW WW RR X
Red camellia Red camellia White camellia White camellia CO CO- -DOMINANCE DOMINANCE RR WW A genetic cross in which both alleles are equally dominant and are expressed equally in the phenotype of offspring in the heterozygous state Red and White camellia Red and White camellia RW https://biologywise.com/codominance-explained-with- examples
Other examples: Blood groups in humans e.g. AB blood group (SEE LATER) CO CO- -DOMINANCE IN ANIMALS DOMINANCE IN ANIMALS
ACTIVITY: GENETICS WORKSHEET 2 ACTIVITY: GENETICS WORKSHEET 2 Genetics Worksheet 2
