Monohybrid cross
Practical 1
Learning objectives:
 By the end of the lesson you should be able to:
 Solve problems in relation to monohybrid crosses
from the P generation to the F2 generation using
dominant and recessive alleles.
 To predict ratios of offspring genotype in
monohybrid crosses.
 State the reasons for differences between observed
and predicted figures in monohybrid crosses.
Introduction
round
 The shape of pea seeds can be either ______________
or
wrinkled
_________________________
Aim
 To carry out a simulation of a monohybrid
cross
Method
 1 The following cross is carried out:
P
True Breeding
Round seeds
x
True breeding
Wrinkled seeds
F1
 2 collect a dish of seeds from the F1 generation
 3 Count the numbers of round and wrinkled seeds and
enter your results in the table
Results
Round seeds
Wrinkled seeds
Interpretation
 The results show that all of the F1 generation have
Round seeds
__________________________
Dominant allele
 This shows that round is the _______________
•This shows that wrinkled is the
Recessive allele
__________________________________
Symbols
 Round = dominant = R
 Wrinkled – recessive = r
Diagram of cross
P phenotype
Genotype
F1 phenotype
genotype
Round seed x wrinkled seed
RR
X
All round
Rr
rr
 4 Members of the F1 are crossed together to
form the F2 generation
 Collect a dish of seeds from the F2 generation
 Count the numbers of each type of seed and
enter them in the table
Results - group
Round seeds
Numbers
Ratio
Wrinkled seeds
Results - class
Group
Round seeds
Wrinkled seeds
1
13
7
2
18
2
3
15
5
4
11
9
5
16
4
6
17
3
7
9
11
8
15
5
9
17
3
Total
131
49
Ratio
3
1
10
Interpretation
 When two of the F1 generation are crossed together some of the
F2 are round and some are wrinkled.
 This can be explained by looking at the diagram of the cross
F1 phenotype
Genotype
F2 Male/Female
R
r
round seed
Rr
x
x
round seed
Rr
R
RR
Rr
r
Rr
rr
Expected phenotype ratio 3 round: 1 wrinkled
 Does this match more closely with the group or the class
results?
The class results
Evaluation
 Why do the class results more closely match with the
expected ratio?
Large numbers are used making the
experiment more reliable
The Backcross
May have the genotype RR or Rr
 Round seeds from the F2 _______________________
Further cross
 To find out which a ______________
was carried out
crossed with wrinkled seeds
• They were ____________________________
as the
genotype of these is known
_____________________________________
•The results of the cross will allow you to tell if the round seed
homozygous or heterozygous
was __________________________
backcross
•This is known as a ____________
 7 Collect two dishes of seeds from cross A and cross B
 Collect the numbers of each type of seed from in each and enter your
results in the table.
Cross
A
B
Round seeds
Wrinkled seeds
Interpretation
 What was the genotype of the cross A parent?
RR
 Give a reason for your answer
All of the offspring were round so all inherited the
R allele from the round parent
•What was the genotype of the cross B parent?
Rr
• Give a reason for your answer
•Some of the offspring were wrinkled so must have
inherited the r allele from the round parent