Essential Biology 10.2 Dihybrid Crosses and Gene Linkage
Due Date:
Student Name:
Candidate Number: 002171-
1. Distinguish between dihybrid and monohybrid crosses.
 Mono Di-
2. When Mendel came upon his law of independent assortment, he was studying sweet-pea colour
and shape. These traits are carried on separate chromosomes. The colour yellow (Y) is dominant
over green (y). Smooth peas (S) are dominant over rough (s).
a. State the law of independent assortment.

b. State the possible genotypes for the following phenotypes:
Yellow, Smooth
Green, rough
Green, Smooth
yyss only
Yellow, rough
c. Use the punnet grid to predict the ratio of phenotypes of offspring in a cross between two
peas which are heterozygous for both genes (SsYy x SsYy).
SY
i.
Sy
sY
sy
SY
Sy
sY
sy
Phenotype
Smooth Yellow
Smooth green
Rough Yellow
Rough Green
Ratio
3. A researcher has some smooth yellow peas. He wants to find out if they are homozygous or
heterozygous for these dominant characteristics, so he performs a test cross.
a. State the genotype and phenotype of the plant that must be used as the test cross.
 Genotype:
 Phenotype:
Stephen Taylor
Bandung International School
http://sciencevideos.wordpress.com
Essential Biology 10.2 Dihybrid Crosses and Gene Linkage
Due Date:
Student Name:
Candidate Number: 002171-
b. Complete a series of punnet grids for this cross.
Deduce the phenotype ratios expected in the following crosses.
i.
Heterozygous for both colour and shape?
ii.
Phenotype
Ratio
ii.
Homozygous for both colour and shape.
iii.
Phenotype
Ratio
Stephen Taylor
Bandung International School
http://sciencevideos.wordpress.com
Essential Biology 10.2 Dihybrid Crosses and Gene Linkage
Due Date:
Student Name:
Candidate Number: 002171-
iii.
Heterozygous for colour, homozygous for shape.
iv.
Phenotype
Ratio
iv.
Homozygous for colour, heterozygous for shape.
CA
CA
Phenotype
Ratio
c. In the cross, the student found 120 yellow-smooth and 124 green-smooth. Deduce the
most likely genotype for the unknown pea. Explain your answer.
 Genotype:
 Explanation:
d. State the statistical test used to test for significance of difference between observed and
expected outcomes.

Stephen Taylor
Bandung International School
http://sciencevideos.wordpress.com
Essential Biology 10.2 Dihybrid Crosses and Gene Linkage
Due Date:
Student Name:
Candidate Number: 002171-
4. Some genes affect the expression of others.
a. Define gene interaction.

b. Identify the phenotype ratios for a cross between two ‘agouti’ (brown) mice, where:
Gene C determines if colour is present:
C = coat has colour
If colour is present, then:
A = agouti (brown)
The mice are both heterozygous for both genes.
c = albino
a = black
CA
CA
Phenotype
Ratio
c. Explain why the phenotype ratio is not 9:3:3:1.


5. Mendel’s law of independent assortment makes the assumption that genes for a pair or group of
traits are being carried on separate chromosomes, and therefore the presence of one allele in a
gamete is not connected to the presence of another.
However, with hundreds of genes per chromosome, it is likely that some genes will be physically
linked and therefore alleles will be inherited together.
a. Define linkage groups.

b. State the consequence of gene linkage in terms of the movement of alleles in anaphase I.
 Linkage groups will not…
Stephen Taylor
Bandung International School
http://sciencevideos.wordpress.com
Essential Biology 10.2 Dihybrid Crosses and Gene Linkage
Due Date:
Student Name:
Candidate Number: 002171-
c. State two plant examples of gene linkage.


The notation shows genes of Zea mays (corn). It is described as “heterozygous at both loci”.
These are both traits related to the corn kernels.
Key:
C = colour, c = no colour;
W = waxy, w = no wax.
Draw some other possible combinations of these linked genes:
Homozygous
dominant at both
loci
Homozygous
recessive at locus 1.
(you decide) –
describe it
d. Complete a punnet grid to show the possible phenotypes produced by a cross between the
corn that is heterozygous at both loci. Use correct notations and show your working.
Phenotype
Ratio
e. List the combinations of alleles which are not possible in the cross above (unless
recombination takes place at Prophase I).


Stephen Taylor
Bandung International School
http://sciencevideos.wordpress.com
Essential Biology 10.2 Dihybrid Crosses and Gene Linkage
Due Date:
Student Name:
Candidate Number: 002171-
f.
A small number of kernels which are coloured but not waxy appear in the offspring.
Explain the process that must have occurred for this to be the case.
 Process:




g. State the stage of meiosis during which crossing over and exchange of alleles can occur.

h. Complete the punnet grid below:
P
p
l
p
l
L
p
l
PpLl
no recombination
with crossing over (recombinants)
6. The term linkage is used in various ways in genetics.
a. Distinguish between autosomes and sex chromomes.


b. Distinguish between gene linkage and sex linkage.


Complete further practice of dihybrid crosses and gene linkage using the faces that you made in the
Genotype to Phenotype activity and on the Practice Problems sheet. Be sure to set out all answers
clearly.
Stephen Taylor
Bandung International School
http://sciencevideos.wordpress.com
Essential Biology 10.2 Dihybrid Crosses and Gene Linkage
Due Date:
Student Name:
Candidate Number: 002171-
Works Cited
1. Allott, Andrew. IB Study Guide: Biology for the IB Diploma. s.l. : Oxford University Press, 2007. 978-019-915143-1.
2. Mindorff, D and Allott, A. Biology Course Companion. Oxford : Oxford University Press, 2007. 978099151240.
3. Clegg, CJ. Biology for the IB Diploma. London : Hodder Murray, 2007. 978-0340926529.
4. Campbell N., Reece J., Taylor M., Simon. E. Biology Concepts and Connections. San Fransisco :
Pearson Benjamin Cummings, 2006. 0-8053-7160-5.
5. Taylor, Stephen. Science Video Resources. [Online] Wordpress, 2010.
http://sciencevideos.wordpress.com.
6. Burrell, John. Click4Biology. [Online] 2010. http://click4biology.info/.
7. IBO. Biology Subject Guide. [Online] 2007. http://xmltwo.ibo.org/publications/migrated/productionapp2.ibo.org/publication/7/part/2/chapter/1.html.
Self Assessment:
Essential Biology
Criterion
Presentation &
Organisation
Academic
Honesty
Objective 1
understanding
Objective 2
understanding
Objective3
understanding
Logic, notation,
mathematical
working
Further research
Assessment
Complete (2)
Partially complete (1)
NA
Complete and neat. All command terms
highlighted, tables and diagrams well presented.
Self
Sources cited using the CSE (ISO 690 numerical)
method, with Works Cited section complete and
correct.
All answers for the following command terms
Most answers for the following command terms
correct:
correct:
Define
Draw
Label
List
Measure State
Most answers for the following command terms
All answers for the following command terms correct:
correct:
Annotate Apply
Calculate Describe Distinguish Estimate Identify
Outline
Most answers for the following command terms
All answers for the following command terms correct:
correct:
Analyse Comment Compare Construct Deduce Derive Design Determine Discuss
Evaluate
Explain Predict
Show Solve
Sketch Suggest
Answers are presented in a logical and concise
manner. SI units used most times, with correct
NA
unit symbols and definitions of terms. All
mathematical working shown.
Evidence is apparent of research and reading
beyond the textbook and presentations to find
correct answers to challenging questions. If any
NA
questions are unanswered, this criterion scores
zero.
NA
Total (max 10):
Stephen Taylor
Bandung International School
http://sciencevideos.wordpress.com
MrT