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Essential Biology 10.2 Dihybrid Crosses and Gene Linkage
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Blog resource: http://tinyurl.com/6jp9zrw
Click4Biology: http://click4biology.info/c4b/10/gene10.htm
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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.
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Essential Biology 10.2 Dihybrid Crosses and Gene Linkage
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a. State the genotype and phenotype of the plant that must be used as the test cross.
 Genotype:
 Phenotype:
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
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Essential Biology 10.2 Dihybrid Crosses and Gene Linkage
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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.

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Essential Biology 10.2 Dihybrid Crosses and Gene Linkage
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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…
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Essential Biology 10.2 Dihybrid Crosses and Gene Linkage
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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).


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Essential Biology 10.2 Dihybrid Crosses and Gene Linkage
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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.
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Essential Biology 10.2 Dihybrid Crosses and Gene Linkage
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Works Cited
1. Allott, Andrew. IB Study Guide: Biology for the IB Diploma. s.l. : Oxford University Press, 2007. 978-0-19-915143-1.
2. Mindorff, D and Allott, A. Biology Course Companion. Oxford : Oxford University Press, 2007. 978-099151240.
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.
Mendelian Genetics Practice Problems
For the problems listed below, you are to solve the type of inheritance and explain the rationale
of your choice.
1. Mice I
o
a) cross 1: red-eyed mouse X white-eyed mouse
gives F1: all red-eyed
cross 2: red-eyed F1 X red-eyed F1
gives F2:
36 red-eyed
13 white-eyed
o
b) cross 1: long-eared mouse X short-eared mouse
gives F1:
12 long-eared
10 short-eared
cross 2: long-eared F1 X long-eared F1
gives F2:
36 long-eared
13 short-eared
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Essential Biology 10.2 Dihybrid Crosses and Gene Linkage
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2. Flowers
cross 1: blue-flowered plant X white-flowered plant
gives F1: all pale-blue-flowered
cross 2: pale-blue F1 X pale-blue F1
gives F2:
27 blue
49 pale-blue
24 white
3. Blood Type
o a) cross 1: person, type A blood X person with type B
gives F1: all type AB blood
cross 2: type AB F1 X type AB F1
gives F2:
2 type A
4 type AB
1 type B
o
b) cross 1: type A blood X type B
gives F1:
2 type A blood
3 type AB blood
1 type B blood
2 type O blood
4. Mice II
cross 1: tail-less mouse X normal mouse
gives F1:
10 tail-less
9 normal
cross 2: tail-less F1 X tail-less F1
gives F2:
10 normal
21 tail-less
9 dead
5. Flies
cross 1: red-eyed female X red-eyed male
gives F1:
50 red-eyed female
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Essential Biology 10.2 Dihybrid Crosses and Gene Linkage
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25 red-eyed male
25 white-eyed male
cross 2: white-eyed male F1 X red-eyed female F1
52 crosses give:
30 red male
33 red female
48 crosses give:
22 red male
24 red female
21 white male
23 white female
6. Pedigrees
7. Mice III
o
a) cross 1: blue-eyed, long-toothed mouse X brown-eyed, short-toothed mouse
gives F1: all blue-eyed, short-toothed
cross 2: blue-eyed, short-toothed F1 X blue-eyed, short-toothed F1
gives F2:
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Essential Biology 10.2 Dihybrid Crosses and Gene Linkage
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92 blue-eyed short-toothed
31 blue-eyed long-toothed
29 brown-eyed short-toothed
9 brown-eyed long-toothed
(NOTE how this result is different from the blood-type result because here we are
dealing with two genes and there we were dealing with multiple alleles of one
gene.)
o
b) cross 1: tall, green plant X short, yellow plant
gives F1:
20 tall green
20 short green
cross 2: tall, green X short, yellow (different plants than (b))
gives F1:
19 tall green
21 tall yellow
8. Pathways I
o a) assume the following pigment-producing pathway in a plant:
o
o
o
o
enz A
enz B
compound 1 ----------> compound 2 ------------> compound 3
(colorless)
(red)
(purple)
Also assume that the A and B alleles produce functional enzyme, while the a and b
alleles produce no functional enzyme. Assume that one functional copy of an enzyme is
sufficient to catalyze the reaction.
cross 1: AaBb X AaBb
predict the colors of the progeny.
o
o
o
o
o
o
o
o
o
b) what if the pathway were:
enz B
enz A
compound 1 ----------> compound 2 ------------> compound 3
(red)
(colorless)
(purple)
c) Assume two parallel pigment pathways:
enz A
enz B
red pigment <----------colorless compound --------------> blue
pigment
cross 1: Aabb X aaBb
gives F1:
33 white
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Essential Biology 10.2 Dihybrid Crosses and Gene Linkage
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32 red
35 blue
9. Linkage:
cross 1: rough, soft pea X smooth, hard pea
gives F1: all rough, hard
cross 2: rough, hard F1 X smooth, soft (note: different mating than usual)
gives F2: 115 rough soft
110 smooth hard
8 rough hard
12 smooth soft
10. A pedigree that illustrates multiple possible models is:
11. A man with a Y-linked disorder has three sons and three daughters by the same mother. His first
son has two sons and two daughters by another woman. Draw the pedigree for this family. How
could you tell if this disorder was Y linked or sex-linked recessive?
12. Three genes in Drosophila with the recessive alleles l, m, and n are known to be linked. You wish
to determine the map distances between them and perform the cross shown below. (Note: The
gene order drawn here is arbitrary - you need to determine the order of these genes on the
chromosome)
Progeny with the following phenotypes are seen:
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Essential Biology 10.2 Dihybrid Crosses and Gene Linkage
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Determine the order of these markers on the chromosome and the distances between them
in centimorgans.
13. This problem appeared on a previous 7.012 Exam.
Alkaptonuria is an extremely rare disease. The gene for Alkaptonuria (ALK) has recently
been shown to lie on human chromosome 9 and to be linked to the gene encoding the
ABO blood group, with a recombination frequency of 11% between the loci.
A pedigree of a family with the disease is shown below, with affected individuals
indicated in black. In addition, the blood type of family members is given.
The two alleles at the ALK locus will be denoted ALK+ and ALK-. The three alleles at
the ABO blood group locus will be denoted IA, IB (which are co-dominant) and i (which
is recessive to IA and IB).
(a) What is the genotype of individual 1 at the ALK and ABO loci?
(b) What is the genotype of individual 2 at the ALK and ABO loci?
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Essential Biology 10.2 Dihybrid Crosses and Gene Linkage
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(c) What is the genotype of individual 3 at the ALK and ABO loci? Which alleles of each
gene are carried on the chromosome he inherited from his father and which alleles are
carried on the chromosome he inherited from his mother?
(d) Individuals 3 and 4 are expecting their fifth child. A physician draws a prenatal blood
sample and determines that the child has blood type B. What is the probability that the
child will have alkaptonuria? Explain your answer.
You can check your answers on the Solutions Page.
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