Essential Biology 10.2 Dihybrid Crosses and Gene Linkage
10.3 Polygenic Inheritance
Student Name:
-
Resources: Clegg pp 333-346, Revision Guide pp 90,92
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
Ratio
Smooth Yellow
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Smooth green
Rough Yellow
Rough Green
Essential Biology 10.2 Dihybrid Crosses and Gene Linkage
10.3 Polygenic Inheritance
Student Name:
-
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:
b. Complete a series of punnet grids for this series of test crosses.
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
iii.
Heterozygous for colour, homozygous for shape.
iv.
Phenotype
Ratio
iv.
Homozygous for colour, heterozygous for shape.
Phenotype
Ratio
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Essential Biology 10.2 Dihybrid Crosses and Gene Linkage
10.3 Polygenic Inheritance
Student Name:
-
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.
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.
CA
CA
Phenotype
Ratio
c. Explain why the phenotype ratio is not 9:3:3:1.
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c = albino
a = black
Essential Biology 10.2 Dihybrid Crosses and Gene Linkage
10.3 Polygenic Inheritance
Student Name:
-
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…
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
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Essential Biology 10.2 Dihybrid Crosses and Gene Linkage
10.3 Polygenic Inheritance
Student Name:
-
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).
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.

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Essential Biology 10.2 Dihybrid Crosses and Gene Linkage
10.3 Polygenic Inheritance
Student Name:
-
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 chromosomes.


b. Distinguish between gene linkage and sex linkage.


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Essential Biology 10.2 Dihybrid Crosses and Gene Linkage
10.3 Polygenic Inheritance
Student Name:
-
Resources: Clegg pp. 343-6, Revision Guide, pp 91. .
7. Define polygenic inheritance.

8. Distinguish between polygenic inheritance and multiple alleles.

9. List one human and one plant example of polygenic inheritance.

10. Define contributing allele.

11. Polygenic inheritance gives rise to continuous variation within a population.
Explain what this means, using skin colour as an example.




12. Assume that two genes (A and B) are responsible for inheritance of skin colour, with two alleles
each and that they are unlinked. The dominant alleles code for melanin production.
a. Calculate the number of possible genotypes.
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Essential Biology 10.2 Dihybrid Crosses and Gene Linkage
10.3 Polygenic Inheritance
Student Name:
-
b. Apply the possible genotypes to the range of phenotypes below:
aabb
Aabb
c. Explain why the phenotypes show a normal distribution.


13. Using a punnet grid, explain why it is possible for children to have skin which is darker or lighter
than both parents.
Mother genotype:
Father genotype:
AB
i.
AB
Ab
aB
ab
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Ab
aB
ab