HIGHER LEVEL TOPIC 10.2 – 10.3: DIHYBRID CROSSES, GENE LINKAGE,
POLYGENIC INHERITANCE
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1. Distinguish between dihybrid and monohybrid crosses.
 Monohybrid cross:

Dihybrid cross:
When Mendel came across 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).
2. State Mendel’s law of independent assortment.
3. State the possible genotypes for the following phenotypes:
Yellow, smooth
Green, rough
yyss only
Green, smooth
Yellow, rough
4. Use a Punnett Grid to predict the ratio of phenotypes of an offspring produced
from a cross between two peas which are heterozygous for both traits: (YySs)
SY
Sy
sY
Smooth
yellow
Smooth green Rough yellow
sy
SY
Sy
sY
sy
Phenotype
Ratio
Rough green
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5. 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 that must be used as the test cross.
Genotype
Phenotype
b) Complete a series of Punnett Grids for this cross.
Deduce the phenotype ratios expected in the following crosses:
I.
Heterozygous for both colour and shape
Phenotype
Ratio
II.
Homozygous for both colour and shape
Phenotype
Ratio
III.
Heterozgous for colour, homozygous for shape
Phenotype
Ratio
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IV.
Homozygous for colour, heterozygous for shape
Phenotype
Ratio
In the cross, the student found 120 yellow-smooth and 124 green-smooth.
V.
Deduce the most likely genotype for the unknown pea. Explain your answer.
6. State the statistical test used to test for significant differences between
observed and expected (ratio) outcomes.
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GENE LINKAGE
Some genes affect the expression of others.
7. Define gene interaction.
7. A) Identify the gene ratios for a cross between two agouti (brown) mice, where
two genes influence coat colour.
The dominant gene C determines if colour is present: C = coat has colour, c = albino
If C, then A = agouti (brown), a = black
The mice are both heterozygous for both genes (CcAa).
Phenotype
Ratio
B) Explain why the phenotype ratio is not 9: 3: 3: 1
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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 certain genes will
be physically linked, and therefore alleles will be inherited together.
8. Define linkage groups.
9. State the consequences of gene linkage in terms of the movement of alleles in
anaphase I
10. Linkage groups will not…
11. 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
a) Draw some other possible combinations of these linked genes
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Homozygous dominant at both loci
Homozygous recessive at locus 1
(you decide – describe it)
b) Complete a Punnett Grid to show the possible phenotype combinations
produced by a cross between the corn that is heterozygous at both loci. Use correct
notations and show your working.
Phenotype
Ratio
c) List the possible combinations that are not possible (unless recombination takes
place at prophase I)
d) A small number of kernels which are coloured but not waxy appear in the
offspring. Explain the process which must have occurred to allow this to happen.
12. State the stage of meiosis in which crossing over and exchange of alleles can
occur.
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13. Complete the Punnet Grid below:
What are the possible combinations (including recombinants)
P L
pl
pl
pl
No recombination
With crossing
over(recombination)
The term linkage is used in various ways in genetics.
14. Distinguish between autosomes and sex chromosomes
15. Distinguish between gene linkage and sex linkage
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16. Define polygenic inheritance
17. Distinguish between polygenic inheritance and multiple alleles
18. List one human and one plant example of polygenic inheritance
19. Define contributing allele
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‘Polygenic inheritance gives rise to continuous variation within a population’.
20. Explain what this means, using skin colour as an example.
21. Assume that two genes (A and B) are responsible for skin colour inheritance,
with two alleles each, and that they are unlinked. The dominant alleles code for
melanin production.
a) Calculate the number of possible genotypes.
b) Apply the possible genotypes to the range of phenotypes below:
c) Explain why the phenotypes show a normal distribution.
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d) Using a Punnett Grid, explain why it is possible for children to have skin which is
lighter or darker than both parents.
Mother
genotype
AB
Ab
aB
ab
AB
Father genotype
Ab
aB
ab
e) Outcome and explanation
22. Compare the advantages and disadvantages of light and dark skin
Light skin
Dark skin
Cold climate
Hot climate
23. State precautionary measures that should be taken by:
a) Pale skinned people in a hot climate
b) Dark skinned people in a cold climate
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24. A characteristic is controlled by two unlinked genes with two alleles.
Deduce the number of possible genotypes and phenotypes. Show your working.
25. A characteristic is controlled by three genes with two alleles each.
Calculate the number of genotypes possible in a cross between a homozygous
recessive father and a homozygous dominant mother. Show your working.
26. Wheat kernel colour is controlled by three unlinked genes (A, B and C). The
dominant alleles code for production of a red pigment.
Deduce the phenotype ratio expected in a test cross between a wheat plant that is
homozygous dominant for A, heterozygous for B and homozygous recessive for C.
Show your working.
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