Essential Biology 10.2: Dihybrid Crosses & Gene Linkage (HL)
1.
A dihybrid cross considers two genes at the same time.
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).
What are the possible genotypes for the following phenotypes:
Yellow, Smooth
Yellow, rough
Green, Smooth
Green, rough
2.
Use the punnet grid to predict the ratio of phenotypes of offspring in a cross between two completely heterozygous peas.
Ratios: Smooth Yellow : Smooth green : Rough Yellow : Rough Green
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Essential Biology 10.2: Dihybrid Crosses & Gene Linkage (HL)
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.
What must be the genotype must he use as the other individual in the test cross? b.
Complete a series of punnet grids for this cross. What phenotypic ratios would he expect to see if the smooth yellow peas were: i.
Heterozygous for both colour and shape? ii.
iii.
iv.
Homozygous for both colour and shape?
Heterozygous for colour, homozygous for shape?
Homozygous for colour, heterozygous for shape?
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Essential Biology 10.2: Dihybrid Crosses & Gene Linkage (HL) c.
In the cross, the student found 120 yellow-smooth and 124 green-smooth.
Which genotype is most likely to have been that of the unknown pea? d.
What statistical test could be used to test the significance of this result?
4.
Define gene interaction.
5.
Complete a punnet grid for a cross between two ‘agouti’ (brown) mice.
What phenotypic ratio is expected?
Why does this differ from the normal 9:3:31 ratio of a heterozygous cross?
Key: C = coat has colour, c = coat has no colour (albino).
A = agouti (brown) colour, a = black coat
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Essential Biology 10.2: Dihybrid Crosses & Gene Linkage (HL)
6.
Define linkage group.
7.
Distinguish between sex linkage and gene linkage.
8.
What is the consequence of gene linkage in terms of the movement of alleles in anaphase I?
9.
Give two plant examples of linked genes.
10.
The notation shows genes of Zea mays (corn). It is described as
“heterozygous at both loci”.
Draw the other possible combinations of these linked genes. a.
Homozygous at both loci b.
Homozygous at locus 1. c.
(write the description)
11.
Complete a punnet grid to show the possible phenotypes produced by a cross between the corn that is heterozygous at both loci.
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Essential Biology 10.2: Dihybrid Crosses & Gene Linkage (HL)
12.
Which combinations of alleles would be ‘impossible’ in the cross above? Why?
13.
What can we assume has happened if a small number of ‘impossible’ phenotypes have appeared in the cross?
14.
At which point of meiosis does recombination due to crossing over occur?
15.
16.
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Essential Biology 10.2: Dihybrid Crosses & Gene Linkage (HL)
17.
A cross is carried out between two organisms with genotypes GgHh and gghh. Which offspring below are recombinants? The genes G and H are linked.
D. gghh, Ggh A.
GgHh, gghh B. Gghh, ggHh C. GGhh, ggHH
Can you explain why the other three options were not correct?
18.
In sweet peas (Lathyrus odoratus), pollen grain shape and flower colour are linked genes. P=purple, p = white L=long pollen, l = short pollen.
The genes are far enough apart that crossing-over occurs occasionally. a.
Which statement is true of the gametes?
A.
They will be all Pl and pL
B.
They will be 25% each of Pl, pL, PL and pl
C.
There will be more pL gametes than PL gametes.
D.
There will be an equal number of PL and pL gametes. b.
Complete a punnet grid in the space below to show the possible genotypes and phenotypes of a test cross with the individual above. Assume no crossing over.
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