Download Autosomal linkage

OCR Biology A
20.4 Phenotypic ratios
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Autosomal linkage
Specification reference

6.1.2 (b) (ii)
Learning outcomes
After completing the worksheet you should be able to:


describe the principles of autosomal linkage
identify autosomal linkage based on inheritance patterns and phenotypic ratios.
Introduction
Two genes are said to be linked when they are located on the same chromosome.
This is significant because:



linked allele combinations will be inherited together (as a single unit)
only crossing over during meiosis can separate linked allele combinations
two alleles are less likely to be separated during crossing over when they are
closer together on a chromosome.
Worked example
The diagram shows three gene loci on a fruit fly (Drosophila melanogaster)
chromosome.
The three genes are linked. Females with no bristles and brown eyes (genotype
bbrr) were bred with long bristled, red-eyed male flies. All the male flies were
heterozygous for both genes, with B and R alleles linked on one chromosome and b
and r alleles linked on the other chromosome.
The table below shows the expected and actual phenotypes of the offspring. Note
that without crossing over no long-bristled, brown-eyed flies or bristleless, red-eyed
flies would have been produced.
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20.4 Phenotypic ratios
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OCR Biology A
Offspring
phenotype
Expected number
without linkage
Actual
number
Genotype
Explanation
Long bristles,
red eyes
100
165
BbRr
B and R are linked on the
father’s chromosome; b and r
are linked on the mother’s
chromosome.
Long bristles,
brown eyes
100
35
Bbrr
Crossing over during meiosis
produces new allele
combinations and breaks the
linkage.
No bristles,
red eyes
100
35
bbRr
Crossing over during meiosis
produces new allele
combinations and breaks the
linkage.
No bristles,
brown eyes
100
165
bbrr
The recessive alleles b and r
are linked on both parental
chromosomes.
Females with short wings and brown eyes (genotype llrr) were then bred with longwinged, red-eyed male flies. Again, all the male flies were heterozygous for both
genes, with L and R alleles linked on one chromosome and l and r alleles linked on
the other chromosome.
The table below shows the expected and actual phenotypes of the offspring.
Crossing over again rearranges the linked alleles in some cases. This enables a few
long-winged, brown-eyed flies and short-winged, red-eyed flies to be produced.
However, because the two genes are closer together in this second example, less
crossing over occurs.
Offspring
phenotype
Expected number
without linkage
Actual
number
Genotype
Explanation
Long wings,
red eyes
100
192
LlRr
L and R are linked on the
father’s chromosome; l and r
are linked on the mother’s
chromosome.
Long wings,
brown eyes
100
8
Llrr
Crossing over during meiosis
produces new allele
combinations and breaks the
linkage.
Short wings,
red eyes
100
8
llRr
Crossing over during meiosis
produces new allele
combinations and breaks the
linkage.
Short wings,
brown eyes
100
192
llrr
The recessive alleles l and r
are linked on both parental
chromosomes.
© Oxford University Press 2017 http://www.oxfordsecondary.co.uk/acknowledgements
This resource sheet may have been changed from the original
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20.4 Phenotypic ratios
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OCR Biology A
Questions
1
a
Draw Punnett square diagrams for both genetic crosses in the worked
example to show:
i inheritance if the genes were not linked and were inherited
independently
br
Ir
BR
Br
bR
br
BbRr
Bbrr
bbRr
bbrr
LR
Lr
IR
Ir
LlRr
Llrr
IIRr
IIrr
(8 marks)
ii
inheritance if crossing over did not occur.
(4 marks)
b Females with short wings and no bristles (genotype llbb) were bred with longwinged, long-bristled male flies. All the male flies were heterozygous for both
genes, with L and B alleles linked on one chromosome and l and b alleles
linked on the other chromosome. Four hundred offspring were produced.
i Draw a Punnett square diagram to show the likely offspring phenotypic
ratios if crossing over did not occur.
Ib
LB
Ib
LlBb
IIbb
(2 marks)
ii
Use the information in the worked example to predict the actual number
of offspring of each phenotype. Explain your answer.
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20.4 Phenotypic ratios
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OCR Biology A
(7 marks)
2
Another D. melanogaster chromosome is shown below.
a
Which two genes show the strongest linkage?
ANS: Body colour and eye colour
(1 mark)
b Which two genes show the weakest linkage?
ANS: Body colour and body size
c
(1 mark)
Red-eyed, bar-eyed flies were bred together. All the flies were heterozygous
for both genes. In all cases, the dominant alleles were linked on the same
chromosome and the recessive alleles were linked on the other
chromosome.
i Draw a Punnett square diagram to calculate the probable offspring
phenotypic ratios that will be produced. Assume no crossing over occurs.
WB
wb
WB
WWBB
WwBb
wb
WwBb
wwbb
(3 marks)
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OCR Biology A
ii
20.4 Phenotypic ratios
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How would crossing over affect the offspring phenotypes?
The W and B and w and b will be separated by crossing over which means that red
eyed, round eyed and some white eyed, bar eyed will be produced.
(3 marks)
d A yellow-bodied, white-eyed male was bred with several grey-bodied,
red-eyed females. The following offspring phenotypes were produced:
33 grey-bodied, red-eyed
32 yellow-bodied, white-eyed
1 grey-bodied, white-eyed
2 yellow-bodied, red-eyed
Explain the results of this genetic cross.
ANS:
(6 marks)
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This resource sheet may have been changed from the original
5