Download Lecture#17 Page 1 BIOLOGY 207 – Dr McDermid Lecture#17

BIOLOGY 207 – Dr McDermid
Lecture#17 Linkage
Readings: Griffiths et al, 7th Edition: Ch. 5 pp 141-149
Problems: Ch 5: Tier 1: #1, 2, 5, 6, 9, 35, 44
Tier 2: #3, 4, 7, 8, 36, 39, 40, 46 (note: 7b answer is wrong)
Concepts: Gene loci on the same chromosome :
1. Gene loci on the same chromosome may show linkage, not independent assortment.
2. Most linkage between gene loci is not complete because crossing over between loci can
occur during meiosis.
3. The extent of linkage between gene loci is expressed as the frequency of recombinant type
progeny (vs. parental type progeny).
Independent Assortment
- two gene loci, on separate chromosomes
- alleles at each locus segregate independently
But if the two gene loci are on the same chromosome then the alleles at the two different gene
loci should segregate together (for the most part) in meiosis.
This would result in an individual having the tendency for particular allele combinations to be
passed on to its offspring.
Example:
AABB
x
AaBb
Then test cross the F1
aabb
F1
P1
A word about notation
A and B -> two loci on separate chromosomes AaBb is A/a B/b
A and B on one chromosome
AB/ab represents alleles A and B on one homolog and a and b on the other homolog.
Coupling conformation
Repulsion conformation
Test cross: AaBb x aabb
gametes
Genotype
Phenotype
ab
gametes
AB
AaBb
AB
Ab
Aabb
Ab
aB
aaBb
aB
ab
aabb
ab
If A + B were assorting independently,
But if A, B were on the same chromosome,
Gametes:
AB from AaBb ab from aabb Ab from Aabb aB from aaBb There are two types of gametes:
parentals - which are the same organization as in parent chromosomes
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recombinants - a cross over occurs between the A and B loci to give recombinant gametes Ab
and aB.
Note:
1) the two parental combinations should occur equally likely (1:1 ratio)
2) the two recombinant combinations should also occur with equal frequency
1) When the two loci are very close together
Most meiotic products :
Ratios of gametes in test cross:
Gametes
Genotyp
# progeny
AB
AaBb
98
ab
aabb
98
Ab
Aabb
2
aB
aBb
2
# recombinants =
total # progeny =
Recombinant Frequency (RF) =
(# recombinants/# total)
2) When the two loci are
further apart on the same chromosome
AC/AC X ac/ac
AC/ac X ac/ac
Ratios of gametes:
Gametes
Genotyp
AC
AaCc
ac
aacc
Ac
Aacc
aC
aCc
Recombinant Frequency
= # recombinants / total # progeny
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Totals
Parentals
196
Recombinants
4
Total= 200
P1
P2
# progeny
72
68
31
29
Totals
Parentals
140
Recombinants
60
Total= 200
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Summarize:
A - B loci
A --------- C loci
Allows us to position gene loci relative to each other along the chromosome
A <---------------------------> C
A <--->B
Construct a genetic map or linkage map
This was first done by Alfred Sturtevant in 1913
A genetic map is a representation of the physical relationships among linked genes along a
chromosome.
Any locus can be mapped relative to another on a chromosome.
Map units
1% recombination is about 1 map unit.
So A --> B
A --> C
Approximate because the frequency of the recombinants only approximates the frequency of
crossing over.
Example
If two loci are unlinked, then the ratio of
parentals: recombinants is
the maximum recombinants is
Two loci that are >50 map units apart
-A-B---------C--------------D---•-----------E-------------F-Summary
Unlinked: shows independent assortment
Non-syntenic
--•-------A--------!!!!!!!!!!!!!!!!!!!!--•------B------•-------a--------!!!!!!!!!!!!!!!!!!!!--•------b----Or syntenic
-----A---------------------•----------------B---------a---------------------•----------------b---->50 map units
Linked: doesn't show independent assortment
Closely linked:
-------•--------------A-B-----------•--------------a-b-----
e.g. 2%
Partially linked:
-------•---------A------B-----------•---------a------b-----
e.g. 30%
"Synteny"
- linked genes are syntenic
- syntenic genes are not necessarily linked
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Lecture notes: Copyright © 2002 Heather McDermid and the Department of Biological Sciences, University of Alberta
Images are Copyright©2000 by W.H. Freeman & Co. in Griffiths et al, Introduction to Genetic Analysis
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