V' Sex Determination and Sex Linkage

1
V. Sex Determination and Sex Linkage -
Overview A. Some Questions About Sex B.
Sex Determination 1. Environmental 2.
Developmental 3. Chromosomal Sex correlates
with a particular complement of chromosomes
suggesting that the genes that govern sexual
development are all on this chromosome.
2
V. Sex Determination and Sex Linkage -
Overview A. Some Questions About Sex B.
Sex Determination 1. Environmental 2.
Developmental 3. Chromosomal Sex correlates
with a particular complement of chromosomes
suggesting that the genes that govern sexual
development are all on this chromosome. NOTE
that this is NOT equivalent to genetic sex
determination. In all cases presented above, sex
determination is influenced by many genes just
that in some organisms the action of those genes
is affected by temperature, or proteins/chemicals
produced elsewhere in the organism, or the genes
are not all concentrated on one chromosome.
3
3. Chromosomal You are familiar with the X
Y system, but there are several a.
Protenor sex determination Sexes differ in
chromosome number
Order Hemiptera True Bugs Family Alydidae
Broad-headed bugs
4
3. Chromosomal You are familiar with the X
Y system, but there are several a.
Protenor sex determination Sexes differ in
chromosome number b. Lygaeus sex
determination Sexes have different types of sex
chromosomes heterogametic and homogametic
sexes (Fowl femaleZW, MaleZZ)
Order Hemiptera Family Lygaeidae
Chinch/Seed Bugs
5
3. Chromosomal You are familiar with the X
Y system, but there are several a.
Protenor sex determination Sexes differ in
chromosome number b. Lygaeus sex
determination Sexes have different types of sex
chromosomes heterogametic and homogametic
sexes (Fowl femaleZW, MaleZZ) - Sex
Determination in Humans
6
- Sex Determination in Humans Lygaeus sex
determination, whereby the presence of the Y
determines maleness.
PAR homologous to X binds and recombines
critical for homolog recognition and tetrad
formation
transcribed
Not transcribed
7
- Sex Determination in Humans Lygaeus sex
determination, whereby the presence of the Y
determines maleness.
MSY 23 million bp (Mb) 3 regions
transcribed
Not transcribed
8
- Sex Determination in Humans Lygaeus sex
determination, whereby the presence of the Y
determines maleness.
MSY 23 million bp (Mb) 3 regions - 15 -
x-transposed region 99 identical to modern X
chromosome probably added by transposition 3-4
million years ago.
transcribed
Not transcribed
9
- Sex Determination in Humans Lygaeus sex
determination, whereby the presence of the Y
determines maleness.
MSY 23 million bp (Mb) 3 regions - 15 -
X-transposed region 99 identical to modern X
chromosome probably added by transposition 3-4
million years ago. 2 active genes (homologous w/
X). - 20 - X-degenerative region still
similar in sequence to X, but older with more
accumulated changes. Contains pseudogenes
similar to active genes on X but rendered
non-functional by mutations. 14 active genes
(transcribed) one is the SRY (sex-determining
region of the Y) only expressed in the testis
(present in all mammalsolder).
transcribed
Not transcribed
10
- Sex Determination in Humans Lygaeus sex
determination, whereby the presence of the Y
determines maleness.
MSY 23 million bp (Mb) 3 regions - 15 -
X-transposed region 99 identical to modern X
chromosome probably added by transposition 3-4
million years ago. 2 active genes (homologous w/
X). - 20 - X-degenerative region still
similar in sequence to X, but older with more
accumulated changes. Contains pseudogenes
similar to active genes on X but rendered
non-functional by mutations. 14 active genes
(transcribed) one is the SRY (sex-determining
region of the Y) only expressed in the testis
(present in all mammalsolder). - 30 -
ampliconic region unique sequences (not
homologous to X). 60 genes in 9 families 98
similarity within families produced by gene
duplication. All active in testes development.
transcribed
Not transcribed
11
- Sex Determination in Humans Lygaeus sex
determination, whereby the presence of the Y
determines maleness.
MSY 23 million bp (Mb) 3 regions - 15 -
X-transposed region 99 identical to modern X
chromosome probably added by transposition 3-4
million years ago. 2 active genes (homologous w/
X). - 20 - X-degenerative region still
similar in sequence to X, but older with more
accumulated changes. Contains pseudogenes
similar to active genes on X but rendered
non-functional by mutations. 14 active genes
(transcribed) one is the SRY (sex-determining
region of the Y) only expressed in the testis
(present in all mammalsolder). - 30 -
ampliconic region unique sequences (not
homologous to X). 60 genes in 9 families 98
similarity within families produced by gene
duplication. All active in testes development.
- 45 - heterochromatin
transcribed
Not transcribed
12
- Sex Determination in Humans Lygaeus sex
determination, whereby the presence of the Y
determines maleness.
SRY codes for a product called the testis
determining factor triggers undifferentiated
gonad to become testis.
13
- Sex Determination in Humans Lygaeus sex
determination, whereby the presence of the Y
determines maleness. Evidence Some XY
individuals lack the SRY region, or have a
mutation in it, and they are phenotypically
female. Some XX individuals have an sry that has
been transposed, and they are phenotypically
male. Experimental insertion of sry-homologs in
mice stimulates XX embryos to become male.
SRY codes for a product called the testis
determining factor triggers undifferentiated
gonad to become testis.
14
3. Chromosomal You are familiar with the X
Y system, but there are several a.
Protenor sex determination Sexes differ in
chromosome number b. Lygaeus sex
determination Sexes have different types of sex
chromosomes heterogametic and homogametic
sexes (Fowl femaleZW, MaleZZ) c.
Balanced sex determination (Drosophila) The
ratio of autosomal sets to X chromosomes
determines the sex
15
3. Chromosomal You are familiar with the X
Y system, but there are several a.
Protenor sex determination Sexes differ in
chromosome number b. Lygaeus sex
determination Sexes have different types of sex
chromosomes heterogametic and homogametic
sexes (Fowl femaleZW, MaleZZ) c.
Balanced sex determination (Drosophila) The
ratio of autosomal sets to X chromosomes
determines the sex
16
3. Chromosomal You are familiar with the X
Y system, but there are several a.
Protenor sex determination Sexes differ in
chromosome number b. Lygaeus sex
determination Sexes have different types of sex
chromosomes heterogametic and homogametic
sexes (Fowl femaleZW, MaleZZ) c.
Balanced sex determination (Drosophila) The
ratio of autosomal sets to X chromosomes
determines the sex
Governed by several genes on autosomes that are
activated differently, and their transcripts are
spliced differently, depending on the ratio of
X/autosomal setssuggesting there is another
x-linked gene that might work in a dosage
dependent way.
17
V. Sex Determination and Sex Linkage -
Overview A. Some Questions About Sex B.
Sex Determination C. Sex Linkage
18
V. Sex Determination and Sex Linkage -
Overview A. Some Questions About Sex B.
Sex Determination C. Sex Linkage Where sex
is determined chromosomally, there are obviously
going to be correlations between sex (a
phenotypic trait determined by those sex
chromosomes) and OTHER TRAITS governed by OTHER
GENES on those SEX CHROMOSOMES. So SEX LINKAGE
is an example of a broader phenomenon of
linkage patterns of correlated inheritance
between traits governed by genes on the same
chromosome. In this case, the correlation is
between sex and other traits governed by the sex
chromosomes.
19
C. Sex Linkage Sex linkage was first described
by Thomas Hunt Morgan at Columbia in 1920s.
Found a novel white-eyed male in his culture.
Mated it with a red-eyed female, and all flies
were red eyed, as expected. Then did the f1 x F1
cross, and got a 31 ratio, as expected.
However, all white eyed flies were MALE.
20
C. Sex Linkage Sex linkage was first described
by Thomas Hunt Morgan at Columbia in 1920s.
Found a novel white-eyed male in his culture.
Mated it with a red-eyed female, and all flies
were red eyed, as expected. Then did the f1 x F1
cross, and got a 31 ratio, as expected.
However, all white eyed flies were MALE. By
crossing the F1 females (heterozygotes) with
white males, he produced some white females that
he could use in a reciprocal cross which
revealed a different pattern, dependent on the
sex of the white eyed fly.
21
C. Sex Linkage Sex linkage was first described
by Thomas Hunt Morgan at Columbia in 1920s.
22
C. Sex Linkage Human x-linked traits are
hemophilia and red-green colorblindness, among
others. Genes on the y are also sex-linked
(holandric), but those with phenotypic effects
other than maleness are rare.
23
V. Sex Determination and Sex Linkage -
Overview A. Some Questions About Sex B.
Sex Determination C. Sex Linkage D.
Dosage Compensation
24
D. Dosage Compensation With chromosomal sex
determination, the sexes have a different
complement of sex chromosomes. Consider X-Y
Lygaeus determination males have 1 X and
females have 2.
25
D. Dosage Compensation With chromosomal sex
determination, the sexes have a different
complement of sex chromosomes. Consider X-Y
Lygaeus determination males have 1 X and
females have 2. One might expect that females
would produce TWICE the amount protein products
from x-linked genes.
26
D. Dosage Compensation With chromosomal sex
determination, the sexes have a different
complement of sex chromosomes. Consider X-Y
Lygaeus determination males have 1 X and
females have 2. One might expect that females
would produce TWICE the amount protein products
from x-linked genes. But for many proteins
(enzymes), correct concentration (dosage) is
critical to function. So, we see different
methods whereby this initial difference in dosage
is corrected or compensated for.
27
D. Dosage Compensation With chromosomal sex
determination, the sexes have a different
complement of sex chromosomes. Consider X-Y
Lygaeus determination males have 1 X and
females have 2. One might expect that females
would produce TWICE the amount protein products
from x-linked genes. But for many proteins
(enzymes), correct concentration (dosage) is
critical to function. So, we see different
methods whereby this initial difference in dosage
is corrected or compensated for. 1. In
Drosophila the X in males is hypertranscribed
transcribed at twice the rate as the Xs in
females to equalize protein concentrations. FYI
dosage compensation in Drosophila
28
D. Dosage Compensation 1. In Drosophila the X
in males is hypertranscribed transcribed at
twice the rate as the Xs in females to equalize
protein concentrations. 2. In mammals all but
one X in each cell is turned off in females.
So, in normal females, one X is active and one is
inactivated. In 47, XXX individuals, 2 Xs are
off in each cell. In 47, XXY males, one X is
turned off.
29
D. Dosage Compensation 1. In Drosophila the X
in males is hypertranscribed transcribed at
twice the rate as the Xs in females to equalize
protein concentrations. 2. In mammals all but
one X in each cell is turned off in females.So,
in normal females, one X is active and one is
inactivated. In 47, XXX individuals, 2 Xs are
off in each cell. In 47, XXY males, one X is
turned off. How? - each X has a gene
the Xic (X-inactivation center).
30
D. Dosage Compensation 1. In Drosophila the X
in males is hypertranscribed transcribed at
twice the rate as the Xs in females to equalize
protein concentrations. 2. In mammals all but
one X in each cell is turned off in females.So,
in normal females, one X is active and one is
inactivated. In 47, XXX individuals, 2 Xs are
off in each cell. In 47, XXY males, one X is
turned off. How? - each X has a gene
the Xic (X-inactivation center). - this is
on in inactivated Xs it produces an RNA that
binds with the X chromosomes, making it
inaccessible to transcription enzymes.
31
D. Dosage Compensation 1. In Drosophila the X
in males is hypertranscribed transcribed at
twice the rate as the Xs in females to equalize
protein concentrations. 2. In mammals all but
one X in each cell is turned off in females.So,
in normal females, one X is active and one is
inactivated. In 47, XXX individuals, 2 Xs are
off in each cell. In 47, XXY males, one X is
turned off. How? - each X has a gene
the Xic (X-inactivation center). - this is
on in inactivated Xs it produces an RNA that
binds with the X chromosomes, making it
inaccessible to transcription enzymes. -
this RNA is NOT translated it is functional as
an RNA molecule.
32
D. Dosage Compensation 1. In Drosophila the X
in males is hypertranscribed transcribed at
twice the rate as the Xs in females to equalize
protein concentrations. 2. In mammals all but
one X in each cell is turned off in females.So,
in normal females, one X is active and one is
inactivated. In 47, XXX individuals, 2 Xs are
off in each cell. In 47, XXY males, one X is
turned off. How? - each X has a gene
the Xic (X-inactivation center). - this is
on in inactivated Xs it produces an RNA that
binds with the X chromosomes, making it
inaccessible to transcription enzymes. -
this RNA is NOT translated it is functional as
an RNA molecule. - of course, this just
pushes the question one step upstream what
determines why Xic is only active in one X
chromosome?
33
D. Dosage Compensation 1. In Drosophila the X
in males is hypertranscribed transcribed at
twice the rate as the Xs in females to equalize
protein concentrations. 2. In mammals all but
one X in each cell is turned off in females.So,
in normal females, one X is active and one is
inactivated. In 47, XXX individuals, 2 Xs are
off in each cell. In 47, XXY males, one X is
turned off. How? When?
34
D. Dosage Compensation 1. In Drosophila the X
in males is hypertranscribed transcribed at
twice the rate as the Xs in females to equalize
protein concentrations. 2. In mammals all but
one X in each cell is turned off in females.So,
in normal females, one X is active and one is
inactivated. In 47, XXX individuals, 2 Xs are
off in each cell. In 47, XXY males, one X is
turned off. How? When? - It seems to
be an imprinted phenomenon, so that daughter
cells have the same X inactivated. However, this
seems to happen at different points in
development for different tissues.
35
D. Dosage Compensation 1. In Drosophila the X
in males is hypertranscribed transcribed at
twice the rate as the Xs in females to equalize
protein concentrations. 2. In mammals all but
one X in each cell is turned off in females.So,
in normal females, one X is active and one is
inactivated. In 47, XXX individuals, 2 Xs are
off in each cell. In 47, XXY males, one X is
turned off. How? When?
Effect? - The inactive X is seen as a
condensed mass on the periphery of the nucleus
a Barr Body
36
D. Dosage Compensation 1. In Drosophila the X
in males is hypertranscribed transcribed at
twice the rate as the Xs in females to equalize
protein concentrations. 2. In mammals all but
one X in each cell is turned off in females.So,
in normal females, one X is active and one is
inactivated. In 47, XXX individuals, 2 Xs are
off in each cell. In 47, XXY males, one X is
turned off. How? When?
Effect? - The inactive X is seen as a
condensed mass on the periphery of the nucleus
a Barr Body - Heterozygous females can be a
mosaic exhibiting one phenotype in some
cells/tissues/body regions (governed by 1 X) and
another phenotype in another region (governed by
the other X).
Calico and Tortoiseshell female cats.
37
VI. Linkage - Overview Linkage is a
pattern of correlated inheritance between traits
governed by genes on the same chromosome.
38
VI. Linkage - Overview Linkage is a
pattern of correlated inheritance between traits
governed by genes on the same chromosome.
Because the genes are part of the same physical
entity, they are inherited together rather than
independently.
39
VI. Linkage - Overview Linkage is a
pattern of correlated inheritance between traits
governed by genes on the same chromosome.
Because the genes are part of the same physical
entity, they are inherited together rather than
independently. Only crossing-over
(recombination) can separate them so they can be
inherited separately.
40
VI. Linkage A. Complete Linkage - if
genes are immediate neighbors, they are almost
never separated by crossing over and are always
inherited together. The pattern mimics that of a
single gene.
AABB
aabb
AB
ab
X
ab
AB
41
VI. Linkage A. Complete Linkage - if
genes are immediate neighbors, they are almost
never separated by crossing over and are always
inherited together. The pattern mimics that of a
single gene.
AABB
aabb
AB
ab
X
ab
AB
Gametes
F1
42
VI. Linkage A. Complete Linkage - if
genes are immediate neighbors, they are almost
never separated by crossing over and are always
inherited together. The pattern mimics that of a
single gene.
F1 x F1
X
Gametes
43
VI. Linkage A. Complete Linkage - if
genes are immediate neighbors, they are almost
never separated by crossing over and are always
inherited together. The pattern mimics that of a
single gene.
31 ratio Aa 31 ratio Bb 31 ratio ABab
F1 x F1
X
Gametes
AABB
AaBb
AaBb
aabb
44
VI. Linkage A. Complete Linkage B.
Incomplete Linkage
45
VI. Linkage A. Complete Linkage B.
Incomplete Linkage - Likelihood of a
cross-over event increases as the distance
between genes increases
46
VI. Linkage A. Complete Linkage B.
Incomplete Linkage - Likelihood of a
cross-over event increases as the distance
between genes increases - So, the frequency of
crossing over can be used as an index of distance
between genes! (Thus, genes can be mapped
through crosses)
47
VI. Linkage A. Complete Linkage B.
Incomplete Linkage - Likelihood of a
cross-over event increases as the distance
between genes increases - So, the frequency of
crossing over can be used as an index of distance
between genes! (Thus, genes can be mapped
through crosses) - How can we measure the
frequency of recombinant (cross-over) gametes?
Is there a type of cross where we can see the
freuqncy of different types of gametes produced
by the heterozygote as they are expressed as the
phenotypes of the offspring?
48
VI. Linkage A. Complete Linkage B.
Incomplete Linkage - Likelihood of a
cross-over event increases as the distance
between genes increases - So, the frequency of
crossing over can be used as an index of distance
between genes! (Thus, genes can be mapped
through crosses) - How can we measure the
frequency of recombinant (cross-over) gametes?
Is there a type of cross where we can see the
freuqncy of different types of gametes produced
by the heterozygote as they are expressed as the
phenotypes of the offspring? Yes.. Test
cross.
49
VI. Linkage A. Complete Linkage B.
Incomplete Linkage - Likelihood of a
cross-over event increases as the distance
between genes increases - So, the frequency of
crossing over can be used as an index of distance
between genes! (Thus, genes can be mapped
through crosses) - How can we measure the
frequency of recombinant (cross-over) gametes?
Is there a type of cross where we can see the
freuqncy of different types of gametes produced
by the heterozygote as they are expressed as the
phenotypes of the offspring? Yes.. Test
cross.
50
VI. Linkage A. Complete Linkage B.
Incomplete Linkage
b
a
A
b
51
VI. Linkage A. Complete Linkage B.
Incomplete Linkage - So, since crossing-over
is rare (in a particular region), most of the
time it WONT occur and the homologous
chromosomes will be passed to gametes with these
genes in their original combinationthese gametes
are the parental types and they should be the
most common types of gametes produced.
b
a
b
A
52
VI. Linkage A. Complete Linkage B.
Incomplete Linkage - Sometimes, crossing
over WILL occur between these loci creating new
combinations of genes This produces the
recombinant types
b
a
b
A
53
VI. Linkage A. Complete Linkage B.
Incomplete Linkage As the other parent only
contributed recessive alleles, the phenotype of
the offspring is determined by the gamete
received from the heterozygote
b
a
b
A
54
VI. Linkage B. Incomplete Linkage - How
can you tell if genes are assorting
independently, or are linked?
55
VI. Linkage B. Incomplete Linkage - How
can you tell if genes are assorting
independently, or are linked? - Do a test
cross.
AaBb x aabb
56
VI. Linkage B. Incomplete Linkage - How
can you tell if genes are assorting
independently, or are linked? - Do a test
cross. - Compare the results with what you
would expect if the genes assorted
independently
AaBb x aabb
57
VI. Linkage B. Incomplete Linkage - How
can you tell if genes are assorting
independently, or are linked? - Do a test
cross. - Compare the results with what you
would expect if the genes assorted
independently
AaBb x aabb
The frequency of AB should f(A) x f(B) x N
55/100 x 51/100 x 100 28
58
VI. Linkage B. Incomplete Linkage - How
can you tell if genes are assorting
independently, or are linked? - Do a test
cross. - Compare the results with what you
would expect if the genes assorted
independently
AaBb x aabb
The frequency of AB should f(A) x f(B) x N
55/100 x 51/100 x 100 28 The frequency of
Ab should f(A) x f(b) x N 55/100 x 49/100
x 100 27
59
VI. Linkage B. Incomplete Linkage - How
can you tell if genes are assorting
independently, or are linked? - Do a test
cross. - Compare the results with what you
would expect if the genes assorted
independently
AaBb x aabb
The frequency of AB should f(A) x f(B) x N
55/100 x 51/100 x 100 28 The frequency of
Ab should f(A) x f(B) x N 55/100 x 49/100
x 100 27 The frequency of aB should f(a) x
f(B) x N 45/100 x 51/100 x 100 23
60
VI. Linkage B. Incomplete Linkage - How
can you tell if genes are assorting
independently, or are linked? - Do a test
cross. - Compare the results with what you
would expect if the genes assorted
independently
AaBb x aabb
The frequency of AB should f(A) x f(B) x N
55/100 x 51/100 x 100 28 The frequency of
Ab should f(A) x f(B) x N 55/100 x 49/100
x 100 27 The frequency of aB should f(a) x
f(B) x N 45/100 x 51/100 x 100 23 The
frequency of ab should f(a) x f(b) x N
45/100 x 49/100 x 100 22
61
VI. Linkage B. Incomplete Linkage - How
can you tell if genes are assorting
independently, or are linked? - Do a test
cross. - Compare the results with what you
would expect if the genes assorted
independently This is fairly easy to do by
creating a contingency table
AaBb x aabb
62
VI. Linkage B. Incomplete Linkage This is
fairly easy to do by creating a contingency
table Add across and down This gives the
totals for each trait independently.
AaBb x aabb
63
VI. Linkage B. Incomplete Linkage This is
fairly easy to do by creating a contingency
table Add across and down This gives the
totals for each trait independently. Then, to
calculate an expected value based on independent
assortment (for AB, for example), you multiple
Row Total x Column Total and divide by Grand
Total. 55 x 51 / 100 28
AaBb x aabb
64
VI. Linkage B. Incomplete Linkage Repeat
to calculate the other expected values (This is
just an easy way to set it up and do the
calculations, but you should appreciate it is the
same as F(A) x f(B) x N)
AaBb x aabb
65
VI. Linkage B. Incomplete Linkage OK so?
Well, now we compare our observed results with
what we would expect if the genes assort
independently.
AaBb x aabb
66
VI. Linkage B. Incomplete Linkage OK so?
Well, now we compare our observed results with
what we would expect if the genes assort
independently. If our results are close to the
expectations, then they support the hypothesis of
independence.
AaBb x aabb
67
VI. Linkage B. Incomplete Linkage OK so?
Well, now we compare our observed results with
what we would expect if the genes assort
independently. If our results are close to the
expectations, then they support the hypothesis of
independence. If they are far apart from the
expected results, then they refute that
hypothesis and support the alternative
linkage.
AaBb x aabb
68
VI. Linkage B. Incomplete
Linkage Typically, we reject the hypothesis of
independent assortment (and accept the hypothesis
of linkage) if are results are so different from
expectations that independently assorting genes
would only produce results as unusual as ours
less than 5 of the time
AaBb x aabb
69
VI. Linkage B. Incomplete
Linkage Typically, we reject the hypothesis of
independent assortment (and accept the hypothesis
of linkage) if are results are so different from
expectations that independently assorting genes
would only produce results as unusual as ours
less than 5 of the time We determine this
probability with a Chi-Square Test of
Independence.
AaBb x aabb
70
VI. Linkage B. Incomplete Linkage We
determine this probability with a Chi-Square Test
of Independence.
71
VI. Linkage B. Incomplete Linkage Our X2
36.38 First, we determine the degrees of
freedom (r-1)(c-1) 1
72
VI. Linkage B. Incomplete Linkage Our X2
36.38 First, we determine the degrees of
freedom (r-1)(c-1) 1 Now, we read across
the first row in the table, corresponding to df
1. The column headings are the probability that
a number in that column would occur at a given
df.
73
VI. Linkage B. Incomplete Linkage Our X2
36.38 Note that larger values have a lower
probability of occurring by chance This should
make sense, and the value increases as the
difference between observed and expected values
increases.
74
VI. Linkage B. Incomplete Linkage Our X2
36.38 So, for instance, a value of 2.71 will
occur by chance 10 of the time.
75
VI. Linkage B. Incomplete Linkage Our X2
36.38 So, for instance, a value of 2.71 will
occur by chance 10 of the time. But a value of
6.63 will only occur 5 of the time... (if the
hypothesis is true and this deviation between
observed and expected values is only due to
chance).
76
VI. Linkage B. Incomplete Linkage Our X2
36.38 So, for instance, a value of 2.71 will
occur by chance 10 of the time. For us, we are
interested in the 5 level. The table value is
6.63. Our calculated value is much greater than
this so the chance that independently assorting
genes would yield our results is WAY LESS THAN
5. Our results are REALLY UNUSUAL for
independently assorting genes.
77
VI. Linkage B. Incomplete Linkage Our X2
36.38 Our results are REALLY UNUSUAL for
independently assorting genes. So, either our
results are wrong, or the hypothesis of
independent assortment is wrong. If you did a
good experiment, then you should have confidence
in your results reject the hypothesis of IA and
conclude the alternative the genes are
LINKED.
78
VI. Linkage B. Incomplete Linkage OK so
we conclude the genes are linked NOW WHAT?
AaBb x aabb
79
VI. Linkage B. Incomplete Linkage OK so
we conclude the genes are linked NOW WHAT? We
map the genes using the knowledge that
crossing-over is rare, and the frequency of
crossing-over correlates with the distance
between genes.
AaBb x aabb
80
VI. Linkage B. Incomplete Linkage OK so
we conclude the genes are linked NOW WHAT? 1.
Crossing-over is rare so the RARE COMBINATIONS
must be the products of Crossing-Over. The
OTHERS, the MOST COMMON products, represent the
PARENTAL TYPES.
AaBb x aabb
81

• VI. Linkage
• B. Incomplete Linkage
• OK so we conclude the genes are linked NOW
  WHAT?
• Crossing-over is rare so the RARE COMBINATIONS
  must be the products of Crossing-Over. The
  OTHERS, the MOST COMMON products, represent the
  PARENTAL TYPES.
• This tells us the original arrangement of
  alleles in the heterozygous parent

AaBb x aabb
82

• VI. Linkage
• B. Incomplete Linkage
• OK so we conclude the genes are linked NOW
  WHAT?
• Crossing-over is rare so the RARE COMBINATIONS
  must be the products of Crossing-Over. The
  OTHERS, the MOST COMMON products, represent the
  PARENTAL TYPES.
• This tells us the original arrangement of
  alleles in the heterozygous parent
• Segregation without crossing over produces lots
  of AB and ab gametes.

AaBb x aabb
83

• VI. Linkage
• B. Incomplete Linkage
• OK so we conclude the genes are linked NOW
  WHAT?
• Crossing-over is rare so the RARE COMBINATIONS
  must be the products of Crossing-Over. The
  OTHERS, the MOST COMMON products, represent the
  PARENTAL TYPES.
• 2. The frequency of crossing-over is used as an
  index of the distance between genes
• The other progeny are the products of crossing
  over, and they occurred 20 times in 100 progeny,
  for a frequency of 0.2. Multiply that by 100 to
  free the decimal, and this becomes 20 map units
  (CentiMorgans).

AaBb x aabb
20 map units
84
VI. Linkage B. Incomplete Linkage So, we
used the chi-square test to identify pairs of
genes on the same chromosome. Then, we mapped
the distance between genes on the chromosome all
by looking at the frequency of phenotypes in the
offspring of a test cross.
AaBb x aabb
20 map units
85
VI. Linkage B. Incomplete Linkage C.
Three-Point Mapping We will do this in lab
.