Chapter 4
overview
Fig. 4-1
Genetic recombination: mixing of genes during
gametogenesis that produces gametes with
combinations of genes that are different from
the combinations received from parents.
• Independent assortment of homologous
chromosomes (Anaphase I). Genes on nonhomologous chromosomes (unlinked genes)
assort independently.
Fig. 4-6
Using a testcross
to distinguish gamete genotypes
Fig. 4-7
50% = independent
assortment
(genes are not linked)
Fig. 4-8
Genetic recombination: mixing of genes during
gametogenesis that produces gametes with
combinations of genes that are different from
the combinations received from parents.
• Independent assortment of homologous
chromosomes (Anaphase I). Genes on nonhomologous chromosomes (unlinked genes)
assort independently.
• Crossing over (recombination among linked
genes)
cis linked: both dominant alleles on the same homolog
trans linked: dominant alleles on different homologs
Fig. 4-2
Fig. 4-3
Crossing over
• Physical exchanges among non-sister chromatids;
visualized cytologically as chiasmata
• Typically, several crossing over events occur within
each tetrad in each meiosis (chiasmata physically
hold homologous chromosome together and assure
proper segregation at Anaphase I)
p. 115
Crossing over occurs at the four-strand stage
(pre-meiotic G2 or very early prophase I)
Fig. 4-4
Crossing over can involve 2, 3, or 4 chromatids in a single meiosis
Fig. 4-5
Crossing over
• Physical exchanges among non-sister chromatids;
visualized cytologically as chiasmata
• Typically, several crossing over events occur within
each tetrad in each meiosis (chiasmata physically
hold homologous chromosome together and assure
proper segregation at Anaphase I)
• The sites at which crossing over occur are random
• The likelihood that a crossover occurs between any
two particular sites (genes) is a function of the
physical distance between those two sites
Crossing over usually affects a minority of chromatids in a collection
of meioses – recombinants are typically a minority of products
Fig. 4-9
<50% = linked genes
Fig. 4-10
A.H. Sturtevant (1911-3): frequency of crossing over
between two genes is a function of their distance
apart on the chromosome; created the first genetic map
number of recombinants
Recombination frequency =
total number of progeny
One map unit = one centimorgan = 1% recombinants
Rationales:
• Crossover events are random
• Greater separation, greater likelihood that crossover will occur
• Map distance should be sum of smaller intervals
Fig. 4-11
• Construct entire chromosome maps by mapping intervals
• Linear map correlates with linear chromosome
Markers used in trihybrid testcross
in Drosophila
v = vermilion eyes (red eyes; v+ are red-brown)
cv = crossveinless (cv+ wings have crossveins)
ct = cut wing (ct+ wings have regular margins)
Data from three-point testcross
v+/ v cv+/ cv ct+/ ct X v / v cv / cv ct / ct
(trihybrid)
(tester)
Progeny phenotypes
v
v+
v
v+
v
v+
v
v+
cv+
cv
cv
cv+
cv
cv+
cv+
cv
ct+
ct
ct+
ct
ct
ct+
ct
ct+
580
592
45
40
89
94
3
5
1448
Steps in solving three-point testcross problem
1. Anticipate and identify eight types of products (23)
2. Identify pairs of reciprocal products
Data from three-point testcross
v+/ v cv+/ cv ct+/ ct X v / v cv / cv ct / ct
(trihybrid)
(tester)
Progeny phenotypes
v
v+
v
v+
v
v+
v
v+
cv+
cv
cv
cv+
cv
cv+
cv+
cv
ct+
ct
ct+
ct
ct
ct+
ct
ct+
580
592
45
40
89
94
3
5
1448
Steps in solving three-point testcross problem
1. Anticipate and identify eight types of products (23)
2. Identify pairs of reciprocal products
3. Identify parental types as the most frequent pair of
products
4. Identify double crossover products as least frequent
pair of products
Data from three-point testcross
v+/ v cv+/ cv ct+/ ct X v / v cv / cv ct / ct
(trihybrid)
(tester)
Progeny phenotypes
v
v+
v
v+
v
v+
v
v+
cv+
cv
cv
cv+
cv
cv+
cv+
cv
ct+
ct
ct+
ct
ct
ct+
ct
ct+
580
592
45
40
89
94
3
5
1448
Parental types - nco
sco
sco
dco
Steps in solving three-point testcross problem
1. Anticipate and identify eight types of products (23)
2. Identify pairs of reciprocal products
3. Identify parental types as the most frequent pair of
products
4. Identify double crossover products as least frequent
pair of products
5. Compare the parental and double crossover products
to deduce the order of the three gene loci
Fig. 4-12
In dco products, the central marker is displaced
relative to the parental types
Fig. 4-13
Steps in solving three-point testcross problem
1. Anticipate and identify eight types of products (23)
2. Identify pairs of reciprocal products
3. Identify parental types as the most frequent pair of
products
4. Identify double crossover products as least frequent
pair of products
5. Compare the parental and double crossover products
to deduce the order of the three gene loci
6. Compute map distances by breaking down the
results for each interval
Fig. 4-12
RF =
183 + 8
1448
85 + 8
1448
(0.132)
(0.064)
Fig. 4-12
RF =
183 + 8
1448
85 + 8
1448
(0.132)
(0.064)
13.2 m.u.
v
6.4 m.u.
ct
cv
Interference: crossing over in one region
interferes with simultaneous crossing over in
adjacent regions
Expected frequency of dco = product of frequency crossovers in two regions
0.132 X 0.064 = 0.0084
0.084 X 1448 = 12 expected (if two sco are independent events)
Interference: crossing over in one region
interferes with simultaneous crossing over in
adjacent regions
Expected frequency of dco = product of frequency crossovers in two regions
0.132 X 0.064 = 0.0084
0.084 X 1448 = 12 expected (if two sco are independent events)
Coefficient of coincidence = observed dco / expected dco
8 / 12 = 0.667
Interference = 1 – coefficient of coincidence
1 – 0.667 = 0.333
Fig. 4-14
Tomato karyotype (n=12)
Tomato linkage map
(1952)
Fig. 4-14
Typical phenotypic ratios for a variety of crosses
(complete allele dominance)
p. 136