Drosophila_X.html: 05_04-Drosophila_X.jpg
A map of the yellow (y), white (w), and
miniature (m) genes on the X chromosome of Drosophila melanogaster.
Each number represents the percentage of recombinant offspring produced in one of three crosses,
each involving two different genes.
1% recombination between two genes on a chromosome is defined as 1 map unit (mu),
or sometimes as 1 centimorgan (cM).
The interloci distances are additive.
video
Neurospora_crossover.html: 05_20-Neurospora_crossover.jpg
Mapping the centromere.
If no crossover event occurs between a gene and the centromere,
the pattern of ascospores within an ascus
is (aaaa++++) due to first-division segregation,
since the two alleles are separated during the first meiotic division. Crossover would produce
recombinant
patterns
due to second-division segregation.
To calculate the distance between the gene and the centromere: d = ((1/2 recombinant asci) / total asci) * 100.
The distance (d in mu) counts half the number of recombinant asci.
since crossing over in each occurs in only two of the four chromatids during meiosis.
Neurospora_crossoverB.html: 05_20-Neurospora_crossoverB.jpg
One possible pattern of ascospores from a crossover event
is (++aa++aa) due to second-division segregation.
Neurospora_crossoverC.html: 05_20-Neurospora_crossoverC.jpg
Another pattern of ascospores from a crossover event
is (++aaaa++). Two other recombinant patterns also occur,
depending on the chromatid orientation during the second meiotic division:
(++aa++aa)
and
(aa++++aa).
Neurospora_tetrads.html: 05_19-Neurospora_tetrads.jpg
The bread mold Neurospora spends most of it life cycle in a multicellular
haploid
stage.
Following fertilization of conidia of opposite mating types, the zygote undergoes meiosis in an ascus, which retains the haploid "tetrad" (do not confuse with tetrad formed in prophase I).
Each cell in the tetrad undergo mitosis to produce eight haploid ascospores.
Because the eight cells reflect the sequence of their formation following meiosis,
the tetrad is "ordered" and can be subjected to ordered tetrad analysis.
Poisson.html: 05_13-Poisson.jpg
The Poisson distribution is a mathematical function that assigns probabilities of
observing random events in a sample.
As the distance between two linked genes increases, a mapping function can help correct for
the underestimate of multiple exchanges observed experimentally.
For example, When about 30% recombinants are detected, the true map distance is 50 mu.
chromosome_X_1.html: 05_24-chromosome_X_1.jpg
Representative regional gene assignments for human chromosome 1 and the X chromosome.
Key:
AMY Amylase (salivary and pancreatic) AT3 Antithrombin (clotting factor IV) CB Color Blindness DMD Duchene Muscular Dystrophy FHM Fumarate Hrdratase (mitochondrial) GDH Glucose Dehydrogenase G6PD Glucose-6-phosphate Dehydrogenase GEMA Hemophilia A (classic) HGPRT Hypoxanthine-Guanine-Phosphoribosyl Transferase PEPC Peptiidase C PGK Phosphoglycerate Kinase PGM Phosphoglucomutase Rh Rhesus Blood Group (erythroblastosis fetalis)
crossesA.html: 05_03-crossesA.jpg
In experiments with Drosophila,
Morgan
(Nobel 1933)
crossed X-linked mutant yellow-bodied (y) and white-eyed (w)
females with wild-type males (gray body and red eyes).
continue
video
crossesA_F2.html: 05_03-crossesA_F2.jpg
A small portion (1.3%) of the F2 flies show recombinant phenotypes,
which express either white eyes (with gray bodies) or
yellow bodies (with red eyes).
crossesB.html: 05_03-crossesB.jpg
In cross B, Morgan crossed white-eye, miniature-wing mutants with
wild-type flies.
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crossesB_F2.html: 05_03-crossesB_F2.jpg
Only 62.8% of all the F2 flies show the parental phenotypes,
a full 37.2% exhibited recombination.
crossover.html: 05_05-crossover.jpg
Single Crossover
parental
gametes are formed, and the exchange is undetected.
double_crossover-gametes.html: 05_09-double_crossover-gametes.jpg
Determining gene sequence in a three-point mapping cross.
First, determine the arrangement of alleles.
Of the 3 possible arrangements of the 3 loci,
arrangement III (with w in the middle) produces the observed
DCO (least frequent) phenotypes: yellow, echinus flies and white flies.
Mapping gene distances.
The map distance between two genes is the sum of all SCO and DCO frequencies between them.
The distance between y and w is the sum of the frequencies
of offsprings (3)/(4) and (7)/(8): 1.50% + 0.06% = 1.56 mu.
The distance between w and ec is the sum of offsprings (5)/(6) and (7)/(8):
4.00% + 0.06% = 4.06 mu.
double_crossover.html: 05_07-double_crossoverA.jpg
A double crossover
of 3 pairs of genes, each heterozygous for two alleles, between two nonsister
chromatids
produces two noncrossover gametes and two double-crossover gametes.
By the product
law, the probability of two crossovers occurring is equal to the product
of the individual probabilities; so a double crossover (DCO) is much rarer than a
single crossover (SCO).
video
double_exchange-four-stranded.html: 05_12c-double_exchange-four-stranded.jpg
A four-strand double exchange yields 100% recombinant chromatids.
double_exchange-three-stranded.html: 05_12b-double_exchange-three-stranded.jpg
A three-strand double exchange yields 50% recombinant chromatids.
double_exchange-two-stranded.html: 05_12a-double_exchange-two-stranded.jpg
A two-strand double exchange yields no recombinant chromatids.
A three-strand
double exchange yields 50% recombinant chromatids.
A four-strand
double exchange yields 100% recombinant chromatids.
Overall, multiple events "even out" and two linked genes yield a theoretical maximum of 50%
recombination.
isogametes.html: 05_18-isogametes.jpg
The single-celled green alga Chlamydomonas forms haploid
colonies which can become isogametes
of opposite mating types, which then fuse to produce a diploid zygote.
linkageA.html: 05_01-linkageA.jpg
Independent assortment of two pairs of chromosomes,
each containing a heterozygous gene pair, shows no linkage patterns:
4 genetically different gametes are formed in equal proportions,
each containing a different combination of alleles of the two genes.
Complete Linkage
linkageB.html: 05_01-linkageB.jpg
If no crossing over occurs between the two genes,
only two genetically different gametes are formed.
This complete linkage produces only parental or noncrossover gametes in equal proportions.
Recombination
linkageC.html: 05_01-linkageC.jpg
Crossing over between two linked genes
generates two new allele combinations, called recombinant or crossover gametes.
The two chromatids not involved in the exchange result in crossover gametes, like those in
complete linkage.
linked.html: 05_02-linked.jpg
Complete linkage produces only parental genotypes and phenotypes.
In the F1 generation, all flies are heterozygous for both gene pairs and exhibit the dominant traits of
red eyes and thin veins.
linkedA.html: 05_02-linkedA.jpg
Each F1 individual forms only parental gametes.
Assuming complete linkage,
the F2 generation will be produced in a 1:2:1 phenotypic and genotypic ratio:
1 brown:2 wild:1 heavy.
linkedB.html: 05_02-linkedB.jpg
A testcross of F1 flies produces a 1:1 ratio of
brown/thin and red/heavy F2, with no recombinants.
Had the genes controlling these traits been incompletely linked
or located on separate autosomes, the testcross would have produced four phenotypes,
rather than two.
maize_recombination.html: 05_15-maize_recombination.jpg
Crossing over between the
colorless (c) / colored (C) and
starchy (Wx) / waxy (wx) endosperm
traits in maize
can be correlated with cytological markers:
a knob at one end of the chromosome and a translocation at the other end.
For example, the colorless, waxy recombinant chromosome (case I) has lost its knob,
providing visual evidence that physical exchange of genetic material had occurred.
mitotic_recombination-twin.html: 05_16-mitotic_recombination-twin.jpg
A SCO between the loci for yellow body (y)
and the centromere
can produce twin spot ((yellow and singed spots)) patches of tissue
where the recombinant cells are homozygous for either the
y or the sn allele.
mitotic_recombination-yellow.html: 05_16-mitotic_recombination-yellow.jpg
A SCO between the loci for yellow body (y)
and singed bristles (sn)
can produce yellow patches of tissue
where the recombinant cells are homozygous for the
recessive y allele.
mitotic_recombination.html: 05_16-mitotic_recombination.jpg
Female flies heterozygous for the X-linked recessive mutations yellow body (y)
and singed bristles (sn) show the wild type phenotype in most tissue cells
(gray body and gray, straight bristles).
Mitotic recombination
can produce some recombinant tissue such as
yellow spot
or
twin spot
patches.
sister_chromatid_exchange.html: 05_17-sister_chromatid_exchange.jpg
Sister chromatids can engage in genetic exchanges (SCEs) during mitosis,
forming harlequin chromosomes.
This does not produce new allelic combinations, though its study may be useful for understanding recombination mechanisms.
synteny_testing.html: 05_23-synteny_testing.jpg
Cells from different organisms can be fused in culture by somatic cell hybridization
to form a hybrid cell with two nuclei called a heterokaryon;
the nuclei can fuse to form a synkaryon in which
chromosomes from from one of the two parental species are gradually lost.
A panel of synkaryons with mostly mouse chromosomes and a few human chromosomes can be used to associate a particular gene product with a specific chromosome.
A, B, C, D be mapped to chromosomes?
A can be assigned to chromosome 5.
B can be assigned to chromosome 3.
C is not on chromosome 1-7.
D can be assigned to chromosome 1.
tetrad.html: 05_06-tetrad.jpg
Since a single crossover occurs between two nonsister chromatids,
the other two chromatids of the tetrad are not involved in the exchange.
So the percentage of tetrads
involved in an exchange between two genes is
twice as great as the percentage of recombinant gametes produced.
The theoretical limit of recombination due to crossing over is 50 percent.
tetrad_analysis-linked.html: 05_22-tetrad_analysis-linked.jpg
The unequal proportions of
parental (P) and
nonparental (NP) ditypes
indicates lack of independent assortment, so the genes are linked.
To calculate the distance between the genes: (NP + 1/2 T / total tetrads) * 100.
Note that T is divided by 2 since only half of the meiotic products represents exchanges.
tetrad_analysis.html: 05_T01-tetrad_analysis.jpg
A cross of ab X ++ using Chlamydomonas
yields three possible patterns of unordered tetrads:
Parental ditypes (P) produce two ++ cells and two ab cells.
Nonparental ditypes (NP) produce two a+ cells and two +b cells.
Tetratypes (T) produce one cell of each of the four possible genotypes.
The data indicates independent assortment of the two genes, suggesting they are not
linked,
while the small number of
tetratypes
(T) is useful for mapping the centromere.
tetrad_analysisAB.html: 05_21-tetrad_analysisAB.jpg
Independent assortment of unlinked genes (located on separate chromosomes)
predicts equal proportions of
parental (P) and
nonparental (NP) ditypes.
tetrad_analysisCD.html: 05_21-tetrad_analysisCD.jpg
The tetratypes (T) are generated by a crossover between either of the genes and the centromere.
The four genotypes are found in a 1:1:1:1 ratio,
as predicted by independent assortment of unlinked genes.
three-point.html: 05_08-three-point.jpg
A three-point mapping cross involving the
yellow (y) body color,
white (w) eye color, and
echinus (ec) eye shape
genes in Drosophila.
In the F1, females are heterozygous at all three loci and males are hemizygous for the mutant alleles.
Most of the F2 show parental (NCO) phenotypes, with DCOs the least common among the recombinants.
Because the F2 phenotypes complement each other
(i.e., one is wild type and the other is mutant for all three genes),
they are called reciprocal classes of phenotypes.
three-point_gene_order.html: 05_11-three-point_gene_order.jpg
three-point_mapping.html: 05_08-three-point_mapping.jpg
three-point_test.html: 05_10a-three-point_test.jpg
A three-point mapping cross involving the
brown midrib (bm),
virescent seedling (v), and
purple aleurone (pr)
genes in maize.
Following a DCO in a three-point mapping cross,
the allele in the middle position will be flanked by the other two alleles.
So a pr–v–bm sequence will show
pr+–v–bm+ after a DCO (least frequent)
event. This is not observed.
three-point_testcross.html: 05_10b-three-point_testcross.jpg
Determining gene sequence in a three-point mapping cross.
First, determine the arrangement of alleles by locating the
reciprocal NCO (most frequent) phenotypes:
we conclude the pr+ is on the other homolog from the v+ and
bm+ alleles, as shown
in (a).
Now find DCO (least frequent) phenotypes among the 3 possibilities in
(b), (c), and (d): pr must be in the middle.
Mapping gene distances.
The map distance between two genes is the sum of all SCO and DCO frequencies between them.
The distance between v and pr is the sum of the frequencies
of SCO (14.5%) and DCO (7.8%), or 22.3 mu.
The distance between pr and bmc is the sum of the frequencies
of SCO (35.6%) and DCO (7.8%), or 43.4 mu.