Down_syndrome.html: 08_05-Down_syndrome.jpg
Down syndrome individuals often exhibit an epicanthic fold of the
eyelid
with a flat face and round head.
They are usually short and have protruding tongues and broad hands.
Physical and mental development is retarded.
Notch_phenotype.html: 08_T02-Notch_phenotype.jpg
Notched-winged females of this X-linked dominant mutation survive only as a heterozygote.
The closely linked recessive white-eye, facet-eye, and split-bristle mutant alleles exhibit the mutant (recessive) phenotype in these heterozygotes.
(The diagram probably has a mistake in locating the N alleles for the lower 2 cases?)
allopolyploidy.html: 08_09-allopolyploidy.jpg
Allopolyploidy results from hybridization of two closely related species.
If the sterile hybrid (AB) undergoes a chromosomal doubling,
a fertile amphidiploid,
with complete chromosome sets (AABB) of the two species,
is produced.
amphidiploid-cotton.html: 08_12-amphidiploid-cotton.jpg
Cultivated American cotton, Gossypium hirsutum.
This species has 26 pairs of chromosomes: 13 are large and 13 are much smaller.
Old World cotton had only 13 pairs of large chromosomes,
while wild American cotton revealed 13 pairs of small chromosomes.
The origin of the cultivated cotton was reconstructed experimentally by treating
a hybrid with colchicine to double the chromosome number and produce a
fertile amphidiploid with characteristics similar to the cultivated variety.
amphidiploid.html: 08_11-amphidiploid.jpg
Species 1 contains genome A consisting of three distinct chromosomes, a1, a2, and a3.
Species 2 contains genome B consisting of two distinct chromosomes, b1 and b2.
Following chromosome doubling of the sterile hybrid (AB),
a fertile amphidiploid (AABB) can be maintained by sexual
reproduction.
aneuploidy.html: 08_T01-aneuploidy.jpg
Aneuploidy is gain or loss of one or more chromosomes, but not a complete set.
Polyploidy is possession of more than two sets:
triploid is three sets and tetraploid is four sets.
video
autopolyploidy.html: 08_09-autopolyploidy.jpg
Autopolyploidy involves the addition of one or more haploid chromosome sets of the same species.
Triploids (AAA) have an uneven number of homologs and are usually not maintained from to future generations.
Tetraploids (AAAA) produce balanced gametes which can be inherited.
auttetraploid.html: 08_10-auttetraploid.jpg
Chromosomal doubling can occur if a cell fails to divide after chromosomes replication.
This can be simulated experimentally by applying colchicine, which interferes with spindle formation,
to somatic cells undergoing mitosis. This prevents replicated chromosomes from migrating to opposite poles at anaphase.
When colchicine is removed, the tetraploid cell can reenter interphase.
chromosome_deletion-intercalary.html: 08_15b-chromosome_deletion-intercalary.jpg
An intercalary chromosome deletion.
Such a deletion involving a large segment will exhibit a deletion loop
during meiosis I.
chromosome_deletion-loop.html: 08_15c-chromosome_deletion-loop.jpg
For synapsis to occur between a chromosome with a large intercalary deficiency and a
normal complete homolog, the unpaired region of the normal homolog must form a deletion
or compensation loop during meiosis I.
video
chromosome_deletion-terminal.html: 08_15a-chromosome_deletion-terminal.jpg
A terminal chromosome deletion.
chromosome_rearrangement.html: 08_14-chromosome_rearrangement.jpg
Errors in crossing over or chromosome breaks may cause chromosome rearrangements.
A chromosome break may produce "sticky ends" that can rejoin other broken
ends.
The rejoined chromosome may exhibit deletions, duplications, inversions, and
translocations.
video
cri-du-chat.html: 08_02-cri-du-chat.jpg
Cri-du-chat (46, –5p) individuals
may exhibit anatomic malformations, especially abnormal development of the glottis and larynx.
They are often mentally retarded.
duplication-origin.html: 08_18b-duplication-origin.jpg
Wild-type (B+) flies possess one copy of region 16A of the X chromosome.
This region is duplicated in B flies.
Unequal crossing over in a B individual can result in a BD chromatid with
a triplicated region 16A and a B+ wild-type, producing a 2:1:1 ratio in the
gametes.
duplication.html: 08_18a-duplication.jpg
Bar-eye (B)
is a dominant X-linked mutation with reduced number of facets in the eye
(wild type B+ has 800 facets).
Homozygous females show semidominance: a more pronounced phenotype (68 facets) than heterozygotes
(350 facets).
BD (double-Bar) females
show even fewer facets (45), due to triplication of region 16A of the X
chromosome
and the position
effect.
video
familial_Down_syndrome.html: 08_25-familial_Down_syndrome.jpg
Familial Down syndrome.
One parent contains a 14/21 translocation and has only 45 chromosomes, but is phenotypically normal.
1/4 of the individual's gametes will have almost 2 copies of chromosome 21.
The resulting zygote has 46 chromosomes, but almost 3 copies of chromosome 21,
and exhibits Down syndrome.
fragile_X_syndrome.html: 08_26-fragile_X_syndrome.jpg
A normal human X chromosome (left) contrasted with a fragile X chromosome (right), which is
prone to breakage in culture.
The syndrome is associated with excessive repeats of the trinucleotide sequence CGG in the FMR1 gene,
which inactivates this gene.
The FMR1 gene codes for an RNA-binding protein; absence of this protein presumably affects brain development.
inversion-paracentric.html: 08_22a-inversion-paracentric.jpg
A single crossover (SCO) within a paracentric inversion
produces two parental chromatids and two
recombinant chromatids with duplications and deletions.
One recombinant chromatid is
acentric (lacking a centromere),
the other is
dicentric (two centromeres);
both pose problems in anaphase.
The acentric chromatid may move randomly to one pole, or it may be lost,
The dicentric chromatid is pulled in two directions and breaks apart, resulting in deletions.
inversion-pericentric.html: 08_22b-inversion-pericentric.jpg
A single crossover (SCO) within a
pericentric inversion also produces two recombinant chromatids,
both with duplications and deletions.
No acentric or dicentric chromatids are produced.
In both paracentric and pericentric inversion, 1/2 the gametes
will inherit duplications and deletions, which reduces viability of the offspring.
inversion-synapsis.html: 08_21-inversion-synapsis.jpg
An inversion heterozygote forms an inversion loop during synapsis.
If crossing over does not occur within the inversion, segregation will produce a 1:1 normal:inverted ratio in the gametes, and 1/2 the offspring will inherit the inversion.
However, if crossing over does occur within the inversion loop,
abnormal
chromatids are
produced,
with reduced gamete viability.
This is likely with large inversions.
video
inversion.html: 08_19-inversion.jpg
An inversion requires two breaks of a chromosome loop, where the "sticky" ends are rejoined
so that the rejoined segment is turned around 180°
This diagram shows a pericentric inversion: the inverted segment includes
the centromere.
video
inversions.html: 08_20-inversions.jpg
A paracentric inversion
does not include the centromere, and the ratio of the lengths of the two chromosome arms is unchanged.
A pericentric inversion
often changes the arm ratio, which may sometimes be detected in metaphase chromosome.
maternal_age.html: 08_06-maternal_age.jpg
The frequency of trisomy-21 for maternal age 30 is about 1 in 1000.
The risk increases tenfold to 1 in 100 at age 40, and continues to increase past that age.
Some techniques for detecting genetic disorders include
Amniocentesis
or Chorionic Villus Sampling (CVS).
nondisjunction.html: 08_01-nondisjunction.jpg
Nondisjunction can occur in the first or second meiotic divisions,
producing gametes that either contain two members of a chromosome or lack that chromosome.
video
Following fertilization by a normal haploid gamete, monosomic, disomic (normal),
or trisomic zygotes are produced.
meiosis I
meiosis II
partial_monosomy.html: 08_02-partial_monosomy.jpg
Partial monosomy, or segmental deletion,
is the loss of a section of a chromosome.
An example is
Cri-du-chat
syndrome (46, –5p),
caused by the loss of a small part of the short ("petite", or "p") arm of chromosome 5.
pseudodominance.html: 08_16-pseudodominance.jpg
If a deletion covers dominant genes, a heterozygous individual may exhibit the
recessive phenotype in those loci due to pseudodominance, as shown by genes linked to the
Notch locus
in D. melanogaster.
D. melanogaster possesses polytene chromosomes
in the salivary glands that allows visualizing the banding pattern of the deficiency loop.
somatic_cell_hybridization.html: 08_13-somatic_cell_hybridization.jpg
Somatic Cel Hybridization
Cells from the leaves of plants can be treated to remove their cell wall, resulting in protoplasts.
These can be fused with protoplasts from a different species in culture, producing somatic hybrid cells that are amphidiploids.
Sometimes entire plants can be derived from cultured protoplasts.
If only stems and leaves are produced, these can be grafted onto the stem of another plant.
If flowers are formed, fertilization may yield seeds which germinate into mature plants.
translocation-Robertsonian.html: 08_24-translocation-Robertsonian.jpg
A Robertsonian translocation (or centric fusion)
involves breaks at the extreme ends of the short arms of two nonhomologous
acrocentric
haploid chromosomes (13, 14, 15, 21, and 22).
Centric fusion of the long arms creates a larger submetacentric
or metacentric chromosome plus 2 acentric fragments.
Familial Down syndrome
is an example of this rearrangement.
translocation-reciprocal.html: 08_23a-translocation-reciprocal.jpg
A reciprocal translocation can occur when two breaks near the ends of two nonhomologous
chromosome arms cause an exchange of segments.
translocation-segregation.html: 08_23c-translocation-segregation.jpg
Following meiosis and independent assortment,
two segregation patterns are possible for gamete formation.
Alternate segregation leads to a normal (1,4 - no translocations) and a balanced
(2,3 - reciprocal translocation) gamete.
Adjacent segregation leads to gametes (1,3 and 2,4) containing duplications and deletions.
video
translocation-synapsis.html: 08_23b-translocation-synapsis.jpg
Synapsis in a translocation heterozygote results in a "cross" pairing of the homologs,
producing unbalanced gametes,
as with inversions, but even without crossing over.
video
trisomy-13.html: 08_07-trisomy-13.jpg
Patau syndrome (trisomy 13 (47,+13)) is associated with numerous
abnormalities.
trisomy-18.html: 08_08-trisomy-18.jpg
Edwards syndrome (trisomy 18 (47,+18)) is associated with numerous
abnormalities.
Most Edwards syndrome individuals are females.
trisomy-21.html: 08_05-trisomy-21.jpg
The karyotype of trisomy-21 (47,+21), showing three members of chromosome 21.
trisomy.html: 08_03-trisomy.jpg
Drawings of capsule phenotypes of the fruits of the Jimson
weed Datura stramonium, as a
result of trisomy (2n + 1) of one of the 12 haploid chromosomes.
trivalent.html: 08_04-trivalent.jpg
Three copies of a single chromosome may synapse during prophase I, forming a trivalent.
In anaphase I, one chromosome moves toward one pole and two chromosomes toward the other pole;
the latter results in a n + 1 cell, preserving trisomy.
unequal_crossing_over.html: 08_17-unequal_crossing_over.jpg
The tetrad at the left is mispaired during synapsis.
A single crossover between chromatids 2 and 3 results in
deficient and duplicated chromosome regions.