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5
Human Karyotypes and
Chromosome Behavior
Human Chromosomes
• Humans contain 46
chromosomes, including
22 pairs of homologous
chromosomes and two
sex chromosomes
• Karyotype = stained and
photographed preparation of metaphase
chromosomes arranged in homologous pairs
in descending size order
Human Chromosomes
• Chromosome maps are prepared by
dividing the chromosome into two regions
(arms) separated by the centromere
• p = short arm (petit); q = long arm
• p and q arms are divided into numbered
bands and interband regions based on
pattern of staining
Centromeres
Centromeres may be located
in different regions of a
chromosome:
• Metacentric = located in
middle of chromosome
• Submetacentric = located closer to one end of
chromosome
• Acrocentric = located near one end of
chromosome
Human X Chromosome
• Females=2 copies of X chromosome
• One copy of X is randomly inactivated in all
somatic cells
• Females are genetic mosaics for genes on the
X chromosome; only one X allele is active in
each cell
• Barr body = inactive X
chromosome
• Dosage Compensation=
dosage equalization for
active genes
Human Y Chromosome
• Y chromosome is largely heterochromatic
(condensed inactive chromatin)
• Important regions of Y chromosome:
-pseudoautosomal region=region of shared
X-Y homology
-SRY=master sex controller gene which
encodes testis determining factor (TDF)
for male development
Abnormal Chromosome Number
• Euploid=balanced chromosome
abnormality=equal number of copies
• aneuploid=unbalanced set of
chromosomes=unequal copy number
• monosomic=loss of a single chromosome
copy
Abnormal Chromosome Number
• Trisomy of chromosome 21 = most
common autosomal (non-sex
chromosome) aneuploidy
• Down Syndrome = genetic disorder
due to trisomy 21 = aneuploidy
• Amniocentesis=fetal cells are
analyzed for abnormalties of
chromosome number and structure
Extra or Missing Chromosomes
• Polysomy = extra copies of single
chromosomes in a cell
• Trisomy = extra copy
of chromosome
• Trivalent = abnormal
pairing of trisomic
chromosomes in cell
division
• Univalent = extra
chromosome in trisomy
is unpaired in cell
division
Sex Chromosome Aneuploidies
• trisomy-X=47, XXX (female)
• double-Y=47, XYY (male)
• Klinefelter Syndrome=47, XXY (male,
sterile)
• Turner Syndrome=45, X (female, sterile)
• Fragile-X syndrome= X chromosome
instability resulting from high copy
number of trinucleotide repeat
Chromosome Deletions
• Deletions = missing chromosome segment
• Polytene chromosomes of Drosophila can be
used to map physically the locations of
deletions
• Testcross mapping
shows which wild-type
alleles are lost by
deletion
• Large deletions are
often lethal
Gene Duplications
• Duplication = chromosome segment present
in multiple copies
• Tandem duplications = repeated segments
are adjacent
• Tandem duplications often result from
unequal crossing-over due to mispairing of
homologous chromosomes during meiotic
recombination
Red-Green Color Vision Genes
• Green-pigment genes may be present in
multiple copies on the X-chromosome due
to mispairing and unequal crossing-over
• Unequal crossing-over between these
genes during meiotic recombination can
also result in gene deletion and colorblindness
• Results in chimeric (composite) gene
Chromosome Inversions
• Inversions = genetic rearrangements in which
the order of genes is reversed in a
chromosome segment
• Inversions do not alter the genetic content
but change the linear
sequence of genetic
information
• At synapsis =
homologous pairing,
inversion loops form
Chromosome Inversions
• Paracentric inversion = does not include
centromere; Pericentric inversion = includes
centromere
• Crossing-over within a
paracentric inversion loop
during recombination
produces one acentric
(no centromere) and one
dicentric (two centromeres)
chromosome
Chromosome Inversions
• Crossing-over within a
pericentric inversion loop
during homologous
recombination results in
duplications and deletions of genetic
information
• Deletions and duplications occur because
inversion loop causes misalignment of
chromosomes during homologous pairing =
synapsis
Reciprocal Translocations
• Reciprocal translocations = exchange of
genetic segments between non-homologous
(unrelated) chromosomes
• There is no loss of genetic information but
the functions of specific genes may be
altered
• Reciprocal translocation may affect one or
both pairs of chromosomes
Reciprocal Translocation
• Heterozygous translocation = only one pair of
non-homologous
chromosomes is affected
• Homozygous
translocation = both pairs
of non-homologous
chromosomes are
affected
Reciprocal Translocations
• Synapsis involving heterozygous
reciprocal translocation results in pairing
of four pairs of sister chromatids =
quadrivalent
• Chromosome pairs may segregate in
several ways during meiosis, with varying
genetic outcomes in gametes
Reciprocal Translocation
3 outcomes of meiotic segregation:
• Adjacent-1 segregation: homologous
centromeres separate at anaphase I;
gametes contain duplications and
deletions
• Adjacent-2 segregation: homologous
centromeres stay together at anaphase I;
gametes have a segment duplication and
deletion
Reciprocal Translocation
• Alternate Segregation: half the
gametes receive both parts of the
reciprocal translocation and the
other half receive both normal
chromosomes; all gametes are
euploid = normal genetic content, but
half are translocation carriers
Robertsonian Translocation
• Robertsonian translocation = fusion of
two acrocentric chromosomes in the
centromere region
• Translocation results in apparent loss of
one chromosome in karyotype analysis
• Genetic information is lost in the tips of
the translocated acrocentric
chromosomes
Robertsonian Translocation
Meiotic segregation (14:21):
• Adjacent-1 segregation: 1/2 gametes have
extra copy of most of chromosome 21
(fertilization will produce Down Syndrome
= trisomy 21); 1/2 gametes have no copy
of 21
• Adjacent-2 segregation: 1/2 gametes
contain duplications; 1/2 lack
chromosome 14
Robertsonian Translocation
• Alternate Segregation: all gametes are
euploid; 1/2 are carriers of the
Robertsonian translocation
• Translocations may produce position
effects = changes in gene function due to
repositioning of gene
• Gene expression = elevated or decreased
in translocated gene
Polyploidy
• Polyploidy = genome composed
of multiple complete sets of
chromosomes; occurs in plants
• Haploid = set of unpaired
chromosomes found in gametes
• Diploid = set of paired
homologous chromosomes
found in most cells; total
number is fixed for a species
Chromosome Number
• monoploid = basic set of chromosomes;
number fixed for species
• diploid = 2 copies of monoploid set
• triploid = 3 copies of monoploid set
• tetraploid = 4 copies of monoploid set
• hexaploid = 6 copies of monoploid set
Endoreduplication
• Endoreduplication=
doubling of the
chromosome
complement
• Tetraploidy=chromosomes fail to segregate
during meiosis or mitosis
• autopolyploidy=chromosomes derived from
single diploid
• Chromosome painting = chromosomes
hybridized with fluorescent dye to show
origins