Transcript first of three for Chapter 8

Chapter 8
Human
Karyotypes and
Chromosome
Behavior
Jones and Bartlett Publishers © 2005
Chromosome stability
• The normal chromosome complement of
mitotic cells of an organism can be
photographed in metaphase.
• Actual cut-outs or digital images can be
arranged to pair homologous chromosomes.
• In humans, chromosome 1 is the longest.
Human karyotype
Human karyotypes: terms
The banding patterns of human chromosomes
The short arm
of a
chromosome is
called “p” (for
‘petite’) and the
long arm “q”.
A, B, C, etc.
refer to
grouping of
chromosomes
by size.
The 3 shapes of chromosomes
based on the location of the centromere
• Acentric chromosomes lack a centromere.
• Dicentric chromosomes have two
centromeres.
– These form a bridge during cell division,
or the chromosome can break.
Chromosome 2 in humans.
The proposed mechanism for the creation of the
long human chromosome 2 by fusion of two
shorter chromosomes
Similarity
between
chromosomes of
the great apes
In human females, one of the 2 X-chromosomes
is inactivated early in development
Dosage compensation:
In Drosophila males, the
X chromosome has its
transcription increased.
C. elegans females
decrease transcription of
both X chromosomes.
In mammals, one X is
inactivated (Barr body).
Calico cat example.
X-inactivation  Barr bodies
Calico Cat
Female cats
heterozygous for
orange and black
hair color develop
patches with either X
inactivated.
(The white area is
another gene, S for
white spotting and is
autosomal.)
Pseudoautosomal inheritance
• At the tips of both arms of the X chromosome are
several megabases that are not inactivated.
• These have homologous regions in the Y
chromosome, so crossing over can occur.
• These pseudoautosomal regions (PARp and PARq)
have a 20-fold higher recombination rate than in
true autosomal chromosomes.
Proposed steps in the creation of the modern
Y-chromosome by rearrangement of segments
in an ancient Y-chromosome
Distribution of Y-chromosome haplotypes,
presumed to have descended from Genghis Khan
Haplotype: the allelic form of each of a set of
linked genes present in a single chromosome.
Human chromosomal anomalies and
spontaneous abortion
The probability of a child with Down syndrome
increases with Mother’s age
Pairing and segregation of 3 homologous
chromosomes in meiosis I
This kind of
chromosome
pairing occurs in
trisomics (2n+1)
or triploids (3n)
Nondisjunction
Environmental risks for
aneuploidy in humans
• Suggested risks: radiation, smoking, alcohol
consumption, oral contraceptives, fertility drugs,
pollutants, pesticides, etc.
• Maternal-age effects on meiosis – maybe estrogen
has a role.
• Bisphenol A is an estrogen mimic found in
polycarbonate plastics.
•
In mice, aneuploid oocytes increased from 1.4%
to 11.6%