Download The Human Chromosome

THE HUMAN CHROMOSOME
WHAT IS KARYOTYPING?

A typical karyotype is a
preparation of an
individual’s metaphase
chromosomes, sorted out
by length, shape,
centromere location, and
other defining features.

Large abnormalities in
chromosome structure or
an altered chromosome
number can be
pinpointed by comparing
an individual’s
karyotype against a
standard karyotype for
the species.
MAKING A KARYOTYPE

Human chromosomes are in
their most condensed form
and easiest to identify when
a cell is at metaphase in
mitosis.

Using blood cells (typically)
scientists induce metaphase,
place the cells in a hypotonic
solution (causing them to
swell). The chromosomes
move away from each other
and then the cells are placed
on a slide.

The cells are then
photographed, and the
photograph is cut with scissors
or with a computers cut-n-paste
tool.
 The chromosomes are then
lined up by size and shape.
SPECTRAL KARYOTYPES

A more recent
diagnostic tool, uses a
range of fluorescent
dyes that bind to
specific parts of
chromosomes.

Analysis of the
resulting rainbowhued karyotype often
reveals abnormalities
that would not
otherwise be
discernable.
This chromosome (9) exchanged a segment
of itself with the non-homologous
chromosome 22.
The end of chromosome 9 affects mitotic cell
division and 22 affects expression of another
gene.
This mutation results in chronic
myelogenous leukemia (CML) in which the
body produces far too many white blood
cells, which give rise to malignant cells in
bone tissues.
AUTOSOMAL INHERITANCE PATTERNS

Your body contains two types of
chromosomes: sex chromosomes and
autosomal chromosomes.
Sex chromosomes determine
whether you are male or female.
 Autosomes determine every other
trait in your body from the color of
your eyes, to how fast your
metabolism is.


Most human traits arise from
complex gene interactions, but
many can be traced to autosomal
dominant or autosomal recessive
alleles that are inherited in simple
patterns.

Some of these alleles cause genetic
disorders.
AUTOSOMAL DOMINANT INHERITANCE

To inherit an autosomal
dominant mutation, you
only need to inherit a
single mutated or altered
allele from either your
mother or your father to
show the trait.

Polydactyly


This mutation causes an
individual to grow more
than 5 fingers or toes.
Only one parent needs to
pass this onto their child
for the child to have it.
One dominant
allele (red) is
fully expressed in
carriers.
Polydactyly, the
inheritance of
more than five
fingers or toes, is
an autosomal
disorder.
AUTOSOMAL RECESSIVE INHERITANCE

Both parents
are
heterozygous
carriers of the
recessive
allele (red),
and a child
must inherit
both recessive
alleles to show
the trait.
To inherit an
autosomal recessive
mutation, you need to
inherit mutated or
altered alleles from
BOTH your mother and
your father to show the
trait.

Albinism
 is characterized by
the complete or
partial absence of
pigment in the skin,
hair and eyes due to
absence or defect of
an enzyme involved
in the production of
melanin.
Tanzania has one of
the highest
incidences of
albanism in the
world! Over 150,000
albinos live there.
SEX DETERMINISM
The Y chromosome carries 255
genes, one of which is the SRY
gene that gives rise to the
formation of testes.
 The X chromosome carries
1,141 genes (no SRY gene).

It includes genes that affect the
distribution of body hair and fat
(which is why if your mother’s
father isn’t bald, you won’t be
either!)
 Most of its genes deal with nonsexual traits like blood-clotting.

X-LINKED INHERITANCE

Sometimes, genetic disorders are
carried on the X-chromosome.

Recessive alleles on the Xchromosome affect more males
than females as the female has a
second X that will mask the
recessive X’s effects.

Males are not protected, because
they only inherit one X
chromosome.
 An affected father cannot pass
on the recessive allele to his son
(because he’ll only pass on the Ychromosome) but he WILL pass
it onto his daughters.
 EXAMPLES:
 Hemophilia A
 Red-Green Color Blindness
 Duchenne Muscular Dystophy
Can you see the number? 1220% of white males cannot!
They have inherited red-green
color blindness from their
mothers, and have the inability
to distinguish some or all
shades of red and green.
ANEUPLOIDY

In aneuploidy, cells usually
have one extra or one less
chromosome.
Autosomal aneuploidy is usually
fatal and linked to most
miscarriages.
 Aneuploidy typically arises
through nondisjunction.



Nondisjunction is where one or
more pairs of chromosomes do not
separate as they should during
meiosis or mitosis.
Polyloidy is where cells have three
or more of each type of
chromosome.
 Examples of Aneuploidy:





Down’s Syndrome
Turner’s Syndrome
Kleinfelter Syndrome
XXX syndrome
XYY condition
Down’s syndrome is caused
by the presence of all or part
of an extra 21 chromosome,
known as trisomy 21. Down’s
syndrome is associated with
some impairment of cognitive
ability and physical growth.
FEMALE SEX CHROMOSOME
ABNOMALITIES

Sometimes females will
inherit different amounts of
X-chromosomes. (This is
called aneuploidy or nondisjunction)

Turner’s Syndrome
A female inherits one X
chromosome and no
corresponding X or Y
chromosome.
 98% of embryos spontaneously
abort.
 Results in several
developmental abnormalities.

One characteristic sign of
Turner’s Syndrome is the
presence of neck webbing and
extra skin.
MALE SEX CHROMOSOME ABNORMALITIES

Kleinfelter Syndrome




1 in 500 males inherit XXY
chromosomes.
Results in normal range of
intelligence, though may also
have some learning
disabilities.
Have several developmental
side effects.
XYY condition
A karyotype may be performed to
determine the presence of either
 1 in 500 to 1,000 males inherit
Kleinfelter or XYY condition.
XYY.
 Mild mental impariment.
MAIN CATEGORIES

One or more changes in
the physical structure
of a chromosome may
give rise to a genetic
disorder or
abnormality.

Such changes are rare,
but they do occur
spontaneously in
nature.

Some can also be induced
due to exposure to
certain chemicals or
irradiation.
DUPLICATION

Even normal
chromosomes have
DNA sequences that
are repeated two or
more times.

Duplication can occur
through unequal
crossovers at prophase
I.


Some duplications cause
neural problems and
physical abnormalities.
Example: Huntington’s
Disease
Normal chromosome
One segment
repeated.
Three
repeats
Huntington’s
Disease causes
progressive
neurological
degeneration.
Symptoms typically
do not appear until
after age 30. It is
cased by a
repeating CAG
sequence that
disrupts normal
brain cell
development.
DELETION

A deletion is a loss of
some portion of a
chromosome, as by
unequal crossovers,
inversions, or chemical
attacks.

Most deletions cause
serious disorders or
death.


Segment C deleted
Missing or broken genes
disrupt the body’s
growth, development,
and metabolism.
Example: Cri-Du-Chat
Syndrome
A tiny deletion from human
chromosome 5 results in an
abnormally shaped larynx and
mental impairment. Crying infants
sound like cats meowing. Hence,
Cri-Du-Chat (cat-cry in French).
Above is a picture of a male infant
diagnose with Cri-Du-Chat, and
the same boy 4 years later.
INVERSION

With an inversion, part of the
sequence of DNA within the
chromosome becomes oriented
in the reverse direction, with
no molecular loss.
 This can cause problems in
meiosis.

Chromosomes can mispair,
and deletions may occur that
can reduce viability of
gametes.
 Some individuals
(carriers) do not even
know that they have an
inverted chromosome
region until a genetic
disorder or abnormality
surfaces in one or more
children.
segments
G, H, I
become
inverted
Inversions and deletions often
occur together, a result of
unequal recombination events.
TRANSLOCATION

With translocation, a broken
part of one chromosome
becomes attached to another
chromosome.
 Most translocations are
reciprocal, in that both of
the two chromosomes
exchange broken parts.

Translocations often
cause reduced fertility,
because affected
chromosomes have
difficulty segregating in
meiosis.
 Example




Some sarcomas
Lymphoma
Myeloma
Leukemia
chromosome
nonhomologous
chromosome
reciprocal translocation