Download File - Down the Rabbit Hole

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Chromosome Basis of Inheritance
GENETIC INFORMATION

Gene – basic unit of genetic
information. Genes determine
the inherited characters.

Genome – the collection of
genetic information.

Chromosomes – storage
units of genes.

DNA - is a nucleic acid that
contains the genetic
instructions specifying the
biological development of all
cellular forms of life
HUMAN GENOME

Most human cells
contain 46
chromosomes

2 sex chromosomes
(X,Y):
XY – in males.
XX – in females.

22 pairs of
chromosomes named
autosomes.
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Chromosome Theory of Inheritance

Genes have specific
locations (loci) on
chromosomes.

It is the chromosome that
segregates and assorts
independently.

How does this relate to
Mendel’s laws of
inheritance?
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What is the difference between an
Autosome and a Sex-chromosome?
 Autosomes
are the first 22
homologous pairs of
human chromosomes that
do not influence the sex of
an individual.
 Sex
Chromosomes are the
23rd pair of chromosomes
that determine the sex of
an individual.
+ Autosomal Traits

Genes located on Autosomes control Autosomal traits and
disorders.

2 Types of Traits:

Autosomal Dominant

Autosomal Recessive
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



Sex Linked Genes in Humans
Characteristics that are
inherited from genes
 Examples
found on the sex
 Red-green color
chromosomes
blindness
Normally on the X
chromosome as the Y
chromosome is small and
has very few genes.
Most seX-linked traits are
recessive
More common in males
than females
 Males only needs to inherit
 Hemophilia
one sex-linked recessive

allele from mom to be
expressed
Females has to inherit two
sex-linked recessive alleles
for expression
+ Sex Linked Traits
X-linked recessive, carrier mother
 It is possible for a female to




be a carrier of an X-linked
trait, but not express it
SRY gene: gene on Y
chromosome that triggers
the development of testes
Fathers= pass X-linked
alleles to all daughters only
(but not to sons
Mothers= pass X-linked
alleles to both sons &
daughters
Men will express all Xlinked traits they inherit
Unaffected
father
Carrier
mother
Unaffected
Affected
Carrier
Unaffected
son
Unaffected
daughter
Carrier
daughter
Affected
son
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Discovery of Sex-Linked Genes
http://nobelprize.org/nobel_prizes/medicine/articles/lewis/index.html

Thomas Hunt Morgan in The Fly Room!
(Columbia University 1910)

Fruit Flies (Drosophila melanogaster)
© 2007 Paul Billiet ODWS
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Chromosomal Linkage
 Thomas Hunt Morgan
 Drosophilia melanogaster
 XX (female) vs. XY (male)
 Sex-linkage: genes located on
a sex chromosome
 Linked genes: genes located
on the same chromosome that
tend to be inherited together
+ Sex-Linked Recessive Pedigree
 Draw
a Pedigree showing a cross between a Red
eyed Male fruit fly and a Carrier Female fruit fly
which have 2 males and 2 females. (Show all
possibilities) Red is dominant to white.
 Genotypes
 Male
of Parents:
= XR Y
Female = XR Xr
XRY
XRY
XRXr
XrY
XRXR
XRXr
+ Rules for Sex-Linked Recessive
Traits
 More
males than females are affected.
 An
affected son can have parents who have the
normal phenotype. (XAY x XAXa)
 For
a daughter to have the trait, her father must also
have it. Her mother must have it or be a carrier.
(XaY, XaXa, XAXa)
 The
trait often skips a generation from the
grandfather to the grandson.
 If
a woman has the trait (XaXa), all of her sons will
be affected.
 Pedigrees
carriers.
show only female carriers but no male
+ Examples of Sex-Linked Recessive
Disorders

Red/Green Colorblindness – Difficulty perceiving differences
between colors (red or green, blue or yellow).

Hemophilia – Absence of one or more proteins necessary for normal
blood clotting.

Deafness

Cataracts – opacity in the lens that can lead to blindness

Night blindness – (Nyctalopia) rods do not work so that can not see in
the dark

Glaucoma – pressure in the eye that can lead to optic nerve damage
and blindness

Duchenne Muscular Dystrophy – progressive weakness and
degeneration of skeletal muscles that control movement due to
absence of dystrophin (protein that maintains muscle integrity).
Mainly in boys, onset 3-5 yrs, by 12 years can’t walk, and later needs
respirator.
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X- Inactivation

X-inactivation is a process by
which one of the two copies of
the X chromosome present in
female mammals is inactivated
(methylation)

X-inactivation occurs so that the
female with two X chromosomes
does not have twice as many X
chromosome gene products as
the male, which only has a single
copy of the X chromosome

Inactivation is random
The coloration of tortoiseshell cats is a
visible manifestation of X inactivation.
The black and orange alleles of fur color
are on the X chromosome. For any
given patch of fur the inactivation of an
X chromosome that carries one gene
results in the fur color of the other active
gene.
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
X- Inactivation
Barr Bodies
 The inactive chromosome
condenses into a Barr body,
which lies along the inside of
the nuclear envelope
 In men and women with
more than one X
chromosome, the number of
Barr bodies visible at
interphase is always one less
than the total number of X
chromosomes. For example,
men with a 47,XXY
karyotype have a single Barr
body, whereas women with a
47, XXX karyotype have two
Barr bodies
 Genes are located on the same chromosome
 Alleles cannot separate according to the laws of
Independent Assortment and Random Segregation
 Unless…crossing over during meiosis I moves
them to a different chromosome.
 We measure the distance between genes by the
frequency of crossing over moving one of them to a
new chromosome, called gene linkage mapping
 Distant genes are separated by crossing over more
often than nearby genes.
 Results in offspring with a new combination of genes
inherited from the parents
 Parental Type
 Some offspring will have same phenotype as one of the
parentslaws of Independent Assortment and Random
Segregation
 Recombinants
 Some offspring will have phenotypes different from both
parents
GENE LINKAGE
 C is farther away from A
than B is
 We figure this out
because a higher
percentage of gametes
are ABc than are Abc.
 In fruit flies, wings and
body color are linked
+ Linkage Map

Genetic map that is based on
the percentage of cross-over
events

A map unit is equal to a 1%
recombination frequency.

Map units are used to express
relative distances along the
chromosome.
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Genetic recombination Summary
 Crossing over
 Genes that DO NOT assort
independently of each other
 Genetic map
 The further apart 2 genes are,
the higher the probability that
a crossover will occur between
them and therefore the higher
the recombination frequency
 Linkage maps
 Genetic map based on
recombination frequencies
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Chromosomal Mutations

Damage to
chromosomes due to
physical or chemical
disturbances or errors
during meiosis.

Two Types of
Chromosome Mutations:
1.
Chromosome Structure
2.
Chromosome Number
+ Problems with Chromosome
Structure:

Deletion – during cell division, especially meiosis, a
piece of the chromosome breaks off, may be an end
piece or a middle piece (when two breaks in a
chromosome occur).

Inversion – a segment of the chromosome is turned
180°, same gene but opposite position

Translocation – movement of a chromosome segment
from one chromosome to a non-homologous
chromosome

Duplication – a doubling of a chromosome segment
because of attaching a broken piece form a
homologous chromosome, or by unequal crossing
over.
+ Structural Errors
+ Problems with Chromosome
Number
Nondisjunction



When members of a pair of homologous
chromosomes do not separate properly during
meiosis I
Sister chromatids don’t separate properly
during meiosis II
+ Chromosomal Mutations

Monosomy and Trisomy due to
Nondisjunction
 members of homologous
chromosomes do not move apart
in Meiosis I or sister chromatids do
not separate during Meiosis II
leaves one cell with too few
chromosomes and one cell with
too many.

Triploids develop from the
fertilization of an abnormal diploid
egg, produced from the
nondisjunction of all chromosomes.

Tetraploids develop from the failure
of a 2n zygote to divide after
replicating its chromosomes,
subsequent mitosis would produce
4n embryo.
+ Chromosomal Mutations

Polyploidy is common in the
plant kingdom, spontaneous
origin of polyploid individuals
plays important role in evolution
of plants.

In the animal kingdom, natural
occurrence of polyploids is
extremely rare.

In general, polyploids are more
nearly normal in appearance
than having monosomy or
trisomy, which is more disruptive
to have one extra chromosome in
a pair.
Summary
 Nondisjunction:
members of a pair of
homologous
chromosomes do not
separate properly during
meiosis I or sister
chromatids fail to separate
during meiosis II
 Aneuploidy: chromosome
number is abnormal
• Monosomy~
missing
chromosome
• Trisomy
~
extra
chromosome (Down
syndrome)
• Polyploidy~ extra
sets of chromosomes
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Genomic imprinting
 Genonmic imprinting is a
parental effect on gene
expression
 Identical alleles may have
different effects on
offspring, depending on
whether they arrive in the
zygote via the ovum or via
the sperm.
 Fragile X syndrome: higher
prevalence of disorder and
retardation in males
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Trisomy 21
Trisomy 21 is the
presence of 3
chromosome 21’s.
Condition is called
Down Syndrome
Characteristics are
specific and familiar
but not all
individuals show the
exact same
characteristics.
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Down Syndrome
Individuals with Down
syndrome have a typical
facial appearance
All have some degree of
mental retardation, but for
most it is mild to moderate.


Can learn to read, write, do
some math, and live day-today with minimal assistance.
Others require a lot of
attention and care
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Down Syndrome
There are several
possible health
concerns, including
heart problems and
hearing loss.


Individuals are now living
longer than in the past, into
their 50’s and 60’s
We are now discovering
that those individuals who
survive to this age are at
very high risk for
developing Alzheimer’s
disease
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Klinefelter Syndrome
People with this disorder
develop as males with subtle
characteristics that become
apparent during puberty.




Often tall and usually don't
develop secondary sex
characteristics, such as facial
hair or underarm and pubic
hair.
Different distribution of body
fat, more than usual in the chest
and hips.
May have some degree of
mental retardation.
Some individuals are never
diagnosed.
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Klinefelter Syndrome
Affects about 1 in
1000 males
The extra X
chromosome
primarily affects the
testes, the most
common feature
being infertility

2% of infertile men
are diagnosed with
Klinefelter syndrome
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Turner Syndrome
Affects about 1 in 5000
newborn girls.
Estrogen and other growth
hormones are not
produced in sufficient
quantity.
When caught in infancy, TS
can be managed and
treated with regular
medical care.
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Turner Syndrome
Females with Turner are
shorter than average and
can have other
noticeable physical
features.

Broad chest, swelling in
hands and feet, webbed
neck.
Other health issues can
include heart disease,
horseshoe shaped
kidneys, learning
disabilities