Transcript The Human Genome Chapter 14

The Human Genome
Chapter 14
By: Nick Thompson
Biology
Period 4
May 5, 2009
Section 1- Human Heredity
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Scientists begin answering the question about what makes us
human by seeing the human cell with a microscope, starting with
the chromosomes.
The cell biologists photograph the cells in mitosis to analyze the
chromes. A picture of chromosomes arranged in pairs is known
as a karyotype.
Two of the 46 chromosomes in a typical human body cell are
called sex chromosomes. They determine the individual’s sex.
The remaining 44 chromosomes are called autosomes.
All human egg cells carry a single X chromosome (23,X).
However, half of all sperm cells carry an X chromosome and half
carry a Y chromosome (23,Y). This means that half of the zygotes
will be 46,XX and half will be 46, XY.
Section 1-Continued
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Human genes are inherited by the same principles Gregor
Mendel discovered with his work on garden peas. A pedigree
chart shows the relationship within a family. It is fittingly used to
study how the trait is passed from one generation to the next.
The human genome – our complete set of genetic information. It
was “a major scientific undertaking” when it was discovered.
Blood Group Genes- knowing a person’s blood group is crucial for
procedures like blood transfusion because, putting the wrong type
of blood into a patient during a transfusion could be fatal. The
best known groups are the ABO and the Rh blood groups.
Rh- stands for “rhesus monkey.” It is determined by single gene
with 2 alleles- positive and negative.
ABO-Three alleles I^a, I^b, and i. Those who are homozygous for
the i allele (ii) produce no antigen and are said to have blood type
O.
Section 1 cont.
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In most cases, the presence of a normal functioning gene is
revealed only when an abnormal or nonfunctioning allele affects
the phenotype.
One of the first genetic disorders to be understood this way was
phenylketonuria, or PKU. People with PKU lack the enzyme that
is needed to break down phenylalanine.
Phenylalanine is an amino acid found in milk and many other
foods.
Not all genetic disorder are caused by recessive alleles. If you
have a dominant allele for a genetic disorder, it will be expressed.
Two examples of genetic disorders caused by autosomal
dominant alleles are a form of dwarfism known as achondroplasia
and a nervous system disorder known as Huntington’s disease.
Section 1 cont.
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In both cystic fibrosis and sickle cell disease, a small change in
the DNA of a single gene affects the structure of a protein,
causing a serious genetic disorder.
Cystic Fibrosis is most common among people whose ancestors
came from Northern Europe. The disease is caused by a
recessive allele on chromosome 7.
Most cases of cystic fibrosis are caused by the deletion of 3
bases in the middle of a sequence for a protein. This protein
normally allows chloride ions to pass across biological
membranes. The deletion of these 3 bases removes just one
amino acid from this large protein, causing it to fold improperly.
Because of this, the cells do not transport the protein to the cell
membrane, and the misfolded protein is destroyed. People with
one normal copy of the allele are unaffected, because they can
produce enough of the chloride channel protein to allow their
tissues to function properly.
Section 1 cont.
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Sickle cell disease, a serious disorder found in about 1 out of 500
African Americans, is caused by a codominant allele.
Sickle cell shaped red blood cells are more rigid than normal cells
and tend to get stuck in the capillaries, the narrowest blood
vessels in the body. As a result, blood stops moving through
these vessels, damaging cells tissues, and organs.
Sickle cell disease produces physical weakness and damage to
the brain, heart, and spleen. In some cases, it may be fatal.
Hemoglobin is the protein in red blood cells that carries oxygen.
The normal allele – just one DNA base is changed. This change
substitutes the amino acid valine for glutamic acid. As a result, th
abnormal hemoglobin is somewhat less soluble than normal
hemoglobin. Low oxygen levels cause some red blood cells to
become sickle shaped.
Section 2- Human Chromosomes
• Intro
– The average human gene consists of about 3000
base pairs, while the largest gene in the human has
more than 2 million base pairs.
• Human Genes and Chromosomes
– Chromosomes 22 contain as many as 545 different
genes, some of which are very important for health.
Genetic disorder on chromosome 22 include and
allele that causes a form of leukemia and another
associated with neurofibromatosis, a tumor
causing disease of the nervous system.
Human Genes and Chromosomes
Continued
• However, chromosome 22 also contains long
stretches of repetitive DNA that do no code for
proteins. These long stretches of a repetitive
DNA are unstable sites where rearrangement
can occur. The structure of chromosome 21 is
similar. It contains about 225 genes, including
one associated with amyotrophic lateral
sclerosis (ALS), also known as Lou Gehrig’s
disease. Chromosome 21 also has many
regions with no genes at all.
Sex-linked Genes
• Chromosomes determine sex, genes located on them
are said to be sex-linked genes.
• More than 100 sex-linked disorders have now been
mapped to the X chromosome.
• The human Y chromosome is much smaller than the X
chromosome and contains only a few genes.
Colorblindness
A sex-linked disorder
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Three human genes associated with vision are located on the X
chromosome.
In males a defective version of this can mean colorblindness.
The most common form of this disorder, red-green
colorblindness, is found in the U.S. Colorblindness is rare in
females, only about 1 in 100 females have colorblindness.
All the X linked alleles are expressed in males, even if they are
recessive.
In order for a recessive allele to be expressed in females, there
must be two copies of the allele, one on each of the two Xchromosomes.
The recessive phenotype of a sex-linked genetic disorder tends to
be much more common among males than among females.
Men pass their X chromosomes along to their daughters, sex
linked genes move from fathers to daughters and possibly to the
daughters sons.
Hemophilia
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Two important genes carried on the X chromosomes help
control blood clotting.
A recessive allele in either of these two genes may produce a
disorder called hemophilia.
In hemophilia, a protein necessary for normal blood clotting is
missing.
About 1 in 10,000 males are born with a form of hemophilia.
People with hemophilia can bleed to death from a minor cut
and can suffer internal bleeding from a bruise or bump.
Hemophilia can be treated by injections of normal blood
clotting proteins,
Which are now produced using recombinant DNA.
Duchenne Muscular Dystrophy
Sex-Linked Disorder
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Duchenne Muscular Dystrophy is a disorder that results in the
progressive weakening and loss of skeletal muscle.
In the U.S., 1of every 3,000 males is born with this condition.
Duchenne muscular dystrophy is caused by a defective
version of the gene that codes for a muscular protein.
Researchers in many laboratories are trying to find a way to
treat or cure this disorder.
Its possible by inserting a normal allele into the muscle cells of
Duchenne muscular dystrophy patients.
X-Chromosome Inactivation
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In female cells, one X chromosome is switched off.
That turned off chromosome forms a dense region in the nucleus
known as Barr body.
Barr bodies are generally not found in males because their X
chromosome is still active.
In cells in some parts of the body, one X chromosome is switched
off.
In other parts of the body the other X chromosome is switched off.
Chromosomal Disorders
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The most common error in meiosis occurs when homologous
chromosomes fail to separate.
This is known as nondisjunction, which means “not coming apart.”
If nondisjunction occurs, abnormal numbers of chromosomes may
find their way into gametes, and a disorder of chromosome
numbers may result.
Down Syndrome
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If two copies of an autosomal chromosome fail to separate during
meiosis, an individual may be born with tree copies of a
chromosome.
This is known as a trisomy, meaning “three bodies.”
The most common form of trisomy involves three copies of
chromosome 21 and is called Down Syndrome.
In the U.S., approximately 1 baby in 800 is born with Down
syndrome.
Down syndrome produces mild to severe mental retardation.
It is also characterized by an increased susceptibility to many
diseases and a higher frequency of some birth defects.
Sex Chromosome Disorders
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Disorders also occur among the sex chromosomes.
Two of these abnormalities are Turner’s syndrome an Klinefelter’s syndrome.
In females, nondisjunction can lead to Turner’s syndrome usually inherits only one X
chromosome.
Women with Turner’s syndrome are sterile, which means that they are unable to
reproduce.
In males, nondisjunction causes Klinefelter’s syndrome.
The extra X chromosome interferes with meiosis and usually prevents these
individuals from reproducing.
Cases of Klinefelter’s syndrome have been found in which individuals were XXXY or
XXXXY.
There have been no reported instances of babies being born without an X
chromosome, indication that the X chromosome contains genes that are vital for the
survival and development of an embryo.
Section 3Human Molecular Genetics
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The Tay-Sachs and Cystic Fibrosis alleles have slightly different
DNA sequences form their normal counter parts, a variety of
genetic tests have been developed that can spot those
differences.
DNA testing can pinpoint the exact genetic basis of a disorder,
making it possible to development more effective treatment for
individuals affected by genetic disease.
DNA fingerprinting-Analysis of sections of DNA that have little or
no known function, but vary widely from one individual to another,
in order to identify individuals.
The DNA sequence of the common bacterium “Escherichia Coli,”
which was determined in 1996, contains only 4,639,221 base
pairs, making it just about as long as this textbook if it were
printed on paper in a readable typeface.
Section 3 Continued
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Gene therapy is the process of changing the gene that causes a
genetic disorder.
The goal of biology is to gain a better understanding of the nature
of life.
As our knowledge increases, however, so does our ability to
manipulate the genetics of living things, including ourselves
The End