Chapter 14 * The Human Genome

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Humans have 23 pairs of chromosomes,
or 46 chromosomes total, in a diploid cell
Karyotype = a picture of the chromosomes
arranged in homologous pairs
2 of the 46 chromosomes are known as sex chromosomes 
they determine and individuals sex
Females have 2 X chromosomes
Males have 1 X and
1 Y chromosome
Males and females are born in almost
a 50:50 ratio because of the way sex
cells segregate during meiosis
The other 44 chromosome
are known as autosomes
Human genes are inherited according
to the Mendelian principles
To apply Mendelian genetics, scientists
have to identify a trait inherited by a
single gene  they must also establish
that the trait is inherited and not the
result of environmental influences
A pedigree chart shows the
relationships within a family and can
help determine if the gene is inherited
Genetic counselors analyze
pedigree charts to infer the
genotypes in family members
If a trait is dominant and an individual shows the recessive
phenotype, they must be homozygous recessive
This also implies that the person who passed the trait on was
heterozygous because they were able to pass along a recessive
allele
An individual’s phenotype is only
partially determined by the genotype
Many traits are strongly influenced
by environmental factors including
nutrition and exercise
These environmental effects are
not inherited; genes are
Genes may be denied a proper environment
in which to reach full expression in one
generation, but given the right environment
can be seen more in later generations
The human genome is our complete set of genetic
information and includes tens and thousands of genes
It took scientists lots of time to identify genes that directly
control a single human trait
Some of the first human
genes identified were those
that control blood type
Human blood has been broken up into different groups –
your blood group is very important because if you get a
transfusion from the wrong blood group it can be fatal
The best known controls of
blood type are the ABO group
and the Rh group
The Rh group is controlled by one gene with 2 alleles;
positive and negative
Positive is dominant
The ABO group has 3 alleles for the gene;
IA, IB , and i
IA and IB are codominant, a person with
both alleles is blood type AB
i is recessive, so a person with two i alleles
makes no antigens and is blood type O
IA IA or IAi would produce blood type A
IB IB or IBi would produce blood type B
If a medical worker tells you
your blood type, they might
say A+, they are referring to
both groups of genes
Many of the human genes that became known
were associated with a genetic disorder
Several examples of genetic disorders
caused by recessive alleles are albinism,
cystic fibrosis, and Tay-Sachs disease
Several examples of genetic disorder
caused by dominant alleles are
achondroplasia and Huntington’s disease
Sickle cell is a disorder associated with codominant alleles
Several of these disorders are caused by
just a small change in DNA that ultimately
affects the structure of a protein  these
proteins are so important that when
changed they can have lethal effects
A human diploid cell contains 46 chromosomes
Chromosomes 21 and 22 are the smallest human
autosomes  they were the first two chromosomes
to be sequenced and their structural features are
representative of other chromosomes
On both chromosomes there are long stretches of
DNA that do not code for genes, these long stretches
represent unstable sites where mutations can occur
and cause problems
Genes that are close enough together on a
chromosome tend to be inherited together
Genes that are located on the
X and Y chromosomes are
sex-linked genes
More than 100 sex-linked genetic disorders
have been mapped to the X chromosome; the
Y chromosome is smaller by comparison and
only contains a few genes
Males have just one X chromosome, so all X alleles
are expressed in males, even if they are recessive
There are 3 genes associated with
color vision on the X chromosome 
in males if there is a defect in any of
the genes it produces colorblindness
Hemophilia is another sex-linked
disorder  a protein for blood
clotting is missing and a person
with hemophilia can bleed to
death from a minor cut
Muscular dystrophy is also a
sex-linked disorder  it causes
a progressive weakening and
loss of muscle tissue
The most common error during meiosis
is when homologous chromosomes fail
to separate (nondisjunction)
If nondisjunction occurs, abnormal
numbers of chromosomes may find
their way into gametes and a disorder
of chromosome numbers may result
Trisomy = when an individual ends
up with 3 copies of a chromosome
The most common form of trisomy
is Down syndrome  it produces
mild to severe mental retardation
and other birth defects
Nondisjunction can occur with the
X and Y chromosomes as well
In Turner’s syndrome, a female is only born
with one X chromosome  as a result her
sex organs do not develop at puberty and she
is sterile
In Klinefelter’s syndrome, males end up with
an extra X chromosome
There has not been a case where
a baby was born without an X
chromosome, showing that its
genes are essential for survival
Knowing more about the sequence of human genes has been
useful in several ways
Parents can test for recessive alleles
that may cause genetic disorders for
their children
If parents are worried their child may
have Tay-Sachs disease, they can have
the fetus tested and make decisions
based on the findings
DNA fingerprinting takes advantage of the fact that
individuals each have their own DNA sequence
DNA fingerprinting does not analyze
important genes (those are usually
identical among the population), but
instead analyzes sections of DNA that
have little or no function and vary widely
DNA fingerprinting uses restriction
enzymes and gel electrophoresis to
analyze and compare DNA
The human genome project was started
in 1990 and completed in 2003
Some of the major goals of the project
were to identify approximately 20,000
to 25,000 genes in human DNA and
help to improve data analysis of DNA
With the information from the
project, some scientists hope to be
able to cure genetic disorders
through gene therapy
In gene therapy, an absent or faulty gene
is replaced by a normal, working gene
Gene therapy is still a high-risk
experimental procedure

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