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Chapter 14 Notes
The Human Genome
Autosomes - Autosomes are all other chromosome pairs except the sex chromosomes.
Sex chromosomes - determine the sex of the organism. Males have XY sex chromosomes and
females have XX sex chromosomes.
I.
Karyotype – Picture of chromosomes arranged in pairs. A karyotype is a grouping of chromosomes
by size and type in a cell. Below is the karyotype from a male and a female.
Male 46XY (Diploid somatic cell)
Female 46XX (Diploid somatic cell)
II.
Pedigree chart – shows how a trait is transmitted from one generation to the next within a family.
Many traits such as the shape of eyes or ears are polygenic, means they are controlled by many
genes and phenotype of an organism is only partly determined by its genotype.
Many traits are strongly influenced by environmental, or nongenetic, factors, including nutrition and
exercise.
III.
The human genome—our complete set of genetic information—includes thousands of genes. Some
of the very first human genes to be identified were those that control blood type.
ABO blood groups and the Rh blood groups.
The Rh blood group is determined by a single gene with two alleles—positive and negative.
The positive (Rh+) allele is dominant, so person with Rh+/Rh+ or Rh+/Rh− are Rh- positive.
Individuals with two Rh− alleles are Rh-negative.
The ABO blood group has three alleles IA, IB, and i. Alleles IA and IB are codominant. These alleles
produce molecules known as antigens on the surface of red blood cells.
IV.
Single-Allele Traits / Disorders
A. Recessive allele disorders 1. Phenylketonuria or PKU –
a.
PKU is caused by an autosomal recessive allele carried on chromosome 12.
b.
lack the enzyme that is needed to break down phenylalanine (amino acid found in
milk and other foods).
c.
phenylalanine may build up in the tissues during the child's first years of life and
cause severe mental retardation.
2. Tay-Sachs Disease
a. A fatal genetic disorder causes nervous system breakdown and death
b. causes an inability to break down a certain type of lipid which accumulates in the brain
c. most often occurs in Jews whose ancestors lived in central Europe
3.
Cystic fibrosis
a. The disease is caused by a recessive allele on chromosome 7.
b. results in the production of a thick, heavy mucus that clogs their lungs and breathing
passageways.
c. caused by the deletion of 3 bases in the middle of a sequence for a protein removes just
one amino acid from this large protein, makes it to fold improperly. Because of this, the
cells do not transport the protein to the cell membrane, and the misfolded protein is
destroyed.
d. this protein normally allows chloride ions (Cl−) to pass across biological membranes.
e. Unable to transport chloride ions, tissues throughout the body malfunction.
B. Dominant-allele disorders –
1. Huntington’s disease
a. progressive loss of muscle control and mental function until death occurs.
b. no symptoms until thirties or older, then gradual damage to the nervous system begins.
2. Dwarfism known as achondroplasia
3. Dominant allele traits
a. dimples
b. widow’s peaks
c. farsightedness
d. broadlip
e. polydactyly (extra fingers and toes)
f. right-handedness
g. unattached ear lobes
h. ability to roll tongue
C. Codominance - both alleles express themselves in the heterozygote.
1. sickle - cell disease. (sickle-cell anemia).
a. genetic disorder that causes red blood cells to be shaped liked crescents or sickle These
sickle-shaped red blood cells are more rigid than normal cells and tend to get stuck in
the capillaries and blood stops moving through these vessels, damaging cells, tissues,
and organs.
b. one DNA base is changed which substitutes the amino acid valine for glutamic acid
which results in the abnormal hemoglobin (red blood cell protein) that less soluble than
normal hemoglobin. Low oxygen levels cause some red blood cells to become sickle
shaped.
c. the sickle-cell allele is codominant to the normal allele
d. the blood of people with this disease contains a mixture of normal hemoglobin and
sickle-cell hemoglobin (codominance)
e. found mainly in Africans and African Americans
f. people can be homozygous for this disease and then all the hemoglobin is the sicklecell type
g. People who are heterozygous for the sickle cell allele are generally healthy. In addition,
they have the benefit of being resistant to malaria. When the body destroys the sickled
cells, it also destroys the parasite that causes malaria.
V.
Sex -Linked genes - genes located on sex chromosomes.
A. Sex-Linked Disorders or Traits - Disorders or traits caused by genes on the sex chromosomes.
A sex-linked trait has alleles on only one of the sex chromosomes—usually the X because it is
much larger than the Y chromosome.
1. color blindness
a. the inability to distinguish between certain colors caused by a X-linked recessive allele
b. caused by defective version of any one of three genes associated with color vision
located on the X chromosome.
c. colorblindness is rare in females - Males have just one X chromosome. Thus, all Xlinked 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 X chromosomes.
This explains how some traits seem to be inherited by one sex more than the other. If a
male inherits a sex-linked trait from his mother, he is much more to show the disorder
than a female because he has only one X chromosome. A woman, who has two X
chromosomes is more likely to be a carrier.
2. hemophilia
a. disorder that prevents blood from clotting properly caused by a X-linked recessive
allele
b. protein necessary for normal blood clotting is missing
3. Duchenne Muscular Dystrophy
a. progressive weakening and loss of skeletal muscle
b. caused by a defective version of the gene that codes for a muscle protein
VI.
X-Chromosome Inactivation- Females have two X chromosomes, but males have only one.
In female cells, one X chromosome is randomly switched off. That turned-off chromosome forms a
dense region in the nucleus known as a Barr body. Barr bodies are generally not found in males
because their single X chromosome is still active.
In cats, a gene that controls the color of coat spots is located on the X chromosome. One X
chromosome may have an allele for orange spots and the other for black spots. 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. As a result, the cat's fur will have a mixture of orange and black spots. Male cats,
which have just one X chromosome, can have spots of only one color.
VII.
Chromosomal disorders
Nondisjunction is the failure of one or more homologous chromosomes (pairs) to separate during
meiosis. It can involve autosomes or sex chromosomes. Abnormal numbers of chromosomes may
find their way into gametes, and a disorder of chromosome numbers may result.
A. Down Syndrome –
1. trisomy on chromosome 21 (3 copies of this chromosome).
2. mild to severe mental retardation.
3. increased susceptibility to many diseases and a higher frequency of some birth defects.
B. Klinefelter syndrome
1. extra X chromosome (karyotype 47,XXY) interferes with meiosis and usually prevents
these individuals from reproducing.
C. Turner's syndrome
1. inherits only one X chromosome (karyotype 45,X).
2. Women with Turner's syndrome are sterile
VIII.
Human DNA Analysis
A. Testing for Alleles - prospective parents can find out if they might be carrying recessive alleles
for a genetic disorder as defective alleles have slightly different DNA sequences from their
normal counterparts. A variety of genetic tests have been developed that can spot those
differences.
B. DNA Fingerprinting - to the identification of individuals.
It does not analyze the cell's most important genes, which are identical among most people. It
rather analyzes sections of DNA that have little or no known function but vary widely from one
individual to another.
IX.
The Human Genome Project – it is an attempt to sequence all human DNA
Rapid Sequencing - Scientists first determined the sequence of bases in widely separated regions of
DNA. These regions were then used as markers which made it possible to locate and return to
specific locations in the genome.
Shotgun sequencing- involved cutting DNA into random fragments and then determining the
sequence of bases in each fragment. Computers put the fragments together by linking the
overlapping areas.
X.
Gene Therapy - process of changing the gene that causes a genetic disorder. In gene therapy, an
absent or faulty gene is replaced by a normal, working gene so that body can make the correct
protein or enzyme it needs, which eliminates the cause of the disorder.