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Transcript
Name _________________________________________ Test Date__________
UNIT IX - HUMAN INHERITANCE
I. HUMAN GENETICS (pp. 270 – 271, 299 - 301)
A. Human Somatic Cells
Human somatic cells (__body____cells) are ___diploid___ or 2n. Each
human somatic cell has __46__chromosomes, or 23 pairs of chromosomes.
Of these 23 pairs, 22 pairs are called __homologous__pairs, meaning they
contain the same genes in the same order. The 44 chromosomes that make
up the 22 homologous pairs in each cell are called _autosomes_______.
The 23rd pair of chromosomes are the __sex__ chromosomes. In female
somatic cells, the sex chromosomes are XX; in a male’s somatic cells, the sex
chromosomes are XY. All human somatic cells contain 44 autosomes and 2
sex chromosomes.
B. Human Gametes
Gametes are __haploid___ or n, and contain _23__ chromosomes. Female
gametes are _egg____ cells, and male gametes are __sperm____ cells.
Gametes are produced through the process of __meiosis___ in the ovaries or
testes, respectively. In meiosis, when the tetrad, or homologous pairs of
chromosomes separate in _anaphase I of meiosis___, the sex chromosomes
separate also. The resulting egg cell can ONLY contain an X chromosome,
while the sperm cell produced has a 50% chance of containing an X, and a
50% chance of containing a Y. Therefore, the _male___ determines the sex
of the offspring.
C. Analyzing Human Inheritance
A pedigree is a diagram that follows the inheritance of a single gene through
several generations of a family. In a pedigree, males are represented by
squares, and females are represented by circles. Individuals who are affected
by the trait are represented with shaded figures. Individuals that are not
affected by the trait are shown by non-shaded figures. Vertical lines
connect parents and children. Horizontal lines connect siblings or spouses.
1
Children are placed in birth order, from left to right.
Male
Female
The following pedigree shows the inheritance of a recessive trait.
1. How many generations are shown in this pedigree? ___V_____
2. How many children did Parents I-I and I-2 have? __3________
How many were boys? _1___ Girls?___2___
3. How many children did Parents II-I and II-II have? ___3_____
How many were boys? __2__ Girls?___1___
4. Key: R = non-affected; r = affected
5. Determine the genotypes of each person in this pedigree. Hint: label all
homozygous recessives first!!!
2
II. NUMBER DISORDERS
(pp. 311 - 315)
Number disorders occur because of a failure of a chromosome pair to separate
correctly in __anaphase I or II ____of meiosis. The most common type of
failure is __nondisjunction____________, which means chromosomes fail to
separate. Nondisjunction can occur during meiosis I, in which
__homologous pairs________ fail to separate or in meiosis II, in which
_sister chromatids______ fail to separate. In both cases, the gametes
produced have an abnormal number of chromosomes.
**Number disorders are NOT inherited; therefore, they CANNOT be
Predicted with _Punnett squares_**
A. Karyotypes
A karyotype is a photograph of chromosome pairs. Cells from the developing
embryo or individual being tested are cultured in a nutrient growth medium.
Then chemically treated to stop _mitosis_____ at ___metaphase_______.
The cells are stained, the chromosomes photographed, and the photograph
is enlarged. The chromosomes are cut out and arranged in
__homologous______ pairs in size order, with the sex chromosomes making
up the 23rd pair. Karyotypes can only be used to detect __number ______
disorders and to determine _gender_____________ of an unborn child. They
do not detect any other type of genetic abnormality; therefore, a normal
karyotype does NOT guarantee a normal child!
3
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B. Autosomal Number Disorders
Most autosomal number disorders are __lethal__. The ONLY autosomal
number disorder that allows survival into adulthood is Down syndrome. Down
syndrome is also known as Trisomy 21 because there are 3 chromosomes at
the 21st position, instead of 2. Individuals have characteristic facial features;
growth, behavior, and mental development are all retarded. There is also a
higher risk of congenital heart defects. The incidence of babies with Down
syndrome is much higher in older mothers.
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C. Sex Chromosome Number Disorders
1. Turner Syndrome – Also called 45 XO because individuals lack a 2nd
__sex chromosome_______. Patients are females, typically short in
stature, underdeveloped sexually, sterile, with a normal life expectancy.
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2. Klinefelter Syndrome – Also called 47XXY, because individuals inherit an
extra sex chromosome. Symptoms do not appear until puberty at which
time affected males show poor sexual development and infertility. Treated
with hormone therapy. Normal life expectancy.
7
III. SEX-LINKED INHERITANCE
(pp. 305 - 308)
A gene is referred to as “sex-linked” if it is located on a sex chromosome
(__X___ or __Y____). In humans, sex-linked genes are almost always located
on the larger __X____ chromosome. The ___Y_____ chromosome is much
smaller and carries only a few genes related to sexual development. Females
have 2 X chromosomes; males have 1 X.
Females will only show recessive traits located on the X chromosome if they
are homozygous recessive. But a male will always show a recessive trait
located on the X chromosome because he only has 1 X. This results in
_males_ having a much higher incidence of sex-linked disorders. Genotypes
for sex-linked traits are written using the X and Y chromosomes to show path of
inheritance. For example, red-green color blindness is a sex-linked recessive
trait. If C = normal vision and c = colorblindness, then a colorblind male = XcY;
colorblind female = __XcXc_. Females can be __carriers_ for sex-linked
recessive disorders. A carrier someone who has the defective allele, but
doesn’t __show the trait__. The genotype of a female carrier is _XCXc______.
Males CANNOT be carriers for a sex-linked trait because they only have one X
chromosome.
8
A. Sex-linked Disorders – All of these disorders are sex-linked RECESSIVE.
1. Color Blindness – Inability to differentiate and distinguish colors; for
example, red-green colorblindness
9
2. Hemophilia – Missing an enzyme required for normal blood clotting.
Have uncontrolled bleeding episodes. Treated with blood transfusions
or Factor VIII injections.
10
3. Duchenne’s Muscular Dystrophy – Symptoms develop at 3-6 years of
age. Causes muscle weakening and the breakdown of muscle fibers,
leading to eventual death. There is no available treatment or cure.
Death usually occurs before adulthood.
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B. Sex-Linked Punnett Squares – In sex-linked traits, probabilities for male
and female offspring must be calculated separately because traits are
inherited differently.
1. A colorblind female marries a man with normal vision. What is the
probability of them having colorblind children?
Key: ________________________________________
Cross: _______________________________________
Probability of having a colorblind daughter = ___________
Probability of having a colorblind son = ____________
2. A genetics counselor interviews a couple with a family history of
hemophilia to evaluate the possibility of having offspring with the disorder.
The woman does not have hemophilia, but states that her father had the
disorder. The man is normal.
Key: ________________________________________
Cross: _______________________________________
Probability of having a daughter with hemophilia = _____
Probability of having a son with hemophilia = ___________
12
V. AUTOSOMAL DISORDERS
(pp. 296 – 304)
Most genes are carried on the __autosomes__, 44 chromosomes other than
the sex chromosomes. So it follows that most genetic disorders are autosomal
disorders. These disorders affect males and females equally. Autosomal
disorders can be divided into three groups based on the pattern of inheritance.
13
A. Autosomal Recessive Disorders
1. Albinism – Characterized by failure to produce the pigment, _melanin_.
Affected individuals lack coloration in skin, hair, and eyes. Very
susceptible to skin cancer. Symptoms appear at birth; they have a
normal life expectancy.
14
2. Tay-Sachs Disease – Characterized by the inability of nerve cells to
break down a specific type of lipid. Symptoms appear between the ages
of 3-6 months. Lipid build-up causes seizures, blindness, degeneration
of mental & motor skills, and death at or before age 5. Historically
associated with Jewish population, although genetic testing has
decreased its incidence.
15
3. Cystic Fibrosis – Characterized by excess mucus production in lungs &
respiratory system. Symptoms appear just after birth and include
frequent respiratory infections, poor nutrition. With treatment, patients
can survive to their 20’s & 30’s. Cystic fibrosis is the most common
_fatal_ genetic disorder in the US among Caucasians.
16
4. Phenylketonuria or PKU – Characterized by an inability to breakdown
the amino acid, phenylalanine. Build-up results in severe brain damage
& mental retardation. All babies born in US hospitals are tested for PKU
because it is easily treated with a diet low in phenylalanine.
17
B. Autosomal Co-Dominant Disorders
Sickle cell anemia is an autosomal co-dominant disorder that affects
__hemoglobin_ production. Hemoglobin is the protein that binds
__oxygen___ to red blood cells. Individuals that are AA produce normal
hemoglobin.
1. Individuals that are SS produce abnormal hemoglobin that causes the red
blood cells to “sickle” when oxygen availability is decreased; for example,
in high altitudes or during periods of stress. Sickled RBC’s are more
fragile, easily destroyed – results in lack of energy due to decreased
_ATP_ production in cells, blockage of blood vessels, and severe brain
damage. Shortened life expectancy (40’s). Most common inherited
disease among African Americans.
18
2. Heterozygotes (AS) produce both normal and abnormal hemoglobin and are
said to have _sickle cell trait_. They do not show symptoms of the disorder.
In certain areas, individuals with sickle cell trait have a benefit over individuals
that lack the sickle cell _allele_ because they are resistant to _malaria_.
Malaria is a serious, sometimes fatal disease spread by mosquitos that affects
millions of people each year in Africa. This increased malarial resistance has
resulted in a very high incidence of AS individuals. If two heterozygotes marry
and have children, they have a _25%_% chance of having a child with sickle
cell anemia.
19
C. Autosomal Dominant Disorders
1. Marfan Syndrome – Dominant disorder that affects the connective tissue
of the skeletal system, eyes, and circulatory system. Affected individuals
have very long limbs, vision problems, and are susceptible to aortic
rupture.
20
2. Huntington’s Disease – Fatal genetic disorder in which symptoms do
not show until _40’s – 50’s_. Characterized by deterioration of nervous
system. Once symptoms begin, they are irreversible and _fatal_ - death
usually results within 10 – 25 years of onset of symptoms. Genetic
testing is available, but controversial because of knowing that you have a
predetermined fatal condition. Individual that has the Huntington allele
has at least a _50%_ chance of passing it on to the children and giving
them the disorder.
21
3. Achondroplasia - dominant disorder that causes a mutation in growth,
causes an abnormality in cartilage formation, severely shortened bones.
The disorder is fatal IF Individuals inherit _2__ dominant alleles.
Individuals with one dominant allele have little treatment available. Ex of
treatments include – hormone therapy, limb lengthening.
22