Transcript Simple Mendelian Inheritance of Human Trait

Human Genetic Disorders
and Genetic Abnormalities
Autosomes
• 22 pairs of matching
homologous chromosomes
• (2 chromosomes in a
homologous pair look exactly
alike.)
Dominant Autosomal Heredity
• Simple dominant traits: free
earlobes, tongue rolling, curly
hair, long eyelashes, dimple, chin
cleft, freckles, Widow’s peak,
Polydactyly (having more than 5
fingers or toes).
* Dominant traits are always
expressed in a person’s
phenotype.
• Ex. Huntington’s disease
Huntington’s Disease
• Lethal genetic disorder
caused by rare autosomal
dominant allele (causes
death)
• Dominant allele on
chromosome 4 is the culprit
• deterioration of nervous
system, particularly the brain
• results in uncontrolled, jerky
movements of head and limbs
• no effective treatment
• occurs between ages of 30 and
50
• Mutation causing the disorder
changes a protein necessary for
normal development of brain cells
(called an expansion mutationmultiple repeats in the same DNA
fragment)
Achondroplasia
• Form of dwarfism
• Inherited as an autosomal dominant
disorder
• Affects approximately 1 in 10,000
people
• Skeletal cartilage does not form
properly
• Have abnormally short arms & legs
relative to other body parts
Hutchinson-Gilford progeria
syndrome
• Mutation that disrupts interactions
among genes that bring about
growth and development
• Observable appearances start
before age 2 – skin starts to thin,
skeletal muscles weaken, hair
loss is pronounced
• There are no documented cases
of progeria running in families, so
they suspect it arises from
spontaneous mutations. Probably
the mutated gene is dominant
over a normal allele on the
homologous chromosome
• Most die in their early teens from
strokes or heart attacks
Dyslexia
• People see and write letters or
parts of words backward
Recessive Autosomal Heredity
• Simple recessive traits:
attached earlobes, straight
hair, non-tongue roller, short
eyelashes, no dimples
• Most genetic disorders are
caused by recessive alleles.
Galactosemia
• Inherited as a recessive
autosomal allele
• About 1 in 100,000 newborns are
homozygous for the recessive
allele
• Do not have working copies of
one of the enzymes that digest
lactose (See page 174)
• High galactose levels causes
malnutrition, diarrhea, vomiting,
and damages to the eyes, liver,
and brain
• When untreated, galactosemics
die early. If placed quickly on a
restricted diet excluding dairy
products, they grow up symptom
free
Ex. Cystic fibrosis
• most common lethal genetic
disorder among White
Americans
• gene for this is located on
region of chromosome 7
• Mucus accumulates in digestive
tract and in lungs; breathing is
difficult, thickened mucus slows
secretion of digestive enzymes,
so food cannot be digested
properly.
• Treatments: Physical therapy,
special diet, and new drug
therapies
Ex. Sickle cell anemia – inherited
as autosomal recessive trait
• most common in black
Americans whose families
originated in Africa and white
Americans whose families
originated in countries
surrounding Mediterranean
Sea
• Red blood cells are shaped
like a sickle (half moon), not
round
• Life span of sickled cells is
shorter, therefore affected
person suffers from anemia
• Clog small blood vessels
causing tissue damage and
deprivation of oxygen and
nutrients
• Treatments: Blood transfusion
and drug therapy
Ex. Tay Sachs: autosomal
recessive disorder
• Affects on central nervous system
• Recessive allele results in
absence of enzymes that
normally breaks down a lipid
produced and stored in tissues of
central nervous system, therefore
lipid accumulates in the brain.
• Results in blindness, loss of
movement, mental deterioration
• Symptoms begin within 1st year
of life.
• Results in death before age 5.
• Allele is common among
Pennsylvania Dutch people and
Ashkenazie Jewish people.
Ex. Phenylketonuria (PKU)
• Results from absence of enzymes
that converts amino acids,
phenylalanine, to tyrosine.
• Phenylalanine accumulates in
body and damages central
nervous system.
• Tests are given at birth –so
infants affected by PKU are given
a diet low in phenylalanine until
brains are fully developed
Ex. Albinism
• Have no skin pigment.
• Have very pale skin and whiteblonde hair
• Affects all races
X-Linked Inheritance
• X-linked gene is found only on the
X chromosome
• Examples:
Colorblindness
Hemophilia
Colorblindness
• Inability to distinguish among
some or all colors
• Mutant forms of the X-linked
genes change the light-absorbing
capacity of sensory receptors
inside the eyes
• Normally, humans can detect
differences among 150 colors.
One with red-green
colorblindness sees fewer than 25
colors
Hemophilia
• Blood-clotting disorder
• About 1 in 7,000 males are
affected
• Treatments: Plasma, gene
cloning, Factor VIII drugs
MISTAKES IN MEIOSIS
(Genetic Disorders)
• During meiosis, sister
chromatids are to pull apart
from each other. If they do not
pull apart, you have an extra
chromosome or a missing
chromosome.
Aneuploidy – cells usually
have one extra or one
less chromosome
Autosomal aneuploidy is
usually fatal form humans
and is linked to most
miscarriages.
Polyploidy
• Cells have 3 or more of each type
of chromosome
• Half of all species of flowering
plants, some species of insects,
fish & other animals are polyploid
Nondisjunction
• Nearly all changes in chromosome
number arise through nondisjunction
whereby homologous chromosomes
fail to separate properly
• Can be trisomic (3 of one type of
chromosome and 2 of every other
type)
• Can be monosomic – lacking a
chromosome
Trisomy
• Have three of an autosome,
instead of 2
• have 47 chromosomes
Examples:
• Down syndrome – Trisomy
21(extra chromosome 21)
• Only autosomal trisomy in which
affected individuals survive to
adulthood.
• 95% of all cases arise through
nondisjunction at meiosis
• Occurs once in every 800 to 1000
births
• Affects more than 350,000 people
in the U.S.
• Affected individuals have upwardslanting eyes, somewhat flattened
facial features, fold of skin that
starts at the inner corner of each
eye, have moderate to severe
mental impairment and heart
defects, skeleton develops
abnormally, muscles and reflexes
are weak, speech & other motor
skills develop slowly
Klinefelter’s syndrome
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affects 1/500 males
Genotype is XXY
Sterile (Sperm count is low)
Taller than average, tend to be
overweight
• Exhibit some degree of mental
retardation
• Testes and prostate gland are smaller
than average
• They make less testosterone & more
estrogen than normal males, with
feminizing effects
• Hair is sparse, voice is high pitched,
& the breasts are a bit enlarged
• Testosterone injections starting at
puberty can reverse the feminized
effects
XYY– affects 1/1000 males
• During anaphase II, 2 Y
chromatids fail to separate and
sperm cell containing 2 Y
chromosomes is formed
• Normal intelligence
• Fertile
• Taller than average
XXX Syndrome
• Occurs at a frequency of 1 in
1000 live births
• Adults are fertile
• Have slight learning difficulties
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Monosomy
About 1 in 2500 to 10,000
newborn girls are XO
Lack a chromosome
Example: Turner’s syndrome
Genotype is XO
Lack functional ovaries and sex
characteristics are not fully
developed
• Sterile
• At least 98% XO embryos may
spontaneously abort early in
pregnancy.
• Only 4’8’’ tall
Karyotype
• Chart of chromosome pairs
used to identify an aneuploidy
(photograph of all an
organism’s chromosomes)
• Metaphase chromosomes are
photographed and chromosome
pictures are enlarged, cut apart,
and arranged in pairs on a chart
according to length and location
of centromere.
• Allows researchers to study
differences in chromosome
shape, structure, and size.
Karyotype