Lesson Overview

Lesson Overview
Human Chromosomes
Lesson Overview
14.2 Human
Genetic Disorders
Lesson Overview
Human Chromosomes
From Molecule to Phenotype
How do small changes in DNA molecules affect
human traits?
–Changes in a gene’s
DNA molecules can
change the proteins
produced by the DNA by
altering their amino acid
sequences this may
directly affect a person’s
–Ex. Sickle cell anemia
Lesson Overview
Human Chromosomes
Sickle Cell Disease
–caused by a defective allele
of the oxygen-carrying
protein (hemoglobin) in red
blood cells.
–The defective allele makes
hemoglobin less soluble,
causing hemoglobin
molecules to stick together
into long fibers, forcing
cells into a distinctive
sickle shape, which gives
the disorder its name.
Lesson Overview
Human Chromosomes
Sickle Cell Disease
–Sickle-shaped cells
are more rigid than
normal red blood cells.
–Sickle-shaped cells
tend to get stuck in the
–If the blood stops
moving through the
capillaries, damage to
cells, tissues, and even
organs can result.
Lesson Overview
Human Chromosomes
Lesson Overview
Human Chromosomes
Cystic Fibrosis
– Cystic fibrosis (CF) is prevalent among European
– Most cases result from the loss of a single amino
acid—phenylalanine, causing the protein to fold
Lesson Overview
Human Chromosomes
Cystic Fibrosis
–Children with CF have
serious digestive
problems and produce
thick, heavy mucus that
clogs their lungs and
breathing passageways.
–People with one normal
copy of the CF allele are
unaffected by CF,
–Two copies of the
defective allele are
needed to produce the
disorder, which means
the CF allele is recessive
Lesson Overview
Human Chromosomes
Lesson Overview
Human Chromosomes
Lesson Overview
Human Chromosomes
Huntington’s Disease
–Huntington’s disease is
caused by a dominant allele
for a protein (glutamine)
found in brain cells.
–Despite intensive study, the
reason this protein is found in
the brain cells is still not clear.
deterioration and
uncontrollable movements,
usually do not appear until
middle age.
Lesson Overview
Human Chromosomes
Genetic Advantages
– Most African Americans
today are descended from
populations that originally
lived in west central Africa,
where malaria is common.
– Malaria is a mosquito-borne
infection caused by a
parasite that lives inside red
blood cells.
Lesson Overview
Human Chromosomes
Genetic Advantages
–Individuals with just one copy of the sickle cell allele
are generally healthy, and are also highly resistant to
the parasite, giving them a great advantage against
Lesson Overview
Human Chromosomes
Genetic Advantages of Cystic Fibrosis
–More than 1000 years ago, the
cities of medieval Europe were
ravaged by epidemics of typhoid
–Typhoid is caused by a bacterium
that enters the body through cells in
the digestive system.
–The protein produced by the CF
allele helps block the entry of this
–Individuals heterozygous for CF
would have had an advantage when
living in cities with poor sanitation
and polluted water, and—because
they also carried a normal allele—
these individuals would not have
suffered from cystic fibrosis.
Lesson Overview
Human Chromosomes
Diploid Cells
•Cells with 2 sets of
•All normal human
cells are diploids
except gametes
•Abbreviated as 2n
Lesson Overview
Human Chromosomes
Haploid Cells
•In humans, sperm &
egg cells are haploids
•They only contain
one set of xm; half
the # in a diploid
•Sperm=n & Egg=n
(n + n= 2n)
Lesson Overview
Human Chromosomes
Chromosomal Disorders
What are the effects of errors in meiosis?
–The most common error
in meiosis nondisjunction
failure of homologous
chromosomes to separate.
– nondisjunction literally
means “not coming apart.”
–Nondisjunction may result
in gametes with an
abnormal number of
chromosomes, which can
lead to a disorder of
chromosome numbers.
Lesson Overview
Human Chromosomes
Chromosomal Disorders
–If two copies autosomes fail
to separate during meiosis,
an individual may be born
with three copies of that
–This is known as a trisomy,
meaning “three bodies.”
–The most common form of
trisomy, involving three
copies of chromosome 21, is
Down syndrome,
–Characteristics: mild to
severe mental retardation
and a high frequency of
certain birth defects.
Lesson Overview
Human Chromosomes
Chromosomal Disorders
–Nondisjunction of the X
chromosomes can lead to
a disorder known as
Turner’s syndrome.
–A female with Turner’s
syndrome usually inherits
only one X chromosome.
–Women with Turner’s
syndrome are: sterile,
which means that they are
unable to reproduce. Their
sex organs do not develop
properly at puberty.
Lesson Overview
Human Chromosomes
• Chromosome disorders are the most common cause of
first-trimester miscarriage.
• Most estimates suggest that at least 50% of all
miscarriages are due to chromosomal factors, with some
estimates putting the number at 75% or higher.
• Trisomies are the most common chromosome-related
finding in post-miscarriage testing.
• Of all trisomies, trisomy 16 seems to be the most
common in humans, occurring in 1% of all pregnancies.
Lesson Overview
Human Chromosomes
Trisomy 16
• A diagnosis of full trisomy 16 would mean that all the cells in
the baby's body were affected by trisomy 16.
• Full trisomy 16 is incompatible with life and nearly all affected
babies are miscarried in the first trimester.
– But it is also possible to have mosaic trisomy 16, meaning
some of the body's cells are affected and other cells are
It is also possible, in rare cases, to have a pregnancy in which the
placental cells have full trisomy 16 or mosaic trisomy 16 even
though the baby is chromosomally normal.
Full trisomy 16 may be diagnosed as the cause of a miscarriage if parents
collect tissue and request chromosomal testing after the loss, but trisomy
16 can also be diagnosed during pregnancy through CVS or amniocentesis.
Lesson Overview
Human Chromosomes
Lesson Overview
Human Chromosomes
Trisomy 18
•Clenched hands, Crossed legs
•Feet with a rounded bottom (rocker-bottom
•Low birth weight, Low-set ears
•Mental delay, Poorly developed fingernails
•Small head (microcephaly)
•Small jaw (micrognathia)
•Undescended testicle
•Unusual shaped chest (pectus carinatum)
Lesson Overview
Human Chromosomes
Trisomy 13 (Patau’s syndrome)
Cleft lip or palate
Clenched hands (with outer fingers on top of the
inner fingers)
Close-set eyes -- eyes may actually fuse together
into one
Decreased muscle tone
Extra fingers or toes (polydactyly)
Hernias: umbilical hernia, inguinal hernia
Hole, split, or cleft in the iris (coloboma)
Low-set ears
Mental retardation, severe
Scalp defects (missing skin)
Single palmar crease
Skeletal (limb) abnormalities
Small eyes
Small head (microcephaly)
Small lower jaw (micrognathia)
Undescended testicle (cryptorchidism)
Lesson Overview
Trisomy 13
Human Chromosomes
Lesson Overview
Human Chromosomes
Trisomy 22
•It is a frequent cause of
spontaneous abortion during
the first trimester of
•Progression to the second
trimester and livebirth are
Lesson Overview
Human Chromosomes
Girl with 8 limbs
Lesson Overview
Human Chromosomes
Chromosomal Disorders
–In males, nondisjunction may
cause Klinefelter’s syndrome.
–Klinefelter’s syndrome
results from having an extra X
–It usually affects males.
–It prevents these individuals
from reproducing.
–There have been no reported
instances of babies being born
without an X chromosome,
indicating that this chromosome
contains genes that are vital for
the survival and development of
the embryo.
Lesson Overview
Human Chromosomes
•They are considered to be males
•As babies have weak muscles and
reduced strength. They may sit up, crawl,
and walk later than other infants.
•Puberty- don't much testosterone as
other boys.
–leads to a taller, less muscular body,
less facial and body hair, and broader
hips than other boys.
•As teens-may have larger breasts, weaker
bones, and a lower energy level than other
•Adulthood-look smilar to males without
the condition, although they are often
taller. They are also more likely than other
men to have certain health problems, such
as autoimmune disorders, breast cancer,
vein diseases
Lesson Overview
Human Chromosomes
Man with enlarge mammary glands
Lesson Overview
Human Chromosomes
If sex cells were diploids…
•The zygote will have 92 xm and will NOT
be viable