Transcript Pedigree Charts and Detecting Disorders

V. Pedigrees & Detecting Disorders
*Human inheritance is hard to study because
humans mature slowly & produce few offspring.
A. Pedigree Charts
-Most knowledge concerning human
inheritance comes from examining the
heredity in human populations & families
(as shown in family trees & pedigree charts).
Pedigree chart – a chart which shows the
relationships within a family.
-Pedigrees help to determine the inheritance
of some alleles/disorders & predict them in
offspring, ex : Recessive hitchhiker’s thumb.
-If a person has a recessive trait that is not
expressed they are called a carrier of that
trait, ex : heterozygous female with normal
vision having the recessive trait for colorblindness.
Reading a Pedigree Chart
1. Circles represent females & squares
represent males.
Below is a pedigree chart of a family
showing four generations. A total of 20
2. A horizontal line connecting a male & individuals.
Generations are identified by Roman
female represents a marriage.
numerals. Individuals in each
3. A vertical line & a bracket connect
generation are identified by Arabic
the parents to their children.
numerals numbered from the left.
4. A shaded circle or square represents Therefore the affected individuals are
II3, IV2 and IV3.
a person who expresses the
trait/disorder.
5. A circle or square that is not shaded
represents a person who does not
express the trait/disorder.
6. If used in a pedigree chart, a
half-shaded circle or square
represents a carrier of the
trait/disorder.
Pedigree Charts
What is the genotype of person #1?
How is person #1 related to person # 5 &
person # 11?
Genetic Counseling
*Couples with genetic disorders in their family histories may
seek genetic counseling to predict the likelihood of having
children with the disorders.
Genetic counselor – a specialist who analyzes a pedigree
chart to infer the genotypes of family members.
-Example : One can infer that if a man passes a dominant trait
onto two of three children, and has a third child who is
recessive for the trait; that he MUST be heterozygous for that
trait.
-Genetic counselors trace both family histories, look for
inherited disorders, develop a pedigree, & provide the
probabilities of passing on the disorders to offspring.
B. Detecting Disorders
Methods of Detection
1. Allele testing – may be done by :
a. Labeled DNA probes – specific
DNA base sequences that detect
complementary base sequences
found in disease-causing alleles.
b. Testing for changes in restriction
enzyme cutting sites.
Restriction enzyme – enzyme that
cuts DNA at a specific sequence of
nucleotides.
c. Comparing lengths of normal vs.
abnormal chromosomes.
Detecting Disorders
2. Karyotype - picture of grouped pairs
of chromosomes.
3. Ultrasound – non-invasive
procedure of bouncing sound waves
off of a fetus to produce its image.
a. Can detect abnormalities in bone &
muscle but may be disruptive to
developing cells (ADD/ADHD).
4. Amniocentesis – invasive procedure
by which a long needle is placed into
the amniotic sac of cells shed by a
fetus & a sample of amniotic fluid is
withdrawn for testing.
*May cause a fetus to abort.
5. Fetoscopy – invasive procedure of
inserting a needle with a light and
special viewing scope into the
amniotic sac.
Detecting Disorders
Possible Solutions :
*The Human Genome
Project (an ongoing effort
to analyze the human DNA
sequence) has made gene
therapy a possibility.
Gene Therapy –
replacement of an absent or
faulty gene with a normal,
working gene.
A new gene is injected into an adenovirus
vector, which is used to introduce the
modified DNA into a human cell. If the
treatment is successful, the new gene
will make a functional protein.