Lab 5: Human Genetic Disorders
Adapted from Learn.Genetics http://learn.genetics.utah.edu/content/disorders/whataregd/
and Bio10 Laboratory Activities by V. Annen
A genetic disorder is a disease that is caused by an abnormality in an individual's DNA.
Abnormalities can range from a small mutation in a single gene to the addition or
subtraction of an entire chromosome or set of chromosomes. Three common categories
of genetic disorders are:
Single gene disorders – These disorders are caused by a mutation in the DNA
sequence of one gene. Genes code for proteins and when a gene is mutated its protein
product can no longer carry out its normal function. Single-gene disorders are inherited
in autosomal dominant, autosomal recessive, and X-linked patterns.
Chromosome abnormalities – In these disorders, entire chromosomes, or large
segments of them, are missing, duplicated, or otherwise altered. Gametes with extra or
missing chromosomes can be formed during meiosis and result in individuals with an
abnormal number of chromosomes. An example is Down syndrome (trisomy 21).
Multifactorial disorders – These disorders are caused by mutations in multiple genes.
Environmental factors also play a role in many multifactorial disorders. Many common
chronic disorders, including hypothyroidism, colon cancer, breast/ovarian cancer,
Alzheimer’s disease are multifactorial. Heritable traits such as height, eye color and skin
color are also multifactorial.
Examples of single gene disorders:
1. Phenylketonuria (PKU) is caused by a mutation in the PAH gene
located on Chromosome 12. The gene codes for a protein called
phenylalanine hydroxylase (PAH). This enzyme breaks down the amino
acid phenylalanine. When this gene is mutated, the shape of the PAH
enzyme changes and it is unable to properly break down phenylalanine.
Excess phenylalanine in the blood is toxic to nervous tissue and can
cause brain damage. PKU is an autosomal recessive disorder.
Answer the questions about this genetic disorder on the Group Lab
Report sheet.
2. Achondroplasia Dwarfism is caused by a mutation in the FGFR3 gene located on
Chromosome 4. Mutations in this gene result in abnormal skeletal formation. 80% of
people with Achondroplasia have average-size parents; these cases result from a new
mutation in the FGFR3 gene. In the remaining cases, Achondroplasia is inherited in an
autosomal dominant pattern. The homozygous dominant genotype is lethal and
individuals are often stillborn or die shortly after birth.
Answer the questions about this genetic disorder on the Group Lab Report sheet.
3. PTC: Genes and bitter taste. PTC stands for phenylthiocarbamide, a chemical
compound made up of carbon, hydrogen, nitrogen, and sulfur. The ability to taste PTC is
determined by a single gene, TAS2R38, is located on Chromosome 7. The gene codes
for a taste receptor on the tongue that binds PTH. There are two common forms (or
alleles) of the RTC gene. One is a ‘tasting’ allele, and the other is a ‘non-tasting’ allele.
About 75% of the population has the ability to taste PTC and the other 25% can't taste
PTC.
Taste PTC paper and record your genotype and phenotype on the Group Lab Report
sheet..
Examples of chromosome abnormalities
Chromosomal abnormalities can be screened for during a woman’s pregnancy by taking
a sample of cells from the placenta, amniotic fluid, or umbilical cord to examine the
baby's chromosomes and determine if he or she has the correct number and the correct
size of chromosomes.
1. Trisomy 18 is a genetic disorder in which a person has a third copy of chromosome
18. It is a relatively common syndrome that is three times more common in girls than
boys. The extra chromosome interferes with normal development.
2. Down syndrome (Trisomy 21) is caused by an extra copy of chromosome 21.
People with Down syndrome have distinct facial features and moderate to severe mental
retardation. They also have an increased risk of developing a number of medical
problems. Because of this, most people with Down syndrome have a decreased life
expectancy. About half live to be 50 years of age.
3. Klinefelter Syndrome is a disorder that affects only males. Males normally have an X
chromosome and a Y chromosome (XY). But males who have Klinefelter syndrome have
an extra X chromosome (XXY), giving them a total of 47 instead of the normal 46
chromosomes. Klinefelter syndrome is one of the most common genetic abnormalities. It
affects between 1 in 500 and 1 in 1,000 males.
4. Turner Syndrome is caused by a missing X chromosome. Females with Turner
syndrome have the XO genotype (only 1 X chromosome). The OY genotype is lethal so
there are no males with Turner Syndrome.
Complete the karyotypes in the Group Lab Report and determine what genetic
disorders, if any, are indicated.
Multifactorial disorders
Example: heart disease. Construct a pedigree using the instructions provided. Turn in
your pedigree with your lab report.
Names ________________________________
________________________________
Lab 5 Group Results Sheet
Single Gene Disorders
1. Determine the probability of two carriers for PKU having a child with PKU.
2. Examine the results of the Silva Family’s genetic testing. The dominant
(normal) gene is larger (4.2 kb) and does not travel as far in a gel during
electrophoresis. Does their baby (C2) have PKU?
P1
P2
C1
C2
P1 and P2 are the parents: C1 is the first child, C2 is the second.
3. Does the first child (C1) have the disorder or is s/he a carrier?
4. What type of diet is prescribed to individuals with PKU?
5. Give the genotype of an adult with achondroplasia dwarfism.
6. What are the chances that two achodroplasic dwarfs will have a normal sized
child?
7. Give the genotypes for individuals with the following phenotypes:
[T = tasting gene, t = non-tasting gene]
a. taster – finds PTC intensely bitter
b. taster – finds PTC somewhat bitter
c. non-taster
8. Taste PTC paper and record your genotype and phenotype.
9. What might be the evolutionary advantage of being an individual capable of
detecting bitter tasting substances?
Karyotypes: Screening for Chromosome abnormalities
A karyotype is an organized profile of a person's chromosomes. In a karyotype,
chromosomes are arranged and numbered by size, from largest to smallest.
10. Complete the Cut ‘n Paste Karyotyping activity and show your work to the
instructor – or hand it in with your lab report.
11. Can you determine if the child is male or female from the karyotype? How?
12. Explain what trisomy is and how it occurs.
13. Examine the 4 karyotypes on the sheet provided in lab. Fill in the following
information about each karyotyype:
Karyotype #1
Karyotype #3
Patient # _______
Patient # _______
Number of chromosomes ______
Number of chromosomes ______
What is the sex? M F (circle one)
What is the sex? M F (circle one)
Normal or mutated (circle one)
Normal or mutated (circle one)
If mutated, name the disorder:
If mutated, name the disorder:
_____________________________
_____________________________
Karyotype #2
Karyotype #4
Patient # _______
Patient # _______
Number of chromosomes ______
Number of chromosomes ______
What is the sex? M F (circle one)
What is the sex? M F (circle one)
Normal or mutated (circle one)
Normal or mutated (circle one)
If mutated, name the disorder:
If mutated, name the disorder:
_____________________________
____________________________
Multifactorial disorders
Construct a pedigree using the instructions provided. Turn in your pedigree with
your lab report.