Human Genetics
Children tend to look like their parents. Often a child may look more like one parent than the
other. The study of how and why children look like their parents is called genetics. How we
look is just a small part of our genetic heritage. Our genes influence the way our bodies work
and thus, our health.
Objectives
By the end of this laboratory you will be able to:
1. determine your phenotype and possible genotype for several physical traits.
2. determine compatible blood types for transfusions.
3. describe why sex-linked traits occur more frequently in males than females.
4. know the significance of the Barr body.
5. know all the boldface terms.
Dominant and Recessive Traits:
We get one half of our genes from each parent. Yet, we may look more like one parent than
the other. Why? ________________________________________________________________
A gene that governs a particular trait may come in alternative forms called alleles.
Not all of
the alleles that we inherit are shown or expressed. Some alleles, the dominant ones, cover the
expression of the recessive alleles. The recessive alleles don’t disappear. They may show up in
future generations when not paired with a dominant allele. When the same allele for a single
gene is on both the paired homologous chromosomes the individual is homozygous for that
trait. When the paired chromosomes carry different alleles for the same gene they are said to be
heterozygous.
Physical Genetic Traits:
Hundreds of traits combine to determine appearance. A single gene determines some traits
like tongue rolling. Others, like skin color, are determined by the combined effects of several
genes. In the following procedure you will determine your phenotype, the detectable expression
of a gene (what is shown), for several physical traits. Then you will determine your possible
genotypes, or the alleles in your genetic makeup, for these traits.
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Procedure:
1. Using Figure 1, have your lab partner help you determine your phenotype for each
characteristic listed and record your phenotype in Table 1.
2. Next determine your possible genotypes for the traits listed and record in Table 1. Note
the dominant alleles are represented by capital letters and recessive alleles by small
letters. For example: “H” for hitchhiker’s thumb and “h” for normal thumb. If the
recessive trait is expressed in the phenotype then it is always homozygous in the
genotype “hh”. If the dominant allele is expressed it may be either homozygous “HH” or
heterozygous “Hh”.
Table 1. Physical traits.
Dominant
Phenotype
Genotype
(there may be several)
Example: (R) Tongue Roller
Roller
R R or R r
Recessive
(r) Can’t roll tongue
(R) Tongue roller
(r) Can’t roll tongue
(W) Widow’s peak
(w) Straight hair line
(E) Unattached earlobes
(e) Attached ear lobes
(P) Ear points
(p) No ear points
(H) Straight thumb
(h) Hitchhiker’s thumb
(C) Cleft chin
(c) “Normal” chin
(R) Right thumb on top
(r) Left thumb on top
(D) Dimples
(d) No dimples
(F) Freckles
(f) No freckles
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Dominant
Tongue roller
Recessive:
Can’t roll tongue
Dominant:
Widow’s Peak
Recessive:
Straight Hairline
Dominant:
Unattached earlobes
Recessive:
Attached earlobes
Dominant:
Has ear points
Recessive:
No ear points
Dominant:
Straight thumb
Recessive:
Hitchhiker’s thumb
Dominant:
“Normal” chin
Recessive:
Cleft chin
Dominant:
Right thumb on top
Recessive:
Left thumb on top
Dominant:
Has dimples
Recessive:
No dimples
Dominant:
Has freckles
Recessive:
No Freckles
Figure 1. Look at Yourself
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Multiple Alleles: Human Blood Groups
Red blood cells have molecules called antigens on their surface. An antigen is a molecule
that causes the immune system to produce antibodies against it. When a foreign antigen enters
the body the immune system will build antibodies against it. ABO antigens are attached to
human red blood cells. The surface antigens on red blood cells are coded for by one gene that
has three different alleles. This is an example of a multiple allelic system. The IA allele
determines the A antigen, the IB allele determines the B antigen, and the “i” allele determines no
antigens (type O).
An individual carries a matched pair of chromosomes and thus has two alleles for the ABO
blood groups. Two alleles may be expressed at the same time. If an individual has IA and IB,
they will have type AB blood. Since both alleles are expressed, this is an example of
codominance. The possible genotypes and phenotypes are listed in Table 2. If red blood cells
with foreign antigens on them enter the body the antibodies produced against them will cause the
blood to clump (not clot—clotting is something quite different). A normal person never makes
antibodies against his own antigens. If this occurred it would be disastrous! Imagine a person
with A antigen making antibodies against A and clumping his own blood cells. Instead, a person
with A would make antibodies against foreign antigens such as B (anti-B antibodies). This is
why only a blood type that is compatible can be used during blood transfusions. For example,
type A blood can only be given to persons with type A or type AB blood (See Table 2). The Rh
system is completely separate, but works in much the same way. If you have the Rh antigen
present on your red blood cells, you are Rh+. If it is absent, you are Rh-.
Genotype
IAIA
IAi
IBIB
IBi
IAIB
ii
Rh+, Rh+
Rh+, RhRh-, Rh-
Table 2. Human Blood Groups
Phenotype
Antigen present
A
A
A
A
B
B
B
B
AB
A&B
O
None
Rh+
Rh
Rh+
Rh
RhNone
Antibody produced
Anti-B
Anti-B
Anti-A
Anti-A
None
Anti-A & Anti-B
None
None
Anti-Rh (after exposure)
Blood type O is sometimes called the universal donor. Why? ____________________________
Type AB is sometimes called the universal recipient. Why? _____________________________
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Determining Your ABO Blood Type:
In this procedure you will use a serum with antibodies in it against a specific antigen.
Conduct a blood typing exercise as demonstrated by your instructor.
Red Blood Cell Antigen
A
B
AB
O
Rh
Table 3. Blood Antigen Test
Antiserum Added
Anti-A
Anti-B
Anti-A & Anti-B
Neither Anti-A nor Anti-B
Anti-Rh
Results
clump
clump
both clump
no clump
clump
What blood type are you? ________________________________________________________
What are your possible genotypes? _________________________________________________
What type(s) of blood could you safely receive in a transfusion? __________________________
To which type(s) of blood could you safely donate in a transfusion? _____________________
Sex Determination in Humans: X and Y chromosomes
Human cells contain 23 pairs of chromosomes and 46 chromosomes in all. One chromosome in
each pair comes from each parent. Twenty-two of the 23 pairs are matched and are called
autosomes. The chromosomes in these 22 pairs are similar in size, shape, and the genes that
they carry. The 23rd pair, XX or XY, determines the sex of the individual and are called sex
chromosomes. Each egg contains one X chromosome. Each sperm contains either one X or one
Y chromosome. If an “X” sperm fertilizes the “X” egg, the child is female (XX). See Figure 2.
If a “Y” sperm fertilizes the “X” egg, the child is male (XY). Notice that the only thing that
determines sex is the presence or absence of the Y chromosome. If you have a Y, you are male.
If you lack the Y, you are female.
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Sperm
Figure 2. Sex determination in humans.
Barr Bodies:
The X chromosome is fairly large and carries numerous genes necessary for life. The Y
chromosome is tiny and has only a few genes. Early in the development of the female embryo
one of the X chromosomes becomes inactive in each cell. This means that only one of the two X
chromosomes is actually working. The inactive X becomes condensed and can be seen in certain
cells as a Barr body. There is always only one active X chromosome in human cells. Thus, if
the person is a normal female there should be one Barr body present and a normal male should
not have any. People with extra X chromosomes will have more than one Barr body. For
example an XXX female would have two Barr bodies. In the past Olympic athletes were
required to have a Barr body test to determine their genetic sex before entering any events.
How many Barr bodies would you expect in a female with 4Xs (XXXX)? ___________________
How many in an XXY male? ______________________________________________________
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Procedure:
1. Obtain a clean microscope slide.
2. Use the blunt end of a toothpick to scrape the inside of your cheek to obtain some
epithelial cells.
3. Smear the cheek scraping onto the slide and add a drop of methylene blue.
4. Stir the dye and cells together and set aside for 2 minutes.
5. Add a coverslip, cover the slide with a piece of tissue and gently press straight down on
the coverslip with your thumb to spread the cells.
6. Examine the slide with your microscope. Can you see any Barr bodies? The Barr body
appears as a darkly stained structure against the nuclear envelope.
7. Exchange slides with someone of the opposite gender.
8. As an alternative, look at one of the prepared demonstration slides.
Which sex has Barr bodies? ________________Why? _________________________________
Draw a cell containing a Barr body in the space provided.
Sketch of a cheek cell with Barr body.
Sex-Linked Traits
Color blindness and hemophilia are examples of genetic disorders that are due to a change in a
gene on the X chromosome. Females rarely show these recessive disorders because a gene on
their second X chromosome may cover the effects of the changed gene. Males have these
disorders more often because the Y chromosome does not have a gene that can cover the effect
of the changed X gene. Color blindness is an X-linked, recessive trait. Therefore, the possible
genotypes are
Females
Males
XCXC = normal vision
XCY = normal vision
XCXc = normal vision (carrier)
XcY = colorblind
XcXc = colorblind
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Procedure:
1. Have your lab partner administer the color blindness test to you. Directions are included
in the test books.
2. Write down the number you see so your partner does not see it.
It is important that you
do not discuss the numbers because colorblind people see a different number or figure
than non-colorblind people.
Number of correct answers ________________Are you colorblind? _______________________
What are your possible genotypes? _________________________________________________
A female who is not colorblind may determine whether she is homozygous or a carrier
(heterozygous) by knowing if any member of her family is colorblind. If her father is colorblind,
what is her genotype? ___________________________. If her mother is colorblind, what is her
probable genotype? _____________________________________________________________.
Why would more males be colorblind than females? ___________________________________
_____________________________________________________________________________
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