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Lab Exercise
15
Human Genetics
Introduction
Contents
Activity 15.1
Activity 15.2
Activity 15.3
Objectives
Introduction
Human Karyotype
Some Human Traits
Pedigree Analysis
Resutls Section
Objectives
- Become familiar with the human karyotype.
- Be able to recognize abnormal karyotypes.
- Be able to diagram a pedigree for a specific trait.
- Be able to interpret a pedigree for a specific trait.
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While Mendelian genetic principles apply to humans just
as they apply to all other organisms, as a society we
constrain ourselves from conducting breeding experiments
on human subjects in an effort to gather information on
the nature of inherited traits. Consequently we rely on
other sources of information to obtain this knowledge.
One source of information comes from study of the human
karyotype, the number and appearance of our chromosomes. Another source of information is from the study
and interpretation of pedigrees.
The number and appearance of the chromosomes as
they enter metaphase is called an organism’s karyotype.
Irregularities seen in an individual’s karyotype usually lead
to abnormalities which in most cases are quite detrimental
to the individual. The first exercise in the lab has you examine the human karyotype and then look at some of the
abnormalities which may occur.
When using pedigree analysis to study the nature of inheritance of particular traits, families are investigated to determine which individuals within the family possess the trait
in question. Then using deductive reasoning, the nature
of the trait’s inheritance in inferred. The second exercise
in this lab asks you to determine your phenotype for some
inherited traits. The third exercise involves the construction
and interpretation of pedigrees.
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Activity 15.1
Human Karyotype
Activity 15.2
Pedigree Analysis
An individual’s karyotype is the appearance and number
of the chromosomes in their cells. The normal human
karyotype consists of 22 pairs of chromosomes of various
sizes plus a pair of sex chromosomes for a total of 46
chromosomes. Examine the normal chromosomes of a
normal human karyotype in the Human Genetics simulation on the BiologyOne DVD. Then examine and read
about some of the abnormal karyotypes that occur. These
abnormalities are the result of chromosomes failing to
separate properly during meiosis, events that are referred
to as nondisjunctions.
Since it is neither practical nor morally acceptable to breed
humans in an effort to determine the nature of genetic
inheritance of specific traits, scientists rely on another
method to infer modes of inheritance. This is the study of
family trees or pedigrees. By analyzing the pedigree, one
may be able to deduce how a gene for a specific trait is
inherited.
Examine the chromosomes displayed in the Results
Section. These are how they would appear through the
microscope when stained. Try cutting these out and arranging these to show the individual’s karyotype. Use the
chromosome size, location of the centromere and banding
patterns to pair chromosomes. Does this individual have a
normal karyotype? What is their sex?
You can also try constructing a karyotype using the illustration of the chromosomes in the Results Section (this
will probably be easier than pairing with the images of the
actual chromosomes). What is this individual’s sex? Is
their kryotype normal?
In a pedigree analysis, individuals are represented by
symbols. Open shapes usually represent individuals who
do not express the trait while filled shapes represent
individuals who express the trait being investigated. If
heterozygous individuals can be determined they are usually represented by partially filled shapes. Unions and the
offspring from unions are represented in the pedigree by
connecting lines as indicated in below.
Study the examples of a pedigrees shown in the Results
Section and then answer the questions concerning the
other pedigrees. Use the information in the Results Section to construct a pedigree.
If you were able to collect information from your family for
some of the genetic traits listed in section 15.2, select one
of those traits and construct the pedigree for your family
for that trait.
Symbols used in Pedigree Charts
male without trait
female with trait
male carrier
individual of unknown sex
male with trait
identical twins
male/femlae union
female without trait
siblings
female carrier
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fraternal twins
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Activity 15.3
Some Human Traits
Through pedigree analysis, a number of human characteristics have been determined to be controlled by a single
gene and are easily demonstrated to follow Mendelian
laws of inheritance and the Hardy-Weinberg predictions.
Several of these traits will be explored in this exercise.
Use the following descriptions of traits to determine
your phenotype and possible genotype(s). Record your
observations in the Report Section. If you are able, also
determine the phenotypes of your relatives. Your instructor may also collect data from the class to determine the
population frequencies for these traits.
Hitch-hiker’s Thumb. If you can bend the tip of your thumb
backwards so that the last joint is nearly a right angle to
the rest of your thumb, you have what is called a hitchhiker’s thumb. Hitch-hiker’s thumb is recessive (h) to a
non-hitch-hiker’s thumb (H).
Dimples. One or more facial dimples (D) is dominant over
no dimples (d).
Tongue Rolling. The ability to roll one’s tongue lengthwise
is a genetic inherited trait. The ability to roll your tongue
(R) is dominant to the inability to roll your tongue (r).
Freckles. Freckles (F) are dominant over no freckles (f).
Widow’s Peak. If your hairline forms a point in the middle
of your forehead you have what is called a widow’s
peak. The allele for a widow’s peak (P) is dominant to no
widow’s peak.
Interlace Fingers. Clasp your hands together and look at
the position of your thumbs. If your left thumb is over the
right thumb, you have the dominant phenotype (II or Ii).
If your right thumb is over your left thumb you have the
recessive phenotype (ii).
Red-green Color Blindness. This is a sex-linked, recessive
trait. Since females have two copies of the X chromosome,
normal color vision females can be either CC or Cc while
color blind females will be cc. Normal males will have the
genotype CY and color blind males will be cY. At the end
of the Human Genetics simulation on the BiologyOne CD
is a color dot pattern you can use to test your color vision
but use the results of this test with caution since computer
color displays are somewhat variable.
Bent Phalanges. If you hold your hands with your palms
toward you and the last joints of your little fingers flare
away from each other, you have the trait known as bent
phalanges. Bent phalanges (B) is dominant over straight
phalanges (b).
Earlobes. If your earlobes hang below the point where
your ear attaches to your head, you have free earlobes. If
they do not hang down, you have attached earlobes. Free
earlobes (E) are dominant to attached earlobes (e).
Long Palmar Muscle. When you clench both fists, palms
up, and slowly rotate your hand, do you see two or three
tendons in your wrist. If you only see two tendons in both
wrists you do not have long palmar muscles and your
genotype is pp. If both wrists have three tendons, you are
probably homozygous for this trait (genotype PP). If only
one wrist has three tendons, you are probably heterozygous (Pp).
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Illustration of Some Human Traits
attached earlobe
free earlobe
straight hairline
bent phalanges
straight phalanges
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widow’s peak
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Lab Exercise
15
Name _______________________
Results Section
Activity 15.1
Human Karyotype
Cut out the photograph of chromosomes on the next page and arrange them in pairs in the spaces below.
Chromosome pairs are numbered from largest chromosomes to smallest with the sex chromosomes shown
last.
________
1
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2
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3
________
4
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5
________
6
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7
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8
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9
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10
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11
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12
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13
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14
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15
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16
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17
________
18
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19
________
20
________
21
________
22
________
sex
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Chromosomes
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Cut out the illustration of chromosomes on the next page and arrange them in pairs in the spaces below.
Chromosome pairs are numbered from largest chromosomes to smallest with the sex chromosomes shown
last.
________
1
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2
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3
________
4
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5
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6
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7
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8
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9
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10
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11
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12
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13
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14
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15
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16
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17
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18
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19
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20
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21
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22
________
sex
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Chromosomes
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Activity 15.2
Pedigree Analysis
Example of pedigree analysis: The following pedigree is for myopia (nearsightedness).
What is the nature of its inheritance?
1
5
2
6
7
8
3
4
9
10
11
12
13
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Analysis:
1. The trait appear to be equally expressed in males and female so it is likely not sex linked.
2. The trait occurs in individual #1 but in none of his children and the trait does not occur in
parents 3 and 4 but does occur in their children. Conclusion: the allele for this trait is
probably recessive.
Test of conclusion: If the allele for myopia is recessive, individauls 1, 11, and 13 must be homozygous
for this trait. Others may be heterozygous or homozygous non-myoptic. Each child from
parents 1 and 2 would be heterozygous, inheriting the myoptic allele from their father. For
individual 11 to inherit the trait from parents 3 and 4, both parents must be heterozygous. They
would then be expected to have 1 out of 4 children with the trait (lucky here). Each normal
child would have a 2/3 chance of being heterozygous.
Diagram a Pedigree
In the below space, diagram a family pedigree in which a couple has 5 children, 3 boys and 2 girls. One
of the girls and a boy are fraternal twins. The mother’s phenotype displays the trait in question. Two of
the boys (not the twin) and the twin girl display this phenotype as well. One of the boys, not the twin,
marries a woman who does not express this phenotype. They have two girls, neither of which has this
phenotype.
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Interpreting a Pedigree
The trait denoted in the following pedigree is known to be recessive.
Answer the below questions.
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3
9
5
4
10
2
6
7
11
1. What is the possible genotype of individual #1? For individual #4?
2. Do you think this is trait is sex-linked?
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If you collected family data for the human traits
- construct a pedigree of your family below
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Activity 15.3
Some Human Traits
Your phenotype
Father's phenotype
& genotype
Mother's phenotype
& genotype
& genotype
Tongue
Rolling
Freckles
Widow's
Peak
Bent
Phalanges
Earlobes
Long
Palmar Muscle
Hitch-hiker's
Thumb
Dimples
Interlace Fingers
Red-gree Color
Blindness
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Sibling phenotypes
% of class
& genotypes
with trait