Human Chromosomes and Genetic Diseases
Name ________________________
In this activity, you will look at some genetic diseases, their causes, their symptoms, and their frequency (how
often they occur).
Goals:
By the end of this activity, you will:
1. Match chromosome pairs from human cells.
2. Use a karyotype to determine the sex of the “unknown person”.
3. Use a karyotype to determine which genetic disease (if any) your “unknown person” has.
4. Become familiar with the causes and symptoms of some genetic diseases.
Part I Procedure: (Using the Human Chromosome Spread and Karyotype Form)
1. You will cut out each pair of chromosomes and glue them on the correct line # on the “Human Karyotype
Form” to make each karyotype. (A hint: Cut and glue one pair at a time.)
2. Using the information from the 3 karyotypes and the information below to help answer the lab questions.
Part II Procedure: Read the background information and data about the 8 genetic diseases and answer
the questions that follow.
Background Information:
It wasn’t until 1956 that scientists were able to observe individual human chromosomes,
which are found in the nuclei of human cells. At this time, it was discovered that human cells
contain 46 chromosomes, which are in pairs.
In females all 23 pairs of chromosomes are alike, meaning that the chromosomes in each pair
look like each other. In males, however, only 22 pairs are alike. The 23rd pair of chromosomes
in males differs in size and shape. The 23rd pair of chromosomes (in males and females) is
called the sex chromosomes. The sex chromosomes- the X and Y chromosomes – determine
the sex of an individual.
Fertilization is the combining of the sperm and egg cell producing a zygote or fertilized egg. At
fertilization, the offspring has an equal chance of becoming male or female. When eggs are
formed in females, each egg carries an X chromosome. When sperm are formed in males, about
half the sperm carry an X chromosome and the rest carry a Y chromosome. Therefore, eggs
(from females) carry only X chromosomes; sperm can carry either X or Y chromosomes.
Genetic diseases are diseases that a person can inherit from his or her parents. During meiosis,
half of the chromosomes (and the genes on them) are given to you by your father and half of the
chromosomes (and genes on them) are given to you by your mother. For this to occur, the
chromosomes go through a process (meiosis) so that they are divided equally- so that the egg
gets copies of one-half of the mother’s chromosomes, and the sperm gets copies of one-half of
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the father’s chromosomes. Meiosis, therefore, is a cell division that allows the mother and
father each to donate copies of half of their chromosomes to their offspring. The egg and sperm
can then combine to form a zygote (or “future child” that results when an egg and sperm
combine). This zygote will contain 46 chromosomes-23 from the mother and 23 from the
father.
Because meiosis is a complicated process, mistakes can occur. For example, a zygote may get
an extra chromosome it doesn’t need, or it may get too few chromosomes, or parts of a
chromosome may be left out (deletions). Since these genetic mistakes that occur in the sex cells
(eggs and sperm) get passed on to the offspring, they are genetic diseases. Some of these
genetic diseases have very severe results. If a child is to be born free of genetic diseases,
several things may occur. First the child should have the correct number of chromosomes. A
chromosome count that is less than or greater than 46 results in a child with a genetic disease.
Second, the chromosomes that the child receives should be whole. If a child receives a
chromosome with a part missing in their set of chromosomes from parents, there is again a
chance of genetic disease. Third, chromosomes must be separate and not stuck to each other
during meiosis.
Data: Genetic Diseases:
Cri-du-chat (Cry of the Cat Syndrome) - A baby with the “cry of the cat syndrome” has a
cry, which sounds like that of a cat in distress because the infant’s larynx (voice box) is not
developed properly. The cause of this condition is that part of chromosome #5 is cut off. Cridu-chat babies are severely mentally disabled and have a small skull. The frequency of this
syndrome is 1 out of 100,000 live births.
Down’s Syndrome (Trisomy 21) - One of the most common causes of mental disabilities is
due to an extra chromosome #21. Therefore, instead of two #21 chromosomes, babies with
Trisomy-21 have three. This is usually caused by an error in egg production, and is more likely
to occur as the mother’s age increases. This results in a child who is short, has broad hands,
stubby fingers and toes, a wide rounded face, folds of skin at the corners of the eyes, a rather
large tongue that makes talking difficult, and mental and learning disabilities. These children
are more likely to have respiratory infections, heart defects, kidney and intestine malformations
and leukemia. The risk for Down syndrome with a mother of age 25 is 1/1,200 births, for a
mother of age 35 it is 1/260 births, and with a mother of age 45, it is 1/25 births.
Edward’s Syndrome - This disorder is caused by an extra chromosome #18 so instead of two
#18 chromosomes, the individual has three. This disorder results in severe mental disabilities
and physical symptoms such as a long skull, a very narrow pelvis, rocker bottom feet, low-set
ears, and small mouth and teeth. Nearly all babies born with this condition die in early infancy.
The frequency of this disorder is 1 out of every 5000 live births.
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Patau Syndrome - An extra chromosome #13 causes this disorder. This baby will be blind,
mentally disabled, have heart defects and very severe lip and palate clefts, a small nose, and a
small cranium. It results in severely abnormal cerebral functions and almost always leads to
death in early infancy. The frequency is 1 out of every 15,000 live births.
Turner’s Syndrome (XO) - This condition occurs when an X-carrying sperm fertilizes an egg
that doesn’t carry an X. It can also occur when a sperm lacking the X or Y chromosome
fertilizes an X-bearing egg, or when some of a received X chromosome is deleted. The
genotype of this individual is XO (the O = missing chromosome) and is a girl. These girls tend
to be shorter and chunkier than normal. At birth, they may have thick folds of skin running on
either side of the neck to the shoulders. At puberty, the secondary sex traits do not develop and
no eggs are produced. There is no menstruation or breast development. Decreases in verbal
performance and learning disabilities are common with this syndrome. Women with Turner
syndrome usually function well in society. The frequency is 1 out of every 2,500 live female
births.
Klinefelter’s Syndrome (XXY) - Males with this condition are tall, with long upper leg bones.
They have poor development of secondary sex characteristics, including having small testicles,
and are sterile (can not reproduce), but otherwise they appear normal. Sometimes these males
show breast development, and there is on average a slight decrease in verbal performance and
learning disabilities are common. These males have an extra X chromosome. Many men with
Klinefelter syndrome function very well in society, and are usually not diagnosed until they are
assessed for infertility as adults. The frequency is 1 out of every 1,000 live male births.
Jacob’s Syndrome (XYY) or Supermale Syndrome - Males with this condition are tall and
have low mental ability. These males have an extra Y chromosome. The frequency is 1 out of
every 1,000 live male births.
Triple X (XXX) or Superfemale Syndrome - Females with this condition appear physically
and mentally normal, but genetically they have three X chromosomes instead of two.
Frequency is 1 out of every 1,000 live female births.
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Questions
1. How many chromosomes are in each normal human body cell?
___________ chromosomes
2. Use the following words in the blanks in the sentence:
cell
chromosomes
DNA
gene
nucleus
___________________ are found in the _______________________, or control center of a
___________________. Chromosomes are made up of strands of ____________________. Sections of the
chromosome that code for traits, which are inherited from the parents are called _____________________.
3. In humans the 23rd pair of chromosomes are called the ____________ chromosomes because they determine
whether the individual is male or female. This pair of chromosomes in a female is _______ and in males is
________.
4. Draw a Punnett square using the sex chromosomes of males
(XY) and females (XX) along with the probability of the
offspring.
5. What are the chances of being born male? ___________
female_______________?
6. Which sex cell (egg or sperm) determines the sex of the offspring (male or female)? ________
Explain:__________________________________________________________________________________
_________________________________________________________________________________________
7. Define meiosis___________________________________________________________________________
8. Define zygote:___________________________________________________________________________
9. Define genetic disease ____________________________________________________________________
10. Define frequency________________________________________________________________________
11. Name three genetic diseases that occur because the person has more than 46 chromosomes.
1)___________________________ 2)_______________________________ 3)________________________
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12. Name one genetic disease that occurs because one part of the affected person’s chromosome is missing.
answer
13. Name two genetic diseases that happen only to males
____ and
14. Name two genetic diseases that happen only to females
_____and
______
15. Name the genetic disease where the person is not normal genetically (too many or too few chromosomes)
but the affected person is normal mentally and physically_________________________
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