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22
Human Genetics
After you have finished reading this chapter, you should be able to:
Discuss
ways human traits are inherited.
Distinguish
between autosomes and sex chromosomes.
Discuss
several sex-linked traits in humans.
Explain
the work of a genetics counselor.
Good health and good sense are two of life’s greatest blessings.
Publilius Syrus, Maxim 827
Introduction
“Is it a girl or a boy?” At the moment of birth, parents have asked this
question in every language for as long as there have been languages. The
next question, whether spoken out loud or whispered, is asked with a
mixture of hope and fear: “Is it healthy?”
Almost always, the process of inheritance results in a wonderful newborn with a unique combination of genetic traits. With these traits and a
supportive environment, the baby will lead a healthy life. But this is not
the case for all newborns. (See Figure 22-1.)
Genetic disorders occur in some infants. These disorders are not caused
by an infectious microorganism. Instead, genetic disorders result from
inborn errors of metabolism that are caused by defects in genes. Can we
predict how often genetic disorders will occur? Can we treat people born
with genetic disorders? Is it possible to learn if a baby has a genetic disorder before birth? These and other important questions will be examined in this chapter on human genetics.
468
Chapter 22 / Human Genetics 469
Figure 22-1 The process of
inheritance results in a newborn
with a unique combination of
genetic traits.
■■ GENETIC COMPLEXITIES:
BEYOND THE PEA PLANTS
Much of Mendel’s success was due to the traits he studied. The pea plants
he worked with were either tall or short, and their seeds were either yellow or green, smooth or wrinkled. And so on. But look at your classmates.
Are people either tall or short? Are there only two skin colors? Do people
have only blond or black hair? Of course not. In humans, and in most
organisms, almost all traits are not as clearly defined as the traits Mendel
studied in pea plants. Was Mendel correct in his explanations? Was
Mendel extraordinarily lucky in picking the traits he studied? The answer
to both questions seems to be yes.
The traits Mendel picked to study were each determined by single
genes. The height of the pea plants is due to a single gene that occurs in
two different versions, or alleles. Human height is a different story.
Height in humans is due to several genes. Pieces of DNA on different
chromosomes code for the proteins that affect human height. A trait
determined by several genes follows a pattern of polygenic inheritance.
If a large group of people were arranged according to height, those with
average height would be the most numerous. There would be fewer
extremely short and fewer extremely tall people. Grouping individuals
in this way produces a bell-shaped curve. (See Figure 22-2 on page 470.)
Figure 22-2
The bell-shaped
curve produced
when a group
of men are
arranged by
height, which
shows the
variation within
a population.
This is typical for polygenic inheritance. The many variations of human
skin color are also due to polygenic inheritance.
■■ LINKED TRAITS
Why do people with red hair also have freckles? Why do people with
blond hair usually have blue eyes? The genetics needed to answer these
kinds of questions was first studied in fruit flies. Shortly after Mendel’s
experiments with pea plants became widely known, geneticists turned to
fruit flies for further research. Fruit flies are often seen flying around pieces
of ripening fruit, where they lay their eggs. These tiny animals have made
great contributions to our understanding of how traits are inherited.
In fruit flies, the inheritance of many different traits has been studied.
These traits include eye color, wing size, body color, number of hairy bristles on the body, and much more. According to Mendel, traits are sorted
and inherited independently. Just because two traits are together in a parent does not mean they will be inherited together in the offspring. But
this is exactly what happens with some traits in fruit flies. In fact, in studying fruit flies, scientists were able to identify four main groups of traits. All
traits in a group tend to be inherited together, not independently. In some
ways, it appears that these traits are linked to each other. Having observed
linkage in fruit flies, scientists realized that chromosomes suddenly made
more sense. These linked traits are inherited together because the genes
for these traits are located on the same chromosome. This knowledge
explains the four groups of linked traits in fruit flies. These animals have
four linkage groups because each cell of a fruit fly has four pairs of chromosomes. (See table on fruit fly traits.)
Chapter 22 / Human Genetics 471
LINKAGE MAP OF FRUIT FLY TRAITS
Chromosome 1
0.0
yellow
Chromosome 2
6.1 Curly
Chromosome 3
Chromosome 4
26.0 sepia
0.0 + shaven
0.0 + scute
13.0 dumpy
40.7 Dichaete
0.0 + cubitus
0.8
prune
48.5 black
44.0 scarlet
1.5
white
54.5 purple
46.0 Wrinkled
0.0 + grooveless
47.0 radius
incompletus
0.0 + sparklingpolished
0.2
13.7
crossveinless
54.8 Bristle
20.0
cut
55.2 apterous
21.0
singed
57.5 cinnabar
52.0 rosy
27.7
lozenge
67.0 vestigial
58.2 Stubble
33.0
vermilion
72.0 Lobe
58.5 spineless
36.1
miniature
75.5 curved
64.0 kidney
51.5
scalloped
100.5 plexus
69.5 Hairless
56.7
forked
104.5 brown
70.7 ebony
57.0
Bar
107.0 speck
79.1 bar-3
64.8
maroonlike
interruptus
eyeless
91.1 rough
100.7 claret
Many human traits are linked because they are found on the same
chromosome. For example, the genes for red hair and freckles are on the
same chromosome. The genes for blond hair and blue eyes are also located
on another single chromosome. These genes are linked and tend to be
inherited together. Now is the time to examine more closely the chromosomes found in human body cells.
■■ IS IT A GIRL OR IS IT A BOY?
As described in Chapter 16, scientists can take a photograph of the chromosomes in a human cell by using a camera attached to a microscope.
The chromosomes can be paired up and then numbered from largest to
smallest. We can create a “picture” of the chromosomes. This picture is
called a karyotype. If we examine a human karyotype, we find 22 perfectly
matched pairs of chromosomes. These chromosomes, numbered from 1
to 22, are called autosomes. This accounts for 44 of the 46 chromosomes
found in a diploid cell. However, the last two chromosomes do not always
make a matched pair. It depends on whether the diploid cell came from
a female or a male. If the cell came from a male, the last two chromosomes will not be an identical match. Cells from males contain an X and
a much smaller Y chromosome. If the cell came from a female, the last two
chromosomes will both be X chromosomes, a matched pair. Differences
472 Genetics and Molecular Biology
in the last pair of chromosomes—whether a person has an X and a Y chromosome, or two X chromosomes—determine the person’s sex. Therefore,
these last two chromosomes are called the sex chromosomes.
Who determines a child’s gender, the father or the mother? Let’s ask
the question another way. Is it the sperm cell from the father or the egg
cell from the mother that determines the child’s gender? (See Figure 22-3.)
P
Gametes
Male
Female
XY
XX
X
X
Y
X
Y
X
XX
XY
X
XX
XY
X
F1
Figure 22-3 The X and the Y
chromosomes are called the sex
chromosomes.
50% Males
50% Females
Egg cells are produced in the mother by meiosis. Her cells have 22
pairs of autosomes and one pair of X’s, her sex chromosomes, for a total
LIVING ENVIRONMENT BIOLOGY, 2e/fig. 22-3 s/s
of 46 chromosomes. Through meiosis, every egg cell gets one of each of
the 22 autosomes and one X chromosome. Now consider sperm cells.
Every cell in the father’s body has 22 pairs of autosomes and an X and a
Y chromosome. During meiosis, 50 percent of his sperm cells get 22 auto-
Figure 22-4 Sex determination in
humans depends on the sperm cell.
X
✕
X
XX
Female
Y
✕
X
XY
Male
somes and an X chromosome, and 50 percent of his sperm cells get 22
autosomes and a Y chromosome. LIVING
Egg cells
have only BIOLOGY,
X chromosomes
ENVIRONMENT
2e/fig. 22-4 in
s/s
addition to the autosomes. So who determines the sex of the child? The
father does. If a sperm with a Y chromosome fertilizes the egg, the zygote
is XY, and a male develops. If a sperm with an X chromosome fertilizes the
egg, the zygote is XX, and a female develops. (See Figure 22-4.)
Chapter 22 / Human Genetics 473
■■ SEX-LINKED TRAITS
Red-green color blindness, an inability to distinguish the colors red and
green, occurs most often in males. Hemophilia, a disease in which blood
does not clot properly, also occurs most often in males. Duchenne muscular dystrophy, a disease that slowly destroys muscles, occurs primarily
in males, too. What accounts for this pattern? These traits are all sexlinked. They occur when a particular allele with a defective portion of
DNA is present on the X sex chromosome. The much smaller Y chromosome lacks alleles for these three traits.
Why do these disorders occur rarely in females? Females have two X
chromosomes. As long as a woman inherits one normal allele on one X
chromosome, the presence of the defective allele on the other X chromosome will be hidden, since it is recessive. The normal allele is dominant.
A male, on the other hand, has only one X chromosome. If the defective
allele is on his single X chromosome, there is no dominant healthy allele
on the Y chromosome. The boy or man will have the disease.
A female who has a recessive defective allele on one X chromosome
but not on the other is called a carrier. These females carry the genetic
disorder, but they do not have the condition in their phenotype. However, they can pass the condition on to offspring. Carriers are heterozygous for the gene that produces a genetic disorder. The only way a female
can be red-green color blind or have hemophilia, for example, is if she is
homozygous for the trait. Her father had the disorder and passed it on to
her on his X chromosome. Also, her mother either had the disease or was
a carrier and also passed on an X chromosome with a defective allele. (See
Figure 22-5.)
Father
–
P gen.
Gametes
Mother
–
X Y
X–
– –
X X
X X
X–
Y
X
X–
F1 gen.
–
– –
X X
X
X–
X–
Y
X–Y
XX–
XY
X
–
– –
X
X–Y
X X
Figure 22-5 The only way females can get hemophilia is if the father was
affected and the mother was a carrier or affected.
LIVING ENVIRONMENT BIOLOGY, 2e/fig. 22-5 s/s (rev.10/22/03)
474 Genetics and Molecular Biology
INHERITANCE PATTERNS FOR SEX-LINKED RECESSIVE GENETIC DISORDERS
Key
X
= X chromosome with normal gene
X– = X chromosome with defective gene
Y
= Y chromosome
XY
→ Normal male
XX
→ Normal female
X– Y
→ Male with genetic disorder
X– X
→ Carrier female
X– X– → Female with genetic disorder
Finally, a sex-linked genetic disorder in a son is always inherited
from his mother. Why is that? Remember that a son can inherit an X
chromosome, where the defective allele is located, only from his
mother. He receives a Y chromosome with no corresponding allele from
his father. He therefore inherits the defective allele from his mother.
(See the table above, which shows the inheritance patterns for sexlinked disorders.)
Check Your Understanding
Explain why there are more male hemophiliacs than female hemophiliacs. You might want to research the occurrence of hemophilia
among the royal families of Europe who descended from Britain’s
Queen Victoria (a carrier of the flawed gene).
■■ CYSTIC FIBROSIS:
AN AUTOSOMAL RECESSIVE TRAIT
In the United States, cystic fibrosis (CF) is the most common genetic disorder in the Caucasian, or white, population. Approximately one in 25
people in this population carries one CF allele. CF is a recessive trait. To
have the disease, a person must be homozygous for the trait. That is, both
parents must be carriers. Unlike the three disorders already mentioned,
the gene for cystic fibrosis is not located on the X or Y chromosomes. It
is on an autosomal chromosome. Cystic fibrosis is therefore called an autosomal recessive trait. Approximately one in every 2000 Caucasian babies is
born with this disease.
Children with cystic fibrosis have many digestive and respiratory prob-
Chapter 22 / Human Genetics 475
Figure 22-6 A physiotherapist is giving
percussion treatment
(to loosen the mucus)
to a 3-year-old cystic
fibrosis patient.
lems. The most important health problem is that the lungs constantly fill
with a thick mucus. It requires constant medical treatment to keep the
lungs clear. People with cystic fibrosis also have frequent lung infections.
These people can also have problems digesting food properly. Until
recently, children with cystic fibrosis rarely lived beyond five years of age.
(See Figure 22-6.)
In 1989, researchers announced with great excitement that they had
found the gene that produces cystic fibrosis. In more than 70 percent of
people with CF, three DNA nucleotides are missing in a single gene. These
nucleotides code for the amino acid phenylalanine, at the 508th position
in the polypeptide of that gene. This one missing amino acid in the
polypeptide causes all the problems. This is a mutation. It is believed to
have first occurred in a person who lived in northern Europe several hundred years ago. Since then, many people have inherited the recessive CF
gene from this person’s descendants.
New, powerful technologies have begun to assist researchers in finding
the genes that cause human genetic disorders. Now that the first draft
sequence of the human genome has been completed, a list of up to 300
possible genes per disorder is often available. A tool called “GeneSeeker,”
which helps narrow the list, was announced in 2003. It uses the Internet
and links together information in an automated way from nine different
databases around the world. The European Journal of Human Genetics called
GeneSeeker a “web-based data mining tool for the identification of candidate genes for human genetic disorders.” This type of informationprocessing tool on the World Wide Web is expected to become a useful
addition to the actual experimentation done in the laboratory.
476 Genetics and Molecular Biology
■■ HUNTINGTON’S DISEASE:
AN AUTOSOMAL DOMINANT TRAIT
Another genetic disorder is inherited in a different way. This disorder,
called Huntington’s disease, causes the nervous system to break down gradually over time. If one parent has the disease, it is very likely that 50 percent of his or her children also will develop the disease. Huntington’s
disease is caused by an autosomal dominant trait. Like cystic fibrosis, the
gene is located on an autosome, not on a sex chromosome. However,
unlike cystic fibrosis, the gene that causes Huntington’s disease behaves
as a dominant trait. As long as a person has one allele for this disease,
even though he or she has another normal allele, he or she will develop
the disease.
The symptoms of Huntington’s disease do not begin to show until a
person is about 40 years old. This late-developing disease creates tremendous worry for people in affected families. If a parent has the disease, it is
quite possible that a son or daughter has inherited the allele. Even if the person is heterozygous for the trait, the disease will begin to show later in life.
■■ KNOWING YOUR FAMILY HISTORY
As we begin to understand more about genetic disorders, it becomes
clearer that there are not only wonderful possibilities but also risks in having children. Parents ask themselves if it is possible for their child to
inherit a genetic disorder. To assess the risks, people must have information about their ancestors. It is necessary to learn about one’s family history. A man and a woman who want to become parents can now go to a
trained genetic counselor for help in determining their risks of having a
child with a genetic disorder. One of the things a genetic counselor does
is to help the couple prepare a chart that shows the occurrence of a disease in past generations of their families. This chart is called a pedigree.
A pedigree uses symbols to represent parents and children in several
generations of a family. Squares represent males, and circles represent
females. If the symbol is half shaded, the individual is heterozygous and
a carrier. A symbol that is fully shaded shows that the individual is
homozygous and has the disease. Sometimes a symbol is half shaded in
a dark color, which means that the individual has the disease even though
only a heterozygote. (See Figure 22-7.)
Why are pedigrees useful? A completed pedigree chart can show how
a genetic disorder in a family is being inherited.
There are three main possibilities, which we have already examined:
Chapter 22 / Human Genetics 477
(a)
(b)
(c)
(d )
(e)
(f )
Key:
M
F (Normal)
M
F (Heterozygote carrier)
M
F (Heterozygote affected)
M
F (Homozygote affected)
Autosomal dominant: (a), (b)
Autosomal recessive: (c), (f )
Sex-recessive: (d ), (e)
Figure 22-7 A completed pedigree chart shows the inheritance of a genetic
disorder in a family.
LIVING ENVIRONMENT BIOLOGY, 2e/fig. 22-7 s/s
◆ The disorder is a dominant condition carried on an autosome. In this
case, roughly equal numbers of males and females are affected. The
condition does not skip generations and then reappear. Also, whenever
the gene is present, the disorder is expressed.
◆ The disorder is a recessive condition carried on an autosome. Again,
roughly equal numbers of males and females are affected. However,
the condition can skip generations when both parents are only carriers, to reappear later in homozygous individuals.
◆ The disorder is due to a recessive allele on the X chromosome. In this
case, mostly males are affected. All daughters of an affected male and
a normal female are carriers. Sons never inherit the disease from their
father, only from their mother who is either a carrier or affected.
A pedigree that shows a person’s family history for a particular trait can
be analyzed, and patterns of inheritance can be determined. From this,
predictions of the probabilities of the trait reappearing in future generations of males and females are made. Consulting with a genetics counselor
enables prospective parents to make informed decisions about having
children.
It is necessary to understand what probability means when making
these kinds of important decisions. For example, a pedigree analysis might
show that the chances of a man and a woman having a child with a particular genetic disorder is one in four. This same probability exists each
478 Genetics and Molecular Biology
A Career in Genetics: Counseling
Joseph and his parents had been referred by their family doctor to the
genetics clinic at a local hospital. The doctor had made this suggestion
after testing the levels of sex hormones in Joseph’s blood and ordering a
chromosome analysis. Joseph, a healthy 16-year-old, was doing well in
school. However, even though he was almost two meters tall, he showed
no signs of sexual maturity. He had not developed additional body hair, and
his voice retained the high pitch of a much younger person. A long talk
took place in the genetics counselor’s office.
The counselor explained that Joseph has one more X chromosome than
is usually found in a male. This is due to an error that occurred during
meiosis when the egg or sperm from which he developed formed. Joseph
and his parents learned from the counselor that about one in every
thousand males has this XXY chromosome makeup—a total of 47
chromosomes instead of the normal 46. The fact that this condition is
called Klinefelter syndrome was not important to Joseph and his family.
However, learning about the characteristics of the syndrome was very
important. The counselor told them that the characteristics of men with
this syndrome usually included tall stature, some minor birth defects, small
time the couple has a child. If one child has already been born with the
disease, the next child still faces exactly the same one-in-four risk. If three
children have been born without the disorder, the chance of the fourth
child having the disorder is no greater; it is still one in four. Think of probability this way. If a coin is tossed once and lands heads up, the chance
of tails the next time is still only 50 percent. If the same coin is tossed fifty
times, and it comes up heads each time, the probability that it will come
up tails on the next toss is still 50 percent. The probability of any event,
whether it is tossing a coin or having a child, is not affected by what happened in previous events.
■■ OTHER GENETIC DISORDERS
Genetic disorders can also result from an abnormal number of chromosomes. As described in Chapter 16, mistakes can happen during meiosis
as the paired homologous chromosomes separate. This type of genetic
disorder is very different from the single gene mutation that produces
hemophilia, cystic fibrosis, or Huntington’s disease. Abnormal chromosome numbers nevertheless cause disorders that can be detected prior to
or at birth.
Chapter 22 / Human Genetics 479
testicles, and sterility. Sometimes, but not in Joseph’s case, mental
retardation begins at birth.
Joseph’s mother asked if the condition could occur in any other children
she might have. It was explained that there are no known examples of
more than one Klinefelter individual in a family. Furthermore, blood tests
done on Joseph’s relatives had not shown any chromosomal disorders.
As a result of the genetics counseling, Joseph realized that he could most
likely look forward to a normal life. Indeed, in a few months, his beard
began to grow. More important, Joseph learned about possible options for
having a family. His parents also learned that amniocentesis might be a
good idea if they decided to have another child, since some families have
shown a tendency to produce other errors in meiosis.
A genetics counselor often has a master’s degree, with special training
in genetics counseling. Some are also physicians. Well-trained and
experienced genetics counselors are in great demand. Many families,
like Joseph’s, rely on their expertise to answer urgent questions that can
have profound effects on their lives.
Down syndrome is due to an extra chromosome number 21. This syndrome, which occurs in about one in 800 births, causes mild to severe
mental retardation. (See Figure 22-8.) Another genetic disorder occurs in
males who are born with an extra X chromosome (XXY). This extra X
chromosome produces Klinefelter syndrome, which results in abnormal
sexual development and infertility.
Figure 22-8 The karyotype of a person
with Down syndrome. Notice that there
are three copies of chromosome 21.
480 Genetics and Molecular Biology
Sickle-cell anemia is a disorder due to a single gene defect, not an
abnormal chromosome number. In Chapter 20, you learned that a mutation in the DNA nucleotide sequence of the gene for hemoglobin causes
this disease. Sickle-cell anemia occurs most often in people whose ancestors came from West and Central Africa.
It is inherited as an autosomal recessive trait, like cystic
fibrosis. In the United States, people who are homozygous
for sickle-cell anemia—approximately 0.2 percent of
Normal red
blood cell
African Americans—have the disease. However, some red
blood cells of people who are heterozygous—approximately 9 percent of African Americans—also become
sickle-shaped under certain conditions. A simple blood
test makes it easy to detect the sickle-cell trait in people.
(See Figure 22-9.)
Sickle-shaped
Phenylketonuria, another autosomal recessive trait, is
red blood cell
one of the most studied genetic disorders. About one in
Figure 22-9
15,000 infants born in the United States has the allele for
The sicklethis disease. This allele prevents the baby from producing
shaped red
LIVING ENVIRONMENT BIOLOGY, 2e/fig. 22-9 s/s
blood cell is
an enzyme that breaks down the amino acid phenylalacharacteristic
nine. As a result, phenylalanine builds up in the baby’s
of sickle-cell
blood. High levels of phenylalanine in the blood interfere
anemia—
with the development of the brain, causing mental retara genetic
dation. Routine tests for this disease are now done on
disorder.
newborns. By detecting the disease early, the baby’s diet
can be changed to prevent the disease’s effects from developing. This early
detection can enable the baby to live a normal life.
Tay-Sachs disease causes a rapid breakdown of the nervous system during the first years of life. Children with Tay-Sachs disease rarely live more
than five years. Tay-Sachs is an autosomal recessive genetic disorder. Until
recently, this disease occurred most often among Jewish people whose
ancestors came from Eastern and Central Europe. Today, however, an
increased number of Tay-Sachs babies are being born to non-Jewish
parents.
■■ GENETIC SCREENING
Consider a husband and wife who know that a severe genetic disorder
such as Tay-Sachs has affected people in their family. Perhaps both parents
are heterozygotes, with one allele for Tay-Sachs and one normal allele.
We know that a child will have the disease only if it has two defective
alleles, that is, if the child is homozygous for the genetic disorder.
Chapter 22 / Human Genetics 481
It has been explained to the couple that their chance of having a
homozygous Tay-Sachs baby is 25 percent; that is, the child has a one in
four chance of having Tay-Sachs disease. A Punnett square can show why
this is so. The couple has a difficult decision to make. A child with TaySachs will probably not survive beyond age five. The child’s few years of
life will be spent with an untreatable disease that destroys the brain. Many
people, faced with this possibility, might choose not to have children.
Since 1968, the presence of some genetic disorders can be determined
before birth. This is called prenatal diagnosis.
The first prenatal diagnosis method that was developed is called
amniocentesis. A needle is used to withdraw amniotic fluid from the
uterus. In this fluid are some cells from the fetus. These cells are removed,
grown in a laboratory dish, and tested. A karyotype of the fetal chromosomes is prepared. Biochemical tests that show the presence of a genetic
disorder may also be done. More often today, the DNA of the fetus is studied directly to see if something is abnormal in the fetus’s genes.
Other prenatal tests are also possible. Sound waves can make images
of the fetus while it remains safely in its mother’s uterus. The procedure
that makes images with sound waves is called ultrasonography. The techniques for taking ultrasound pictures have improved a great deal. It is
now possible to see some types of abnormalities in the fetus with these
images. For example, defects in the kidneys, brain, and heart can be
observed. Genetic disorders may also damage fetal muscles and nerves.
When this happens, the fetus is not able to move normally. These abnormal movements can be recognized with ultrasonography. (See Figure
22-10.)
Chorionic villus sampling (CVS) is a technique that can be performed
on an embryo two months earlier than amniocentesis. A CVS test removes
cells from the chorion layer outside the amnion. Cells from this area are
Figure 22-10 This
ultrasonogram shows the
outline of a developing
fetus. (Note the head—
which is facing to the
right—and the chest and
right arm, on the left side
of the image.)
482 Genetics and Molecular Biology
rapidly growing into the wall of the mother’s uterus. These fetal cells can
be analyzed for genetic abnormalities.
Another technique analyzes blood from the fetus. In this method,
called cordocentesis, a needle is used to remove fetal blood from the
umbilical cord. Remember that blood goes back and forth between the
fetus and the placenta through the umbilical cord. Fetal blood cells can
be studied for chromosomal abnormalities. Also, the blood itself can be
checked with biochemical tests.
It is important to know that there is some risk involved in amniocentesis and CVS. Infections may occur, or the fetus itself may be harmed by
the needle used to remove material to be studied. The chance of this happening is between one in 100 and one in 200. Newer, safer techniques
are now being developed that will test the few fetal cells that are found
in the mother’s blood. These cells can be captured, and their DNA can
then be studied for genetic disorders.
■■ DIFFICULT DECISIONS
Most of the time, prenatal diagnosis determines that the developing fetus
is normal. The couple can relax and prepare for the birth of a healthy
child. However, when the results of a prenatal diagnosis indicate that the
fetus is not genetically normal, difficult decisions have to be made. It is
important for the couple to be able to discuss their course of action with
their physician and with trained genetics counselors.
Often, the couple will use the information from the diagnosis to prepare for the birth of their child. Caring properly for the fetus during pregnancy is very important. Special plans can be made to have the
appropriate medical personnel available at the time of birth. Specialists
who can treat a particular disorder may be needed, too. Parents need to
learn about the disorder and how best to care for the infant after it is
born.
Amazingly, some inherited conditions can be treated and corrected in
the fetus before birth. In February 1995, a four-month-old fetus was given
a successful bone marrow transplant. Prenatal diagnosis had shown that
the fetus had SCIDS, severe combined immunodeficiency syndrome, as
described in Chapter 14. Children with SCIDS rarely survive because they
have no natural protection against disease. The first son born to this couple had the disorder and had died at seven months of age. With the second baby, normal bone marrow that produces white blood cells was
transplanted from the father into the fetus during the fourth month of
pregnancy to prevent the deadly condition from developing. By Decem-
Chapter 22 / Human Genetics 483
ber 1996, when this procedure was reported, the couple had a healthy
18-month-old baby who was cured of the genetic disorder.
■■ ETHICAL QUESTIONS CONCERNING
PRENATAL DIAGNOSIS
Many birth defects still cannot be detected by prenatal diagnosis. The
decision by a couple to proceed with prenatal diagnosis is made only after
very careful evaluation by doctors and genetics counselors. From the
results, difficult questions may arise. Individuals and society as a whole
need to consider the ethics involved in these questions.
Sometimes, the couple faces the very difficult question of whether to
continue the pregnancy. If the disorder is extremely serious, the couple
may decide that an abortion should be performed. This is not an easy
decision to make, and it is very much influenced by the couple’s moral
and religious values.
Should a woman be urged to have an abortion if testing shows that the
fetus will be born with severe mental or physical handicaps? If a doctor
is opposed to abortion, should he or she be allowed to withhold results
of prenatal diagnosis that might cause a woman to choose an abortion?
In addition to information about genetic disorders, prenatal testing can
determine the child’s gender. Should couples be informed of the fetus’s
gender? Should decisions to abort a pregnancy based only on the fetus’s
gender be made illegal? Should a couple be allowed to abort a pregnancy
that involves twins if one of the fetuses is severely abnormal but the other
one is normal?
As in many other areas of biology today, knowing the science of genetics is not enough. An important part of biology education is learning how
to think and make decisions about difficult ethical issues.
LABORATORY INVESTIGATION 22
How Is a Human Karyotype Prepared?
INTRODUCTION
A karyotype is a picture or photograph that shows the chromosomes of
an organism. It was not until 1956 that the human karyotype was determined. This karyotype established the human diploid number to be 46
chromosomes—44 autosomes and two sex chromosomes. Karyotyping
has become a routine procedure for fetuses of couples that are at risk of
having children with syndromes or conditions that are related to abnormal chromosome numbers.
To prepare a karyotype, a geneticist interrupts cell division at metaphase
by the use of a special chemical. The treated cells are placed on a glass
slide and broken open. Then they are stained. A group of chromosomes
that has been freed from a cell is photographed through a microscope.
The photograph is enlarged and the chromosomes are rearranged and
counted. In this investigation, you will use a photograph of chromosomes
to complete a human karyotype.
MATERIALS
Photocopy of “Set of Human Chromosomes” (from Teacher’s Manual),
scissors, glue or tape, paper
PROCEDURE
1. Carefully cut out the chromosomes from your photocopy of “Set of
Human Chromosomes.”
2. To aid in sorting, scientists have assigned human chromosomes to
seven groups according to the size of a chromosome and the location
of its centromere, the point where the two chromosomes are joined.
Place the chromosomes in pairs; then decide in which of the seven
groups the chromosomes belong.
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Group
Chromosomes
Characteristics
A
1–3
Very long, with centromeres midway from the ends
B
4 and 5
Long, with centromeres somewhat closer to one end
C
6–12 and X
Medium length, with centromeres somewhat closer to one end
D
13–15
Medium length, with centromeres at or very near the end
E
16–18
Somewhat short, with centromeres somewhat closer to one end
F
19 and 20
Short, with centromeres midway from the ends
G
21 and 22 and Y
Very short, with centromeres at or very near the end
3. Arrange the pairs of chromosomes in order from the largest to the
smallest, and in their groups. Arrange the chromosomes in a line with
the centromeres on the line. Identify each chromosome with a number or letter. Glue or tape the chromosomes in place on your sheet of
paper to complete your karyotype.
INTERPRETIVE QUESTIONS
1. From your karyotype, determine if the individual these chromosomes
came from was male or female. How would the chromosomes differ if
they came from a member of the opposite sex?
2. Explain how the karyotype would be different if its chromosomes
came from a person with Down syndrome.
3. Describe how a genetics counselor might use the information from a
karyotype to guide prospective parents.
Chapter 22 / Human Genetics 485
486 Genetics and Molecular Biology
■■ CHAPTER 22 REVIEW
Answer these questions on a separate sheet of paper.
VOCABULARY
The following list contains all of the boldfaced terms in this chapter. Define
each of these terms in your own words.
amniocentesis, autosomes, carrier, chorionic villus, cordocentesis,
linkage, pedigree, polygenic inheritance, prenatal diagnosis, sampling,
sex chromosomes, sex-linked, ultrasonography
MULTIPLE CHOICE
Choose the response that best completes the sentence or answers the question.
1. Red-green color blindness is an example of a genetic disorder
that is a. sex-linked dominant b. sex-linked recessive
c. autosomal dominant d. autosomal recessive.
2. What is the probability of a person with Huntington’s disease
transmitting the disease to his or her child? a. 0 percent
b. 25 percent c. 50 percent d. 100 percent
3. Genes that are found on the X and Y chromosomes are said to be
a. autosomal b. sex-linked c. polygenic d. teratogenic.
4. A chart that shows the occurrence of a genetic disorder in a family
is called a a. pedigree b. Punnett square c. karyotype
d. sonogram.
5. Genetic disorders are caused by a. bacteria b. viruses
c. prenatal exposure to certain chemicals d. defects in genes.
6. Which of the following is an example of polygenic inheritance?
a. height in humans b. flower color in peas c. hemophilia
d. sickle-cell anemia
7. An example of a disorder caused by a dominant allele is
a. cystic fibrosis b. Duchenne muscular dystrophy c. sickle-cell
anemia d. Huntington’s disease.
8. Linked traits a. are located on the same chromosome b. do not
follow the law of independent assortment c. are inherited
together d. all of these.
9. How many pairs of autosomes are found in humans?
a. 1 b. 2 c. 22 d. 46
10. In humans, the gender of the offspring is determined by
a. the mother’s egg b. the father’s sperm c. the time of day
fertilization takes place d. whether the egg is fertilized.
Chapter 22 / Human Genetics 487
11. A person who is heterozygous for a recessive allele that causes a
genetic disorder a. usually shows moderate symptoms of the
disorder b. is called a carrier c. will not transmit the disorder
to his or her offspring d. all of these.
12. A sample of the fluid surrounding the fetus is removed in the
diagnostic procedure called a. amniocentesis b. chorionic
villus sampling c. cordocentesis d. ultrasonography.
13. A female with Down syndrome would have the chromosomal
makeup a. 45XY b. 44XX c. 45XX d. 44XXY.
14. What must be true of the parents of a female hemophiliac?
a. mother is a carrier, and father is normal b. mother is normal,
and father has hemophilia c. mother has hemophilia, and father
is normal d. mother is a carrier, and father has hemophilia.
15. Which of these disorders is caused by an autosomal recessive allele?
a. cystic fibrosis b. red-green color blindness c. hemophilia
d. Duchenne muscular dystrophy
PART B—CONSTRUCTED RESPONSE
Use the information in the chapter to respond to these items.
This pedigree shows the incidence of polydactyly (extra fingers or toes)
in a family.
Key:
Male
5
6
13
14
1
2
3
7
8
9
10
11
12
15
16
17
18
19
20
24
25
4
Female
Affected
individuals
21
22
23
16. Based on the findings in this pedigree, polydactyly is
a. sex-linked recessive b. sex-linked dominant c. autosomal
recessiveLIVING
d. autosomal
dominant.
How
can you
ENVIRONMENT
BIOLOGY, 2e/fig.
22-Q16
s/s tell?
17. Describe the genotypes of as many individuals in the pedigree as
possible. What can you conclude about individual 17’s genotype if
he and his wife have a fourth child who is not polydactyl?
18. How is PKU passed from parents to children? Why is early
detection of this disease important?
19. Why is prenatal diagnosis important for a couple with a family
history of genetic disorders? Briefly describe four techniques used
in prenatal diagnosis.
488 Genetics and Molecular Biology
20. A grandmother fears that her school-age grandchildren may catch
cystic fibrosis or Tay-Sachs disease from their classmates. What
would you say to reassure her?
PART C—READING COMPREHENSION
Base your answers to questions 21 through 23 on the information below and
on your knowledge of biology. Source: Science News (October 19, 2003): vol.
162, p. 254.
Long Live the Y?
Rumors of the human Y chromosome’s eventual death may be exaggerated. David Page of the Whitehead Institute for Biomedical Research
in Cambridge, Mass., and his colleagues have identified a means by
which the Y chromosome may forestall, or at least delay, the gradual
degradation that some biologists argue will ultimately delete it from the
human genome.
In the Feb. 28 Nature, two Australian scientists summarized recent
research showing that the human Y chromosome gradually accumulates mutations that deactivate its few genes. Non-sex chromosome pairs
have a chance to replace mutation-ridden sections by swapping DNA
with each other. But most of the Y is unable to exchange DNA with its
partner, the X chromosome. “At the present rate of decay, the Y chromosome will self-destruct in around 10 million years,” the two
researchers said.
Page’s group isn’t so sure. These researchers have sequenced all the
DNA on the Y and discovered that many of its genes have neighboring
doubles. A gene and its copy often form DNA palindromes, in which a
gene’s DNA sequence is followed by nearly the same sequence, but in
reverse. (Palindromes in literature are words or phrases that read the
same forward or backward, such as, “Was it a rat I saw?”).
Further scrutiny of the Y palindromes led the researchers to conclude
that each gene and its backward copy are so similar in sequence that
they must regularly exchange DNA in a novel form of the recombination
observed in non-sex chromosomes.
“There is, in fact, intense recombination within the arms of the palindrome,” says Page. “It really changes the way we think about the Y chromosome.” He speculates that the palindromes enable the human Y
chromosome to constantly repair mutations, staving off its own
degradation.
“It’s a great story,” comments Eric Green of the National Human
Genome Research Institute in Bethesda, Md. He suggests that
researchers should look for such palindromes in sex chromosomes of
other animals.
Chapter 22 / Human Genetics 489
21. Explain the conclusion that was reached by two Australian
scientists about the human Y chromosome and the deactivation of
its genes.
22. State what Page and his research team have discovered next to
many of the genes on the human Y chromosome.
23. Explain the importance of recombination between sections of DNA
on the human Y chromosome.