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Growing Up With Us...
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A Newsletter For Those Who Care For Children
Volume 15, Issue 11
TRANSMISSION OF GENETIC DISORDERS
November 2009
Editor-in-Chief: Mary Myers Dunlap, MAEd, RN
BEHAVIORAL OBJECTIVES
Common methods of genetic transmission of disorders
will also be described.
AFTER
READING THIS NEWSLETTER THE
LEARNER WILL BE ABLE TO:
1.
Discuss principles of human genetics, including the
role of DNA, chromosomes, and genes in the
development of individual characteristics.
2.
Describe common methods of genetic transmission
of disorders.
A 6 month old is diagnosed with cystic fibrosis. The
mother, crying, asks, “I don’t have it, nor does my
husband. How did she get this?”
A 4 year old, Jimmy, is admitted with sickle cell anemia.
The father says, “Since he has the disease, at least we
know any other children we have will be normal.”
Parents of a 4 year old boy, admitted with hemophilia,
are expecting another child and ask, “What are the
chances of this baby having hemophilia? We know it’s a
girl.”
Everyday, such questions and realities are faced by
parents across this country. Some genetically
transmitted diseases or disorders are apparent at birth,
such as Down Syndrome. In others,
the manifestation does not appear
for weeks, months or years. Some
genetic diseases, although
genetically determined, do not
become apparent until
environmental factors precipitate
the onset of symptoms. For
example, PKU is a disorder in which the enzyme to
metabolize phenylalanine, a protein, is lacking. Also, the
acute symptoms of sickle cell anemia are often
precipitated by certain conditions, such as low
oxygenation, infection or dehydration. Healthcare
professionals, caring for children in a variety of settings,
must understand the role that genetics play in human
reproduction.
This newsletter will discuss principles of human
genetics, including the role of DNA, chromosomes, and
genes in the development of individual characteristics.
THE BASICS OF HUMAN GENETICS
Genetics, the science of heredity, focuses on the
passing on of traits, diseases and abnormalities from
one generation to the next. Although its roots were
evident in ancient history, it was not until the mid 1800’s
when Gregor Mendel, a monk, first demonstrated the
effects of dominant and recessive heredity. In the mid
1950’s, DNA was discovered.
Contained within the nucleus of body cells are more
than 200,000 genes. These genes are composed of
thousands of tiny segments of deoxyribonucleic acid
(DNA), the hereditary material that forms the “blueprint”
for an individual. This information programs the body’s
physiologic processes and characteristics. The DNA
strands are arranged into
packages called chromosomes.
Alterations of a whole
chromosome, a part of a
chromosome or even a single
gene can manifest as a genetic
disorder.
Humans normally have 46
chromosomes arranged into 23 matched pairs, including
22 pairs of “regular” chromosomes called autosomes,
and 1 pair of specialized sex chromosomes, XX in
females and XY in males, that determine gender. This
genetic material is passed on to offspring through the
ova and sperm, when conception occurs. So that the
correct numbers of chromosomes will be provided, the
ovum and sperm cells divide in half prior to conception,
giving each 23 unpaired chromosomes. Therefore,
when the ovum and sperm unite, their combined genetic
material will equal the normal 46 total, or 23 pairs of
chromosomes. Characteristics of the offspring are
determined by the genetic material brought together
when the two pairs of 23 chromosomes unite at
conception. The inheritance of genetic diseases,
abnormalities, or traits is determined by both the type of
chromosome on which the abnormal gene resides
(autosomal or sex chromosome), and by whether the
gene, itself, is dominant or recessive. This is due to
whether a single gene from one parent (dominant
inheritance) or both copies of the gene (one from each
parent) are defective (recessive inheritance).
Copyright © 2009 Growing Up With Us, Inc. All rights reserved.
Page 1 of 4
Autosomally inherited diseases are inherited through the
non-sex chromosomes, pairs 1 through 22. X-linked
diseases are inherited through the X sex chromosome.
CHROMOSOMAL ABNORMALITES
An abnormality in the number or structure of
chromosomes may produce genetic disorders.
For example, the correct number of chromosomes, 23 from
each parent, may not be passed on during conception. Or,
an extra chromosome may be passed on at conception,
resulting in 47, instead of the normal 46. This is termed a
trisomy. Trisomy 21, in which a third chromosome occurs at
pair #21, is known as Down Syndrome. A missing
chromosome, or monosomy, may also occur. An example
of this is Turner’s syndrome, characterized by a single X
chromosome, rather than the normal two Xs. Since a single
X chromosome is involved, resulting offspring are all
female.
SINGLE GENE INHERITANCE
Each trait, such as eye color, height, or blood type, has
one or more pairs of genes that control it. When two genes
are contributed that produce the same
characteristics for a given trait, they are
said to be homozygous. Two genes
that code for different characteristics of
the same trait are said to be
heterozygous. For example, if each
parent contributes a gene for a
different trait, for example the father
contributes a gene for blue eyes, and
the mother for brown eyes, the dominant gene will be
evident in the offspring. In eye color, brown is dominant,
while blue is recessive. Therefore, if the father provides the
gene for blue eyes, and the mother provides brown, the
child will have brown eyes. However, remember, both the
mother and father received two genes for eye color from
their parents. If the parents received heterozygous genes
for eye color, say one brown (B) and one blue (b), they
could each contribute the recessive gene for blue eyes,
resulting in a blue-eyed child (bb). With single gene
inheritance, males and females are affected equally.
Autosomal Recessive Disorders: Approximately 1400
autosomal recessive traits are known. Examples of these
disorders include sickle cell disease, cystic fibrosis, Tay
Sachs disease and PKU - phenylketonuria. Autosomal
diseases are inherited through the non-sex chromosomes.
Like blue eyes, this recessive gene must be contributed by
both parents (Aa) in order for the child to have this trait.
People who do
Autosomal
Maternal
not have the
Recessive
A
a
disease, but
A
AA
Aa
Paternal
who do have
the abnormal
a
Aa
aa
gene, are
termed carriers. Thus, two carrier parents, who are not
affected by the disease, have a 25% chance with each
pregnancy of having an affected child. There is a 50%
chance the child will be a carrier (Aa), and a 25% chance of
the child being disease free (AA).
A common misunderstanding is, “Since I’ve had one
child with cystic fibrosis, therefore, the next one shouldn’t
have it.” Much like the spin of a roulette wheel, the chances
are for each pregnancy, and are unrelated to subsequent or
prior pregnancies.
Autosomal Dominant Disorders: Dominant inheritance
occurs when an abnormal gene from ONE parent is
capable of causing disease. Autosomal dominant disorders
occur even though the matching gene from the other parent
is normal. The abnormal gene dominates the outcome of
the gene pair. Dominant genes exhibit their traits,
regardless of the gene contributed by the other parent.
Examples of autosomal dominant disorders include
Huntington
Autosomal
Maternal
disease,
Dominant
familial
h
h
colorectal
Hh
Hh
Paternal
H
cancers and
neurofibromat
h
hh
hh
osis.
With each pregnancy, there is a 50% chance that a
heterozygous affected parent will pass on the defective
gene to the child. This Punnett square demonstrates the
probability of an affected father (Hh) and unaffected mother
(hh) passing Huntington Disease on to a child.
X-Linked Disorders: In X-linked genetic disorders, the
defective gene resides on the X chromosome. The Y
chromosome carries no known medically significant
characteristics. The two chromosomes in the female are
alike in gene make-up, with two genes for each trait.
Therefore, since females have two copies of the X
chromosome (XX), the normal copy typically overrides the
abnormal gene. Females are usually not affected by these
disorders, except to be carriers. However, with males,
genes on the X chromosome have no counterpart on the Y
chromosome. Therefore, characteristics determined by a
gene on the X chromosome are always expressed in the
male. One of the significant aspects of X-linked inheritance
is the absence of father-to-son inheritance. The defective
gene is passed to males from their carrier mothers.
Hemophilia and Duchenne’s muscular dystrophy are
two examples of X-linked inheritance. There is a 50%
chance that a carrier mother will pass the defective gene on
to her son or daughter. If a daughter receives the defective
gene, she will become a carrier, like her mother. If a son
receives it, he will have the disorder.
Understanding the principles of genetics helps healthcare
professionals provide optimal care to children and their
families. Such knowledge helps in providing parental
support and education, as well as referrals, when
necessary.
Growing Up With Us, Inc.
PO Box 481810 • Charlotte, NC • 28269
Phone: (919) 489-1238 Fax: (919) 321-0789
Editor-in-Chief: Mary M. Dunlap MAEd, RN
E-mail: [email protected]
Website: www.growingupwithus.com
GUWU Testing Center
www.growingupwithus.com/quiztaker/
Copyright © 2009 Growing Up With Us, Inc. All rights reserved.
Page 2 of 4
Name:_____________________________________________________
Date:___________________________________
Employee ID#:____________________________________________
Unit:____________________________________
GROWING UP WITH US...
Caring for Children
November 2009
Competency: Demonstrates Age-Specific Competency by correctly answering 9
out of 10 questions related to Transmission of Genetic Disorders.
TRANSMISSION OF GENETIC DISORDERS
1. Genetic disorders are immediately apparent at birth.
a. True
b. False
2. A pair of genes is said to be heterozygous when they code for:
a.
b.
c.
d.
the same trait, such as eye color or height.
two different characteristics of a trait, such as blonde versus brown hair.
the same characteristics of a trait, such as two genes for blue eyes.
different characteristics in different people, such as a tall person versus a short person.
3. A dominant gene is one that:
a.
b.
c.
d.
causes various genetic diseases.
breaks off and attaches to another area of the chromosome.
is unlikely to be passed on from parent to child.
exhibits its characteristics regardless of other genes.
4. Which of the following is an example of an autosomal dominant disease?
a.
b.
c.
d.
Tay Sachs disease
Down syndrome
Huntington disease
hemophilia
5. A chromosome is:
a.
b.
c.
d.
the same as a gene.
a package of DNA.
any normal body cell.
unpaired until conception.
Copyright © 2009 Growing Up With Us, Inc. All rights reserved.
Page 3 of 4
Name:_____________________________________________________
Date:___________________________________
Employee ID#:____________________________________________
Unit:____________________________________
POPULATION/AGE-SPECIFIC EDUCATION POST TEST
GROWING UP WITH US... Caring For Children
TRANSMISSION OF GENETIC DISORDERS
6. Humans normally have 46 chromosomes arranged in 23 matched pairs.
a. True
b. False
7. An 8 month old, Meg, is diagnosed with cystic fibrosis. Her mother states, “How did she get this? I
don’t have it, nor does my husband.” The healthcare professional knows, since this is an
autosomal recessive trait,:
a.
b.
c.
d.
both parents are carriers for the genetic disease.
someone in previous generations must have had cystic fibrosis.
it was passed from the mother to her daughter.
an abnormal gene from one parent caused the disease.
8. If Meg’s parents become pregnant again, what is the probability of that child having cystic fibrosis?
a.
b.
c.
d.
0% - since they have already had one affected child
25%
50%
75%
9. Meg has a 4 year old brother who does not have cystic fibrosis. This healthcare provider knows he
may be a carrier.
a. True
b. False
10. Parents of a 4 year old boy, admitted with hemophilia, are expecting their second child. They ask,
“What are the chances of this baby having hemophilia? We know we’re having a girl.” The
healthcare professional knows:
a.
b.
c.
d.
since the gene is passed to males from their carrier mother, there is a 0% chance.
there’s a 25% chance the new baby won’t have hemophilia.
there’s a 50% chance that the new baby won’t have hemophilia.
all of these parents’ children will have hemophilia.
Copyright © 2009 Growing Up With Us, Inc. All rights reserved.
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