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Growing Up With Us... © 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. Page 4 of 4