Gale Encyclopedia of Genetic Disorder / Gale Encyclopedia of Genetic Disorders, Two Volume Set - Volume 2 - M-Z - I

Abnormalities can also arise when the zygote begins to divide. This type of abnormality is usually severe, eventually leading to a miscarriage. If an abnormality occurs after the zygote has divided one or more times, the baby will have some normal cells and some abnormal cells. This situation is referred to as “mosaicism” and “mosaic” may be used to describe the person’s condition.

Molar pregnancies

Molar pregnancies can occur in one of two ways. Sometimes the original cell that duplicates and divides to form the fetus is completely of paternal origin. The chromosomes in a sperm duplicate themselves, then proceed to divide as if they were a normal zygote. These pregnancies are completely abnormal and miscarry. Another type of molar pregnancy occurs when two sperm fertilize one egg. The zygote is triploidy and has 69 chromosomes instead of 46. Although some fetal parts can be seen, these pregnancies normally miscarry in the first or second trimester.

Birth defects

The term birth defect describes many different types of abnormalities, including physical malformations. Abnormalities of anatomical structures may be significant or insignificant; minor variations in structure are common. Approximately 3% of newborns have major malformations. The causes are: chromosome abnormalities (6–7%), inherited genetic conditions (7-8%), environmental factors (7–10%), and multifactorial causes (20–25%). The cause of the remaining 50–60% of malformations is unknown. Multifactorial refers to causes with both genetic and environmental components. Environmental factors include exposures to drugs, chemicals, or other substances that affect the development of the fetus while he/she is in the uterus. Substances that cause birth defects are referred to as teratogens.

Artificial reproductive technology

Couples may pursue assisted reproductive technologies for a number of reasons. If a couple has artificial insemination, the sperm is inserted into the uterus when the woman in ovulating. Fertilization then occurs as it would normally. If a couple has in vitro fertilization (IVF), the egg and sperm are mixed outside the body in the laboratory. The zygote forms in a petri dish if fertilization occurs. After a number of cell divisions, the developing embryo is placed in the woman’s uterus. If the sperm are incapable of fusing with the egg themselves, the sperm may be injected into the egg. This additional step to the IVF procedure is called intracytoplasmic sperm injection (ICSI).

A human zygote. (Photo Researchers, Inc.)

In the year 2001, preimplantation diagnosis is possible for a number of genetic diseases. Couples may pursue this if they are at a significant risk for having a child with a disease that could be diagnosed prior to becoming pregnant through preimplantation diagnosis. The procedure is like that of in vitro fertilization, with an additional step. After fertilization occurs and the zygote has begun to divide, a single cell is removed. Removing the cell does not harm the other cells. The cell that is removed is tested for the genetic disease for which the couple is at risk. Multiple developing embryos are tested. Only the embryos that do not have the condition are placed in the woman’s uterus to complete development.

The development of a person from the zygote is a fascinating and amazing process. It is a difficult area to study because scientists cannot manipulate human embryos to observe the effects, and the development of the fetus cannot be directly observed. Researchers still have many unanswered questions. Following a doctor’s recommendations from prior to the pregnancy throughout pregnancy (such as folic acid intake and avoidance of alcohol and other drugs) increases the chances that the development of a zygote into a full-term infant will be normal. However, there are many babies born with severe birth defects or genetic diseases despite the parents’ efforts at doing everything in their power to prevent a problem. Most birth defects and genetic disorders occur because of an event out of control of the parents.

Zygote

Zygote

Resources

BOOKS

Agnew, Connie L. Twins!: Expert Advice From Two Practicing Physicians on Pregnancy, Birth, and the First Year. New York: HarperCollins, 1997.

Brasner, Shari E. Advice From a Pregnant Obstetrician. New York: Hyperion, 1998.

Nathanielsz, P.W. Life in the Womb: The Origin of Health and Disease. Ithaca, NY: Promethean Press, 1999.

Vaughn, Christopher C. How Life Begins: The Science of Life in the Womb. New York: Times Books, 1996.

PERIODICALS

Check, Erika. “What Moms Can Do Now.” Newsweek (27

September 1999): 57–58.

Christensen, Damaris. “Sobering Work.” Science News (8 July

2000): 28–29.

Kowalski, Kathiann. “High-tech Conception in the 21st

Century.” Current Health (January 2000): 1–4.

Miller, Annetta, and Joan Raymond. “The Infertility

Challenge.” Newsweek (Spring/Summer 1999 Special

Edition): 26–28.

ORGANIZATIONS

American College of Obstetricians and Gynecologists. PO Box 96920, 409 12th St. SW, Washington, DC 20090-6920.http://www.acog.org .

American Society for Reproductive Medicine. 1209 Montgomery Highway, Birmingham, AL 35216-2809. (205) 978-5000. asrm@asrm.org. http://www.asrm

.org .

RESOLVE, The National Infertility Association. 1310 Broadway, Somerville, MA 02144-1779. (617) 623-0744. resolveinc@aol.com. http://www.resolve.org .

WEBSITES

The InterNational Council on Infertility Information Dissemination, Inc. http://www.inciid.org .

Maternal and Child Health Bureau.

http://www.mchb.hrsa.gov/ .

Organization of Teratology Information Services.

http://www.otispregnancy.org/index.html .

Michelle Queneau Bosworth, MS, CGC

Pedigree charts are a visual tool for documenting biological relationships in families and the presence of disorders. Using these charts, a medical professional such as a geneticist or genetic counselor, can analyze the genetic risk in a family for a particular trait or condition by tracking which individuals have the disorder and determining how it is inherited.

A standard set of symbols has been established for use in creating pedigree charts. Those found within the body of several entries in the encyclopedia follow the symbol guide explained on the next page. The exact style and amount of information presented on the chart varies for each family and depends on the trait or condition under investigation. Typically, only data that is directly related to the disorder being analyzed will be included.

Symbol Guide for Pedigree Charts

A chromosome map indicates the relative positions of the genes that code for certain characteristics. The basic format for writing a gene position is the chromosome number, arm, band, sub-band, and sub-sub-band, if known. An example is 3p22.5.

The chromosome number refers to one of the 22 autosomal chromosomes (numbered 1-22) or one of the sex-determining chromosomes known as X or Y. In the example, the gene is on chromosome 3.

Each chromosome has two arms, separated by a centromere, the pinched-in area toward the top of the chromosome. The short arm, labeled “p”, is above the centromere and the long arm, “q”, is below it. In the case of the example gene, it is found on the short arm (p) of chromosome 3, or 3p.

The arms are further divided into cytogenetic bands (regions) numbered 1, 2, 3, etc... The numbers start at the

centromere and increase to the end of the arm, known as the telomere. These bands can only be seen when stained and viewed under a microscope. Sub-bands, which are numbered the same way as bands, may be visible within bands at greater magnifications. Therefore, the exact location of the example gene is the short arm (p) of chromosome 3, band 2, sub-band region 2, and a sub-sub band 5.

The following 24 illustrations demonstrate the approximate gene location for several of the genes relating to disorders mentioned in this encyclopedia. Disorders known to be related to a specific chromosome but not necessarily at an exact location have been placed below the chromosome. These chromosome maps are in no way complete, rather, they provide an introduction to understanding relative size differences of human chromosomes and where geneticists have located the genes associated with the source of certain genetic disorders.

Chromosome Map

Chromosome 4

q11

112

13

Map Chromosome

Chromosome Map

Chromosome 5

SRD51A: Steroid 4-alpha reductase 1 15 Cri du chat syndrome

14

1

13

12

p 11 CSA: Cockayne syndrome q

11

Familial adenomatous polyposis (5q)

12

113

14

15

21

222

23

Asthma

31

Corneal dystrophy

Chromosome 6

p 1 11

q11

12

113

14

15

16

21

22

223

24

EPM2A: Epilepsy

25

26

27

Major histocompatibility complex (6)

Chromosome 7

22

2 21

15

GCK: Diabetes

14

113

12

p 11

q

Chromosome 9

Distal arthrogryposis syndrome (9)

Chromosome 10

111

21

22

223

24

25

Chromosome Map

Chromosome 11

HRAS: Harvey ras oncogene

LQT1: Long QT syndrome

15

IDDM2: Diabetes

114

13

12

12DHC: Smith-Lemli-Opitz syndrome

1MEN1: Multiple endocrine neoplasia

13 MKS2: Meckel syndrome, type 2

14

21

22

ATM: Ataxia telangiectasia

223

24

25

Chromosome 13

13

1 12

13

1 RB1: Retinoblastoma

14

ATP7B: Wilson disease

21

2

22

31

332

33

34

Chromosome 12

q11

12

113

14

15

221

22

Chromosome 14

13

1 12 p 11

q

11

12

1

13

21

222

23

24

PS1(AD3): Alzheimer disease

31

3Krabbe disease

32