Genetics, Conception, and Fetal Development

Genetics, Conception, and Fetal Development

Shannon E. Perry

Key Terms and Definitions

Web Resources

Additional related content can be found on the companion website at image

This chapter presents a brief discussion of genetics and an overview of the process of fertilization and of the development of the normal embryo and fetus.

Genetics image

Genetics, the study of a single gene or gene sequences and their effects on living organisms, is a contributing factor in virtually all human illnesses. In maternity care, genetics issues occur before, during, and after pregnancy. With growing public interest in genetics, increasing commercial pressures, and web-based opportunities for individuals, families, and communities to participate in the direction and design of their genetic health care, genetic services are rapidly becoming an integral part of routine health care.

For most genetic conditions, therapeutic or preventive measures do not exist or are very limited. Consequently, the most useful means of reducing the incidence of these disorders is by preventing their transmission. It is standard practice to assess all pregnant women for heritable disorders to identify potential problems.

Genetic disorders affect people of all ages, from all socioeconomic levels, and from all racial and ethnic backgrounds. Genetic disorders affect not only individuals, but also families, communities, and society. Advances in genetic testing and genetically based treatments have altered the care provided to affected individuals. Improvements in diagnostic capability have resulted in earlier diagnosis and enabled individuals who previously would have died in childhood to survive into adulthood. The genetic aberrations that lead to disease are present at birth but may not develop clinically for many years, possibly never.

Some disorders appear more often in ethnic groups. Examples include Tay-Sachs disease in Ashkenazi Jews, French Canadians of the Eastern St. Lawrence River valley area of Quebec, Cajuns from Louisiana, and the Amish in Pennsylvania; beta thalassemia in Mediterranean, Middle Eastern, Transcaucasus, Central Asian, Indian, and Far Eastern groups, as well as those of African heritage; sickle cell anemia in African-Americans; alpha thalassemia in those from Southeast Asia, South China, the Philippine Islands, Thailand, Greece, and Cyprus; lactase deficiency in adult Chinese and Thailanders; neural tube defects in Irish, Scots, and Welsh; phenylketonuria (PKU) in Irish, Scots, Scandinavians, Icelanders, and Polish; cystic fibrosis (CF) in Caucasians, Ashkenazi Jews, and Hispanics; and Niemann-Pick disease type A, in Ashkenazi Jews (Hamilton & Wynshaw-Boris, 2009; Solomon, Jack, & Feero, 2008) (Cultural Considerations box).


Genomics addresses the functions and interactions of all the genes in an organism. It is the study of the entire DNA structure. New fields incorporating genomic knowledge are emerging, for example, nutrigenetics, the study of the effect of genetic variations on diet and health with implications for susceptible subgroups; nutrigenomics, the study of the effect of nutrients on health through alteration of genome, proteome, and metabolome and noting changes in physiology that result; and pharmacogenetics/pharmacogenomics, the study of inherited variations in drug metabolism and response to the drug. Genomic health care incorporates assessment, diagnosis, and treatment that use information about gene function. It is highly individualized because treatment options are based on the phenotypic responses of an individual. Genetic information includes personal data, as well as information about blood relatives.

Genetics and Nursing

Genetic disorders span every clinical practice specialty and site, including school, clinic, office, hospital, mental health agency, and community health settings. Because the potential impact on families and the community is significant (Box 5-1), genetic information, technology, and testing must be integrated into health care services, and genetics must be integrated into nursing education and practice.

Although diagnosis and treatment of genetic disorders requires medical skills, nurses with advanced preparation are assuming important roles in counseling people about genetically transmitted or genetically influenced conditions. Nurses with expertise in genetics and genomics function in many areas of maternity and women’s health nursing. Examples include preconception counseling and preimplantation diagnosis for patients at risk for the transmission of a genetic disorder, prenatal screening and testing, prenatal care for women with psychiatric disorders that have a genetic component such as bipolar disorder and schizophrenia, newborn screening and testing, the care of families who have lost a fetus or a child affected by a genetic condition, the identification and care of children with genetic conditions and their families, and care of women with genetic conditions who require specialized care during pregnancy.

Nurses are usually the ones who provide follow-up care and maintain contact with the patients. Community health nurses can identify groups within populations that are at high risk for illness, as well as provide care to individuals, families, and groups. These nurses provide a vital link in follow-up for newborns who may need newborn screening (

Referral to appropriate agencies is an essential part of the follow-up management. Many organizations and foundations (e.g., the Cystic Fibrosis Foundation, the Muscular Dystrophy Association) help provide services and equipment for affected children. Numerous parent groups exist in which the family can share experiences and derive mutual support from other families with similar problems.

Probably the most important of all nursing functions is providing emotional support to the family during all aspects of the counseling process. Feelings that are generated under the real or imagined threat posed by a genetic disorder are as varied as the people being counseled. Responses may include a variety of stress reactions such as apathy, denial, anger, hostility, fear, embarrassment, grief, and loss of self-esteem.

In 2005 a panel of more than 50 nursing leaders from clinical, research, and academic settings developed and came to consensus on a document, “Essential Nursing Competencies and Curricula Guidelines for Genetics and Genomics.” The competencies in the document reflect the minimal amount of genetic and genomic competency expected of all nurses. The competencies are not intended to replace or recreate current standards of practice. The document is available online at Examples of competencies in the professional practice domain include that the registered nurse:

Genetics-related activities that all nurses should be able to provide are further delineated in Genetics/Genomics Nursing: Scope and Standards of Practice (2nd ed.) (International Society of Nurses in Genetics [ISONG] and American Nurses Association [ANA], 2006). This document includes standards and levels of practice for genetics nursing that were established cooperatively by ISONG and ANA. These activities are not limited to particular practice settings, nor are they limited to specific specialty areas.

Genetic History-Taking and Genetic Counseling Services

Determining whether a heritable disorder exists in a couple or in anyone in either of their families is standard practice in obstetrics. The goal of screening is to detect or define risk for disease in low risk populations and identify those for whom diagnostic testing may be appropriate. Obtaining a complete three-generation medical history that includes ethnicity information is the best genetic “test” applicable to preconception care (Solomon Jack, & Feero, 2008). The U.S. Surgeon General Family History Initiative has posted My Family Health Portrait at Nurses can recommend to their patients that they complete a family history using this website. In addition, nurses can obtain a genetic history using a questionnaire or a checklist such as the one in Fig. 5-1.

Individuals and families seek out, or are referred for, genetic counseling for a wide variety of reasons and at all stages of their lives. Some seek preconception or prenatal information; others are referred after the birth of a child with a birth defect or a suspected genetic condition; still others seek information because they have a family history of a genetic condition. Regardless of the setting or the individual and family’s stage of life, genetic counseling should be offered and available to all individuals and families who have questions about genetics and their health.

Genetic counseling that follows may occur in the office, or referral to a geneticist may be necessary. The most efficient counseling services are associated with the larger universities and major medical centers. These facilities are also where support services are available (e.g., biochemistry and cytology laboratories), usually from a group of specialists under the leadership of a physician trained in medical genetics. Health professionals should become familiar with people who provide genetic counseling and the places that offer counseling services in their area of practice.

Estimation of risk

Most families with a history of genetic disease want an answer to the following question: What is the chance that our future children will have this disease? Because the answer to this question may have profound implications for individual family members and the family as a whole, health care professionals must be able to answer this question as accurately as they can in a timely manner.

If a couple has not yet had children, but they are known to be at risk for having children with a genetic disease, they will be given an occurrence risk. Once the mating of a couple has produced one or more children with a genetic disease, the couple will be given a recurrence risk. Both occurrence and recurrence risks are determined by the mode of inheritance for the genetic disease in question. For genetic diseases caused by a factor that segregates during cell division (genes and chromosomes), risk can be estimated with a high degree of accuracy by application of mendelian principles.

In an autosomal dominant disorder, both the occurrence and recurrence risk is 50%, or one in two, that a subsequent offspring will be affected. The recurrence risk for autosomal recessive disorders is 25%, or one in four. For X-linked disorders, recurrence is related to the child’s sex. Translocation chromosomes have a high risk of recurrence.

The risk of recurrence for multifactorial conditions can be estimated empirically. An empiric risk is based not on genetics theory but rather on experience and observation of the disorder in other families. Recurrence risks are determined by applying the frequency of a similar disorder in other families to the case under consideration.

Disorders in which a subsequent pregnancy would carry no more risk than the risk for pregnancy alone (estimated at 1 in 30) include those resulting from isolated incidences not likely to be present in another pregnancy. These disorders include maternal infections (e.g., rubella, toxoplasmosis), maternal ingestion of drugs, most chromosomal abnormalities, and a disorder determined to be the result of a fresh mutation.

Interpretation of Risk

The guiding principle for genetics counselors is the principle of nondirectiveness. According to this principle, the individual who is providing genetic counseling respects the right of the individual or family being counseled to make autonomous decisions. Counselors using a nondirective approach avoid making recommendations, and they try to communicate genetics information in an unbiased manner. The first step in providing nondirective counseling is becoming aware of one’s own values and beliefs. Another important step is recognizing how one’s values and beliefs can influence or interfere with the communication of genetics information.

Counselors explain the risk estimates and provides appropriate information about the nature of the disorder, the extent of the risks in the specific case, the probable consequences, and (if appropriate) alternative options available. The final decision to become pregnant or to continue a pregnancy must be left to the family. An important nursing role is reinforcing the information the families are given and continuing to interpret this information at their level of understanding.

It must be emphasized to families is that each pregnancy is an independent event. For example, in monogenic disorders, in which the risk factor is one in four that the child will be affected, the risk remains the same no matter how many affected children are already in the family. Families may make the erroneous assumption that the presence of one affected child ensures that the next three will be free of the disorder. However, “chance has no memory.” The risk is one in four for each pregnancy. On the other hand, in a family with a child who has a disorder with multifactorial causes, the risk increases with each subsequent child born with the disorder.

Ethical Considerations

Learning about conditions that might affect the pregnancy in the preconception period is ideal (Solomon et al., 2008). Individuals can then make informed reproductive decisions that might include adoption, surrogacy, or use of donor sperm. However, most genetic testing is offered prenatally so as to identify genetic disorders in fetuses (Wapner, Jenkins, & Khalek, 2009). When an affected fetus is identified, parents can be prepared for birth of such an infant. Termination of the pregnancy is also an option. Other requests for genetic testing occur for sex selection or for late-onset disorders. An ethic of social responsibility should guide genetic counselors in their interactions with patients while recognizing that people make their choices by integrating personal values and beliefs with their new knowledge of genetic risk and medical treatments.

Other ethical issues relate to autonomy, privacy, and confidentiality. Should genetic testing be performed when no treatment is available for the disease? When should family members at risk for inherited diseases be warned? When should presymptomatic testing be performed? Some who might benefit from genetic testing choose not to have it, fearing discrimination based on the risk of a genetic disorder. Several states have prohibitions against insurance discrimination; other states are expected to follow their lead. Until guidelines for genetic testing are created, caution should be exercised. The benefits of testing should be weighed carefully against the potential for harm. The American Academy of Pediatrics (2001) and the Canadian College of Medical Geneticists (2003) recommend against genetic testing of children for disorders that have a late-onset and for which no treatment exists.

Preimplantation genetic screening (PGS) is available in a limited number of centers. In this procedure, embryos are tested before implantation by in vitro fertilization (IVF) (Wapner et al., 2009). PGS has the potential to eliminate specific disorders in pregnancies conceived by IVF. Work is ongoing to test fetal cells and nucleic acid retrieved from the maternal blood samples as means of noninvasive prenatal diagnosis (Wapner et al.).

The Human Genome Project

The Human Genome Project was a publicly funded international effort coordinated by the National Institutes of Health (NIH) and the U.S. Department of Energy. Not only was the Human Genome Project responsible for a long list of amazing genetics discoveries, but it also stimulated and facilitated the work of thousands of scientists worldwide. Within 24 hours after a piece of DNA had been sequenced by Human Genome Project scientists the results were posted on a public database (; no restrictions on its use or redistribution have been enacted.

Two key findings from initial efforts to sequence and analyze the human genome are that (1) all human beings are 99.9% identical at the DNA level, and (2) approximately 30,000 to 40,000 genes (pieces or sequences of DNA that contain information needed to make proteins) make up the human genome (International Human Genome Sequencing Consortium, 2001). The finding that human beings are 99.9% identical at the DNA level should help to discourage the use of science as a justification for drawing precise racial boundaries around certain groups of people. The vast majority of the 0.1% genetic variations are found within and not among populations. The finding that humans have 30,000 to 40,000 genes, which is only twice as many as roundworms (18,000) and flies (13,000), was unexpected. Scientists had estimated the human genome contained 80,000 to 150,000 genes. The assumption was that humans are more evolved and more highly sophisticated than other species because they have more genes.

Initial efforts to sequence and analyze the human genome have proven invaluable in the identification of genes involved in disease and in the development of genetic tests. More than 100 genes involved in diseases such as Huntington disease (HD), breast cancer, colon cancer, Alzheimer disease, achondroplasia, and CF have been identified. Genetic tests for 1672 inherited conditions are commercially available; of these, 1379 are clinical tests and 293 are research tests (

Genetic testing

Genetic testing involves the analysis of human DNA, ribonucleic acid (RNA), chromosomes (threadlike packages of genes and other DNA in the nucleus of a cell), or proteins to detect abnormalities related to an inherited condition. Genetic tests can be used to examine directly the DNA and RNA that make up a gene (direct or molecular testing), look at markers that are coinherited with a gene that causes a genetic condition (linkage analysis), examine the protein products of genes (biochemical testing), or examine chromosomes (cytogenetic testing).

Evidence-Based Practice

Genetic Risk Assessment for Breast Cancer

Critically Analyze the Data

Women seeking a risk assessment for breast cancer now have a new and powerful tool: genetic testing. The BRCA1 mutation can predispose a woman to breast cancer, whereas the BRCA2 can increase the risk for breast or ovarian cancer or both. An accurate reflection of risks could lead to greater psychologic well-being and less worry. In a meta-analysis reported in the Cochrane database, women who received genetic risk assessment reported less distress, a more accurate perceived risk, and increased knowledge about breast cancer and genetics (Sivell, Iredale, Gray, & Coles, 2007).

Women with a high risk of breast or ovarian cancer have the choice of risk-reducing surgery or more frequent screening. A study of 517 women, cancer-free but positive for BRCA1 or BRCA2, found that women were more likely to have prophylactic mastectomy and oophorectomy if they have a close family history with breast or ovarian cancer (Metcalfe, Foulkes, Kim-Sing, Ainsworth, Rosen, Armel, et al., 2008). A smaller study of 272 female carriers of the gene noted that predictors for prophylactic surgery were age less than 60 years and previous history of breast or ovarian cancer and that most made their decision within a median of 4 months (Beattie, Crawford, Lin, Vittinghoff, & Ziegler, 2009).

Women who have already had breast cancer can now assess their risk for recurrence. In a study by the Hereditary Breast Cancer Clinical Study Group an international cohort of 927 women with hereditary breast cancer were much more likely to undergo prophylactic mastectomy in North America than in Europe (Metcalfe, Lubinski, Ghadirian, Lynch, Kim-Sing, Friedman, et al., 2008). Similarly, the same study revealed that older women and women who chose mastectomy over breast-conserving surgery at the time of original diagnosis were more likely to choose the prophylactic contralateral mastectomy.


Beattie, M. S., Crawford, B., Lin, F., Vittinghoff, E., Ziegler, J. Uptake, time course, and predictors of risk-reducing surgeries in BRCA carriers. Genetic Testing and Molecular Biomarkers. 2009; 13(1):51–56.

Metcalfe, K. A., Foulkes, W. D., Kim-Sing, C., Ainsworth, P., Rosen, B., Armel, S., et al. Family history as a predictor of uptake of cancer preventive procedures by women with a BRCA1 or BRCA2 mutation. Clinical Genetics. 2008; 73(5):479–499.

Metcalfe, K. A., Lubinski, J., Ghadirian, P., Lynch, H., Kim-Sing, C., Friedman, E., et al. Predictors of contralateral prophylactic mastectomy in women with a BRCA1 or BRCA2 mutation: The Hereditary Breast Cancer Clinical Group Study Group. Journal of Clinical Oncology. 2008; 26(7):1093–1097.

Sivell, S., Iredale, R., Gray, J., & Coles, B. (2007). Cancer genetic risk assessment for individuals at risk of familial cancer. In The Cochrane Database of Systematic Reviews, 2007, Issue 2, CD 003721.

Most of the genetic tests now being offered in clinical practice are tests for single-gene disorders in patients with clinical symptoms or who have a family history of a genetic disease. Some of these genetic tests are prenatal tests or tests used to identify the genetic status of a pregnancy at risk for a genetic condition. Current prenatal testing options include maternal serum screening (a blood test used to see if a pregnant woman is at increased risk for carrying a fetus with a neural tube defect or a chromosomal abnormality such as Down syndrome) and invasive procedures (amniocentesis and chorionic villus sampling) (see Chapter 19). Other tests are carrier screening tests, which are used to identify individuals who have a gene mutation for a genetic condition but do not show symptoms of the condition because it is a condition that is inherited in an autosomal recessive form (e.g., CF, sickle cell disease, and Tay-Sachs disease) (Peach & Hopkin, 2007).

Predictive testing is used to clarify the genetic status of asymptomatic family members. Predictive testing is presymptomatic or predispositional. In presymptomatic testing, if the gene mutation is present, symptoms are certain to appear if the individual lives long enough (e.g., HD). Predispositional testing differs from presymptomatic testing in that a positive result indicating that a mutation is present (e.g., BRCA-1) does not indicate a 100% risk of developing the condition (breast cancer).

In addition to using genetic tests to test for single-gene disorders, genetic tests are being used for population-based screening, for example, state-mandated newborn screening for PKU and other inborn errors of metabolism (IEMs), and for testing for common complex diseases such as cancer and cardiovascular conditions. Genetic tests also are being used to determine paternity, identify victims of war and other tragedies, and profile criminals.

Gene therapy (gene transfer)

The aim of gene therapy is to correct defective genes that are responsible for disease development. The most common technique is to insert a normal gene in a location within the genome to replace a gene that is nonfunctional. In the early 1990s a great deal of optimism existed about the possibility of using genetic information to provide quick solutions to a long list of health problems. Although the early optimism about gene therapy was probably never fully justified, the development of safer and more effective methods for gene delivery will likely ensure a significant role for gene therapy in the treatment of some diseases. Major challenges include targeting the right gene to the right location in the right cells, expressing the transferred gene at the right time, and minimizing adverse reactions. No human gene therapy product has yet been approved by the U.S. Food and Drug Administration for sale. Current research includes treatment for inherited blindness, lung cancer tumors, melanoma, myeloid disorders, deafness, sickle cell disease, and other blood disorders.

Ethical, legal, and social implications

Because of widespread concern about misuse of the information gained through genetics research, 5% of the Human Genome Project budget was designated for the study of the Ethical, Legal, and Social Implications (ELSIs) of human genome research.

Two large ELSI programs were created to identify, analyze, and address the ELSIs of human genome research at the same time that the basic science issues were being studied. The two ELSI programs are separate but complementary. During the past decade, issues of high priority for these programs have been privacy and fairness in the use and interpretation of genetic information; clinical integration of new genetics technologies; issues surrounding genetics research, such as possible discrimination and stigmatization; and education for professionals and the general public about genetics, genetics health care, and ELSIs of human genome research. Both ELSI programs have excellent websites that include vast amounts of educational information, as well as links to other informative sites.

The ELSI programs address the potential that genetic information may be used to discriminate against individuals or for eugenic purposes. Informed consent is very difficult to ensure when some of the outcomes, benefits, and risks of genetic testing remain unknown. Some ethical considerations include: What is normal or a disability, and who decides? Do disabilities or diseases exist that need to be prevented or cured? Who will have access to these expensive therapies, and who will pay for them? Continued awareness of and vigilance against misuse of information is the collective responsibility of health care providers, ethicists, and society.

Factors Influencing the Decision to Undergo Genetic Testing

The decision to undergo genetic testing is seldom an autonomous decision based solely on the needs and preferences of the individual being tested. Instead, a decision is often based on feelings of responsibility and commitment to others. For example, a woman who is receiving treatment for breast cancer may undergo BRCA1/BRCA2 mutation testing not because she wants to find out if she carries a BRCA1 or BRCA2 mutation, but because her two unaffected sisters have asked her to be tested, and she feels a sense of responsibility and commitment to them. A female airline pilot with a family history of HD, who has no desire to find out if she has the gene mutation associated with HD, may undergo mutation analysis for HD because she believes she has an obligation to her family, her employer, and the people who fly with her.

Decisions about genetic testing are shaped, and in many instances constrained, by factors such as social norms, where care is received, and socioeconomic status. Most pregnant women in the United States now have at least one ultrasound examination, many undergo some type of multiple-marker screening, and a growing number undergo other types of prenatal testing. The range of prenatal testing options available to a pregnant woman and her family may vary significantly, based on where the pregnant woman receives prenatal care and her socioeconomic status. Certain types of prenatal testing may not be available in smaller communities and rural settings (e.g., chorionic villus sampling and FISH analysis [fluorescent in situ hybridization]). In addition, certain types of genetic testing may not be offered in conservative medical communities (e.g., preimplantation diagnosis). Some types of genetic testing are expensive and typically not covered by health insurance. Because of this, these tests may be available only to a relatively small number of individuals and families: those who can afford to pay for them.

Cultural and ethnic differences also have a significant impact on decisions about genetic testing. When prenatal diagnosis was first introduced, the principal constituency was a self-selected group of Caucasian, well-informed, middle to upper-class women. Today the widespread use of genetic testing has introduced prenatal testing to new groups of women, women who had not previously considered genetics services. The fact that many of the women currently undergoing prenatal testing may not share mainstream U.S. views about the role of medicine and prenatal care, the meaning of disability, or how to respond to scientific risks and uncertainties further amplifies the complexity of ethical issues associated with prenatal testing.

Genes and Chromosomes

The hereditary material carried in the nucleus of each somatic (body) cell determines an individual’s physical characteristics. This material—DNA—forms threadlike strands known as chromosomes. Each chromosome is composed of many smaller segments of DNA referred to as genes. Genes or combinations of genes contain coded information that determines an individual’s unique characteristics. The code consists of the specific linear order of the molecules that combine to form the strands of DNA. Genes never act in isolation; they always interact with other genes and the environment.

All normal human somatic cells contain 46 chromosomes arranged as 23 pairs of homologous (matched) chromosomes; one chromosome of each pair is inherited from each parent. There are 22 pairs of autosomes, which control most traits in the body, and one pair of sex chromosomes, which determines sex and some other traits. The large female chromosome is called the X; the tiny male chromosome is the Y. When one X chromosome and one Y chromosome are present, the embryo develops as a male. When two X chromosomes are present, the embryo develops as a female.

Homologous chromosomes (except the X and Y chromosomes in males) have the same number and arrangement of genes. In other words, if an autosome has a gene for hair color, its partner also has a gene for hair color—in the same location on the chromosome. Although both genes code for hair color, they may not code for the same hair color. Genes at corresponding loci on homologous chromosomes that code for different forms or variations of the same trait are called alleles. An individual with two copies of the same allele for a given trait is homozygous for that trait. With two different alleles, the person is heterozygous for the trait.

The term genotype is typically used to refer to the genetic makeup of an individual when discussing a specific gene pair, but at times, genotype is used to refer to an individual’s entire genetic makeup or all the genes that the individual can pass on to future generations. Phenotype refers to the observable expression of an individual’s genotype, such as physical features, a biochemical or molecular trait, and even a psychologic trait. A trait or disorder is considered dominant if it is expressed or phenotypically apparent when only one copy of the gene is present. It is considered recessive if it is expressed only when two copies of the gene are present.

Chromosomal Abnormalities

The incidence of chromosomal aberrations is estimated to be 0.6% in newborns. Approximately 62% of miscarriages and 10% of stillbirths and perinatal deaths are caused by chromosomal abnormalities (Hamilton & Wynshaw-Boris, 2009). Errors resulting in chromosomal abnormalities can occur in mitosis or meiosis. These errors occur in either the autosomes or the sex chromosomes. Even without the presence of obvious structural malformations, small deviations in chromosomes can cause problems in fetal development.

The pictorial analysis of the number, form, and size of an individual’s chromosomes is known as a karyotype. Cells from any nucleated, replicating body tissue (except red blood cells, nerves, or muscles) can be used. The most commonly used tissues are white blood cells and fetal cells in amniotic fluid. The cells are grown in a culture and arrested when they are in metaphase, and then the cells are dropped onto a slide. This process breaks the cell membranes and spreads the chromosomes, making them easier to visualize. The cells are stained with special stains (e.g., Giemsa stain) that create striping or “banding” patterns. These patterns aid in the analysis because they are consistent from person to person. Once the chromosome spreads are photographed or scanned by a computer, they are cut out and arranged in a specific numeric order according to their length and shape. The chromosomes are numbered from largest to smallest, 1 to 22, and the sex chromosomes are designated by the letter X or Y. Each chromosome is divided into two “arms” designated by p (short arm) and q (long arm). A female karyotype is designated as 46, XX and a male karyotype is designated as 46, XY. Fig. 5-2 illustrates the chromosomes in a body cell and a karyotype. Karyotypes can be used to determine the sex of a child and the presence of any gross chromosomal abnormalities.

Autosomal abnormalities

Autosomal abnormalities involve differences in the number or structure of autosomal chromosomes (pairs 1-22) resulting from unequal distribution of the genetic material during gamete (egg and sperm) formation.

Abnormalities of chromosome number.

Euploidy denotes the correct number of chromosomes. Deviations from the correct number of chromosomes or the diploid number (2N, 46 chromosomes) can be one of two types: (1) polyploidy, in which the deviation is an exact multiple of the haploid number of chromosomes or one chromosome set (23 chromosomes); or (2) aneuploidy, in which the numerical deviation is not an exact multiple of the haploid set. A triploid (3N) cell is an example of a polyploidy. It has 69 chromosomes. A tetraploid (4N) cell, also an example of a polyploidy, has 92 chromosomes.

Aneuploidy is the most commonly identified chromosome abnormality in humans. Aneuploidy occurs in at least 5% of all clinically recognized pregnancies, and it is the leading known cause of pregnancy loss. Aneuploidy is also the leading genetic cause of mental retardation. The two most common aneuploid conditions are monosomies and trisomies. A monosomy is the product of the union between a normal gamete and a gamete that is missing a chromosome. Monosomic individuals have only 45 chromosomes in each of their cells. Limited data are available concerning the origin of monosomies because when an embryo is missing an autosomal chromosome, the embryo never survives.

The product of the union of a normal gamete with a gamete containing an extra chromosome is a trisomy. Trisomies are more common than monosomies. Trisomic individuals have 47 chromosomes in each of their cells. Most trisomies are caused by nondisjunction during the first meiotic division. That is, one pair of chromosomes fails to separate. One of the resulting cells contains two chromosomes, and the other contains none.

The most common trisomal abnormality is Down syndrome, or trisomy 21 (47, XX+21, female with Down syndrome; or 47, XY+21, male with Down syndrome). Although the risk of having a child with Down syndrome increases with maternal age (incidence is approximately 1 in 1200 for a 25-year-old woman, 1 in 350 for a 35-year-old woman, and 1 in 30 for a 45-year-old woman), children with Down syndrome can be born to mothers of any age. Eighty percent of children with Down syndrome are born to mothers younger than 35 years (National Down Syndrome Society, 2006) (Nursing Care Plan).

Other autosomal trisomies that have been identified are trisomy 18 (Edwards syndrome) and trisomy 13 (Patau syndrome). Infants with trisomy 18 and trisomy 13 are usually severely to profoundly retarded. Although both conditions have a very poor prognosis, with the vast majority of affected infants dying within the first few days of life, a significant percentage of these infants will survive the first 6 months to 1 year of life; some children with trisomy 18 and trisomy 13 have lived beyond age 10 years.

Nondisjunction can also occur during mitosis. If it occurs early in development, when cell lines are forming, the individual has a mixture of cells, some with a normal number of chromosomes and others either missing a chromosome or containing an extra chromosome. This condition is known as mosaicism. Mosaicism in autosomes is most commonly seen as another form of Down syndrome. Approximately 1% to 2% of individuals with Down syndrome have mosaic Down syndrome.

Oct 8, 2016 | Posted by in NURSING | Comments Off on Genetics, Conception, and Fetal Development

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