Biological Sciences
Chromosomal Disorders
Chromosomal disorders are genetic conditions caused by abnormalities in the number or structure of chromosomes. These disorders can result in a wide range of physical and developmental abnormalities, such as Down syndrome and Turner syndrome. They are typically diagnosed through genetic testing and can have significant impacts on an individual's health and well-being.
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9 Key excerpts on "Chromosomal Disorders"
eBook - PDF- Angela L. Williams(Author)
- 2019(Publication Date)
- Omnigraphics(Publisher)
The test examines the fetus s DNA in the mother s blood. What Are Chromosome Abnormalities? There are many types of chromosome abnormalities. However, they can be organized into two basic groups: numerical abnormalities and structural abnormalities. Numerical abnormalities : When an individual is missing one of the chromosomes from a pair, the condition is called “monosomy.” When an individual has more than two chromosomes instead of a pair, the condition is called “trisomy.” An example of a condition caused by numerical abnormalities is Down syndrome, which is marked by mental retardation, learning difficulties, a characteristic facial appearance, and poor muscle tone (hypotonia) in infancy. An individual with Down syndrome has three copies of chromosome 21 rather than two; for that reason, the condition is also known as “Trisomy 21.” An example of monosomy, in which an individual lacks a chromosome, is Turner syndrome. In Turner syndrome, a female is born with only one sex chromosome, an X, and is usually shorter than average and unable to have children, among other difficulties. Structural abnormalities : A chromosome s structure can be altered in several ways. Deletions : A portion of the chromosome is missing or deleted. Duplications : A portion of the chromosome is duplicated, resulting in extra genetic material. Translocations : A portion of one chromosome is transferred to another chromosome. There are two main types of translocation. In a reciprocal translocation, segments from two different chromosomes have been exchanged. In 446 Genetic Disorders Sourcebook, Seventh Edition a Robertsonian translocation, an entire chromosome has attached to another at the centromere. Inversions : A portion of the chromosome has broken off, turned upside down, and reattached. As a result, the genetic material is inverted. Rings : A portion of a chromosome has broken off and formed a circle or ring. This can happen with or without loss of genetic material.
eBook - ePub- Tom Lissauer, Will Carroll, Tom Lissauer, Will Carroll(Authors)
- 2016(Publication Date)
- Elsevier(Publisher)
Chapter 9Genetics
Richard H Scott, Shereen TadrosLearning objectivesAfter reading this chapter the reader should:• Understand the scientific basis of genetic disorders and the patterns of inheritance they show• Know how to use this knowledge to frame the important clinical aspects of the disorders commonly encountered in paediatric practice• Understand the principles underpinning malformation disorders• Understand the scientific basis and clinical use of chromosome and molecular genetic tests including karyotype, fluorescence in situ hybridization, microarrays, DNA sequencing and testing for imprinting disorders• Understand the principles of predictive and diagnostic genetic testing and the ethical dilemmas related to genetic testing in childhoodChromosomal Disorders
Chromosomes
The term ‘chromosome’ comes from the Greek for ‘colour’ and ‘body,’ and was coined in the late 19th century in reference to their staining behaviour when dyes were applied. Chromosomes are string-like bodies present in the nucleus of every nucleated cell. Each chromosome consists of DNA that is tightly coiled around proteins known as histones that support its structure. Much of what we know about chromosomes comes from observations during cell division, when the DNA becomes more tightly coiled and is thus visible under a microscope. Humans normally have 22 pairs of numbered chromosomes – autosomes – and one pair of sex chromosomes – XX or XY. Each parent contributes one chromosome to each pair; therefore, children get half of their chromosomes from their mother and half from their father.Each chromosome has a constricted region known as a centromere. The regions on either side of the centromere are the chromosome's arms. The shorter arm is the ‘p arm’ (for ‘petit’ or small) and the longer arm is the ‘q arm’ (as next letter in the alphabet!). The ends of the chromosomes are called telomeres.
eBook - PDFChild Psychology
A Canadian Perspective
- Alastair Younger, Scott A. Adler, Ross Vasta(Authors)
- 2014(Publication Date)
- Wiley(Publisher)
The problems that we consider now are therefore the exception to the general rule. Some human disorders are entirely hereditary and are passed along according to the same principles of inheritance that determine eye colour and nose shape. Other genetic disorders are not inherited, but may result from errors during cell division in meiosis. Chromosomes and the genes they carry can also be made abnormal by radiation, drugs, viruses, chemicals, and perhaps even the aging process. In this section, we examine the various kinds of genetic disorders and discuss some examples of each. Table 3.2 summarizes both the disorders discussed in the text and several other important examples. HEREDITARY DISORDERS According to Mendel’s principles, just as a child may inherit genes for brown or blue eyes, ab- normal genes can also be passed along to offspring. Whether defective genes are expressed in the phenotype depends on whether they are dominant or recessive. If a defective gene inherited from one parent is recessive, the dominant (and usually normal) allele from the other parent can prevent the problem. Of course, the problem gene still exists in the genotype and will be passed on to half the person’s offspring. Most of the offspring will themselves be unaffected; those who receive the defective gene from both parents, however, will develop the disorder. Learning Objective 3.2 Describe different types of genetic disorders and their impact on child development. 75 Genetic and Chromosomal Disorders DOMINANT TRAITS Dominant genes that cause severe problems typically disappear from the species because the affected people usually do not live to reproduce. In a few cases, however, severely disabling dominant genes are passed on because they do not become active until relatively late in life. People with these genes may reproduce before they know that they have inherited the disease.
eBook - ePubAnatomy and Physiology for Midwives E-Book
Anatomy and Physiology for Midwives E-Book
- Jane Coad, Kevin Pedley, Melvyn Dunstall(Authors)
- 2019(Publication Date)
- Elsevier(Publisher)
Box 7.11 ). If the changes can be seen by light microscopy such as a marked structural abnormality or an atypical number of chromosomes, they are termed ‘gross aberrations’ and can be detected from an examination of the karyotype. Chromosomal abnormalities can be classified as numerical or structural, affecting either the autosomes or the sex chromosomes. These types of abnormality are easier than a single-gene abnormality to detect.BOX 7.11 Incidence of Chromosomal Abnormalities- • Incidence of major chromosomal abnormality
-
- • About 1 in 200 live births
- • About 1 in 20 perinatal deaths (stillbirths and early neonatal deaths)
- • About 1 in 2 early spontaneous abortions
- • About 1 in 100 births: single-gene (unifactorial) disorder
- • About 1 in 50 births: + major congenital abnormality
Numerical Abnormalities
The loss or gain of one or more chromosomes is described as aneuploidy (wrong number of chromosomes), whereas cells with the correct number of chromosomes are euploidic. It is estimated that 10–25% of all human fetuses are aneuploidal, predominantly due to nondisjunction in maternal meiosis, although trisomy 18 most often results from nondisjunction in meiosis II. Aneuploidy appears to occur more frequently in humans than in other species, possibly because of deliberate or occupational exposure to environmental factors such as tobacco smoke, alcohol, oral contraception use, radiation exposure and industrial chemicals, which is probably one of the reasons for a high rate of miscarriage in humans.Aneuploidy is usually due to nondisjunction in the formation of the gametes resulting in a zygote that does not have 46 chromosomes. Monosomy describes the loss of a complete chromosome and trisomy describes the addition of a single chromosome, as in Down syndrome (trisomy 21 or T21 caused by an additional copy of chromosome 21) (see Table 7.1
eBook - ePub- Chang-Hui Shen(Author)
- 2019(Publication Date)
- Academic Press(Publisher)
Chromosome abnormalities arise from genomic variants. Because most chromosome abnormalities occur accidentally in the ovum or sperm, the abnormality is present in every cell of the body. Some abnormalities arise after birth, however, resulting in a condition in which a few cells have the abnormality and others do not. Chromosomes abnormalities can be either inherited from a parent (e.g., translocation) or develop spontaneously for the first time. This is why chromosome studies are performed on parents when a child is found to have an abnormality. Advances in molecular biology and cytogenetic techniques permit the identification of many diverse types of SVs, which contribute to human disease, phenotypic variation, and karyotypic evolution.Errors in Cell Division
Chromosome abnormalities usually occur when there is an error in cell division (Fig. 13.30 ). There are two kinds of cell division. Meiosis results in cells with half the number of usual chromosomes, 23 instead of the normal 46. These are the eggs and sperm. Mitosis produces two cells that are duplicates (46 chromosomes each) of the original cell. This kind of cell division occurs throughout the body, except in the reproductive organs. In both processes, the correct number of chromosomes appears in the daughter cells. Errors in cell division, however, can result in cells with too few or too many copies of a chromosome.Fig. 13.30 Effects of nondisjunction of sex chromosomes during cell division.Maternal Age
Additional factors, such as maternal age, can increase the risk of chromosome abnormalities. Women are born with all the eggs they will ever have. Therefore, when a woman is at certain age, so are her eggs. Chromosomal errors can appear in eggs as they age. Thus, older women are at greater risk of giving birth to babies with chromosome abnormalities than younger women. A second factor might be environmental conditions, although conclusive evidence is currently lacking.
- K.V. Chaitanya(Author)
- 2022(Publication Date)
- CRC Press(Publisher)
Genetic disorders are the conditions which produce specific morphological and physiological changes as a response to a change in the genetic makeup of an organism. These changes are due to either excess or lack of genetic information in an individual, depending on multiple factors such as the genetic combination of the parents, individual, and affected chromosomal pairs. Studies on single gene disorders will provide invaluable insight into the underlying molecular mechanisms responsible for their increased understanding. Currently, these genetic disorders do not have any permanent cure. The treatment provided for these disorders is only palliative to improve their health condition. Even though the cytogenetic analysis is the fundamental beginning point for uncovering the mechanisms and etiology of genetic disorders, the amount of gene expression and its possible interaction with the environment is insufficient for disease diagnosis and its designation at the present stage of medicine. Not much knowledge on the nature of the genetic disorders and the mechanisms of the phenotypic outcomes and cellular pathways to disease is available. Single gene mutations are responsible for most chromosomal or genetic instability due to their manifestations in cellular and molecular pathways and alterations in their regulatory regions. The diseases associated with a single enzyme defect, or a pathway defect are attributed to a cascade of abnormal molecular and cellular events.Single gene disorders are well understood and most studied sections of genetic disorders, with specific inheritance properties such as dominant and recessive and a very simple genetic etiology. Every individual carries two sets of 22 autosomes and either XX or XY allosomes, with each set inherited from each parent. It also carries two copies of every gene on these chromosomes, divided into autosomal, X-linked, and Y-linked inheritance. Autosomal and X-linked can be further divided into dominant and recessive, depending on whether one or two mutant alleles are required to cause disease. Even though a single gene primarily causes these diseases, several different mutations on the same gene can even lead to the same disease, with varying degrees of severity and phenotype. Sometimes, the same mutation can result in different phenotypes, mainly due to the environment in which the patient lives and the genetic variations, which influence the disease phenotype or outcome. Single gene disorders are described in the following sections.
eBook - PDF- Kenji Ikehara(Author)
- 2011(Publication Date)
- IntechOpen(Publisher)
Currently cytogenetics is paving its way into the molecular approaches in deciphering the structure, function and evolution of chromosomes. Still, conventional cytogenetics where routine banding techniques are employed remains a Cytogenetic Techniques in Diagnosing Genetic Disorders 61 simple and popular technique to get an overview of the human genome as a whole (Thirumulu Kannan Ponnuraj & Zilfalil Alwi, 2009). Routine banded karyotype analysis can now be combined with M-FISH and other molecular techniques leading to more precise detection of various syndromes in children. Through the analysis of chromosome banding patterns, thousands of chromosomal abnormalities have been associated with inherited or de novo disorders, generating many leads to the underlying molecular causes of these disorders and today, when high resolution genetic linkage analysis can be conducted easily, the discovery of a patient whose disorder is caused by a gross chromosomal abnormality is heralded as a valuable resource for locating the disease gene. Solid tumors also present a myriad of complex chromosomal aberrations and each is a possible clue to tumor initiation and progression. The challenge is to navigate from the visible morphological alteration to the DNA sequence level. In other words, chromosomal abnormalities exist as nature’s guide to the molecular basis of many unexplained human disorders. Hence, cytogenetics continue to remain as indispensable tools for the diagnosis of various genetic disorders which gives an overall picture of the whole genome for analysis. This could possibly also pave a way for treatment and management related to Chromosomal Disorders. 7. References Albertson, D. & Pinkel, D. (2003). Genomic microarrays in human genetic disease and cancer. Human Molecular Genetics 12: 145–152. Arrighi, F.E. & Hsu, T.C. (1971). Localization of heterochromatin in human chromosomes.
eBook - ePub- Reed E. Pyeritz, Bruce R. Korf, Wayne W. Grody(Authors)
- 2018(Publication Date)
- Academic Press(Publisher)
Finally, we briefly review the various types of human chromosomal abnormalities. Keywords Chromosomal abnormalities; Chromosome banding; Chromosomal microarray; Chromosome structure and function; FISH; Meiosis; Mitosis 9.1. Introduction The human genome is packaged into a set of chromosomes as in other eukaryotes. Chromosomes are thus the vehicles of inheritance as they contain virtually the entire cellular DNA, with the exception of the small fraction present in the mitochondria. The structure, function, and behavior of chromosomes are therefore of much interest and importance. Chromosomes are derived in equal numbers from the mother and the father. Each ovum and sperm contains a set of 23 different chromosomes, which is the haploid number (n) of chromosomes in humans. The diploid fertilized egg and virtually every cell of the body arising from it has two haploid sets of chromosomes, resulting in the diploid human chromosome number (2 n) of 46. The human karyotype consists of 22 pairs of autosomes and a pair of sex chromosomes. The correct chromosome number in humans was determined and confirmed in 1956 [ 1, 2 ]. The behavior of chromosomes during meiotic cell division provides the basis for the Mendelian laws of inheritance, whereas their abnormal behavior in cell division leads to abnormalities of chromosome number. In this chapter, we examine the current understanding of the structure, molecular organization, and behavior of human chromosomes and explore how these features contribute to chromosomal diseases. 9.2. Chromosome Structure Although the structure of human and other eukaryotic chromosomes is not understood in full detail, recent investigations have provided insights into several aspects of chromosome structure at the molecular level. The haploid human genome consists of about 3 × 10 9 base pairs (bp) of DNA. Since 3000 bp of naked DNA are ∼1 μm long, the total length of the diploid human genome is about 2 m
eBook - PDF- Jorge J. Yunis(Author)
- 2013(Publication Date)
- Academic Press(Publisher)
Therefore, the incidence figures may not be applicable to other ethnic groups. It is well known, for instance, that elevated maternal age is associated with an in-creased incidence of the various trisomies known in man, and non-Caucasian mothers generally bear children at a younger age. Similarly, environmental influences such as exposure to radiation, socioeconomic factors, or disease in the parents may play a less obvious but significant role in the frequency of chromosomal abnormalities. Knowledge contributed by human cytogenetics may be of value in the general field of genetics: chromosome and sex chromatin studies are being Human Chromosomes in Disease 189 used in the evaluation of homotransplantation barriers (Peer, 1958; Wood-ruff and Lennox, 1959) ; in the study of dosage compensation as impli-cated in Lyon's ( 1 9 6 2 ) hypothesis; in the study of drug effect (Chevre-mont, 1961) and radiation (Bender and Gooch, 1963) on mitosis; and in the study of mammalian cell genetics in tissue culture (Moorhead, 1962). The field of human cytogenetics has grown so rapidly in the last few years that only a brief survey of the significant data can be accomplished in this chapter. Cytogenetics as it pertains to clinical problems will be emphasized, and variations of the main chromosome genotypes will be stressed because these patterns provide knowledge of fundamental bio-logical significance. IL S E X C H R O M O S O M A L A B N O R M A L I T I E S At the early stages of the human chromosome study, it was hoped that sex chromosomal aberrations would sharply define disease entities such as Turner's syndrome, Klinefelter's syndrome, and true hermaphroditism. The last few years of investigation in this field have revealed such an impressive array of genotypes and phenotypes for each of the clinical entities mentioned that some confusion has arisen. The data collected are vast, not always comparable or acceptable, and still incomplete.
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