Biological Sciences
X-linked Genes
X-linked genes are genes located on the X chromosome. In humans, males have one X and one Y chromosome, while females have two X chromosomes. As a result, X-linked genetic disorders are typically more common in males, as they only need to inherit one copy of the mutated gene. This inheritance pattern is known as X-linked inheritance.
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6 Key excerpts on "X-linked Genes"
eBook - PDF- William K. Scott, Marylyn D. Ritchie, William K. Scott, Marylyn D. Ritchie(Authors)
- 2021(Publication Date)
- Wiley-Blackwell(Publisher)
Basic Concepts in Genetics 28 X-linked Inheritance The majority of the genes located on the X chromosome do not have a complementary gene on the Y chromosome. Therefore, males are “hemizygous” at these loci because they have only a single copy of the gene. X-linked conditions were historically described as “X-linked dominant” or “X-linked recessive.” In an X-linked recessive condition, a single abnormal allele was sufficient to (a) (b) (c) (d) (e) Figure 2.10 Pedigrees consistent with (a) autosomal dominant inheritance, (b) autosomal recessive inheritance, (c) X-linked recessive inheritance, (d) X-linked dominant inheritance, and (e) mitochondrial inheritance. Here and elsewhere squares indicate males, circles indicate females, open symbols indicate unaffected individuals, and solid symbols indicate affected individuals. Genetic C Can nes Nssociate n itC iseasee/ hait Cenot tpes 29 cause disease in a hemizygous male, while females needed two. On the other hand, in “X-linked dominant” conditions, a single abnormal allele was sufficient to cause a condition in both males and females. Although these patterns of inheritance hold true for many conditions, female carriers may exhibit features of an X-linked recessive condition, due to skewed X-inactivation. For exam-ple, in Duchenne muscular dystrophy, approximately 25% of female carriers will exhibit some symptoms of the condition, ranging from muscle weakness to features as severe as seen in males (Hoogerwaard et al. 1999). This has led many researchers to argue that conditions with loci on the X-chromosome should simply be termed, “X-linked.” Mitochondrial Inheritance Mitochondrial inheritance is a non-classical pattern of single gene inheritance that is observed in conditions in which the causative allele is located in the mitochondrial DNA (mtDNA). In humans, each mitochondrion has approximately 2–10 copies of the mtDNA, which contains 37 genes.
eBook - PDFChild Psychology
Development in a Changing Society
- Robin Harwood, Scott A. Miller, Ross Vasta(Authors)
- 2012(Publication Date)
- Wiley(Publisher)
Researchers and physicians who work in these com- munities emphasize that “the burden of genetic disease is high and the need for special- ized and comprehensive care is essential to the health community” (Morton et al., 2003, p. 7). Populations such as the Old Order Amish and Mennonites provide a unique op- portunity to study the biology of rare genetic disorders and also point to the importance of genetic diversity in maintaining the health of a population. Many new findings are on the horizon of genetic research. Within the last few years, investigators have discovered the gene for a type of Alzheimer’s disease that runs in families. They also have located the genes for cystic fibrosis and for amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig’s disease. The genetic locus of many other diseases may soon be discovered as part of the Human Genome Project, discussed in the Research & Society box. X-Linked Disorders As can be seen in the photograph, the Y chromosome is only about a third the size of the X chromosome. This size difference between the chromosomes has implications for the transmission of certain kinds of disorders called x-linked disorders. One result of the larger X is that it contains more genetic information. As a consequence, some genes that travel on the X chromosome do not have a corresponding allele on the Y chromosome. In a female, if there is a recessive allele on one X chromosome, there is a good chance that the corresponding allele on her other X chromosome will override its expression. Males X-linked disorders Disorders that travel on the X chromosome, leaving males more vulnerable to their expression. Genetic Disorders 85 have no such protection and are thus more vulnerable than females to recessive disorders that travel only on the X chromosome. An example of an x-linked disorder is the allele responsible for normal blood clotting.
- Janette B. Benson, Marshall M. Haith(Authors)
- 2009(Publication Date)
- Academic Press(Publisher)
Finally, the last type of Mendelian transmission is exemplified by Y-linked disorders caused by mutations on Y chromosome. Although Y chromosome is very small and home to relatively few genes (compared with other chro-mosomes), it is far from being the genetic ‘badlands’ it was once considered. Clearly, this type of inheritance is relevant to males only; females do not have Y chromosome and, thus, cannot transmit or manifest a disorder. Y-linked disorders are rare and there are few examples of them. One such example is a late-onset sensorineural deafness. In addition, Y chromosome contains a number of genes involved in spermatogenesis; mutations in these genes have been reported to lead to genetic disorders of infertility. Repeat Expansions Single-Gene Disorders The 24 500 genes and DNA between them form 3 billion DNA bp. Of note is that a large portion of these bp are composed of recurring motifs of repetitive DNA (e.g., STRPs, see above; repetitive DNA sequences in specific parts of the human chromosomes, the telomeres and cen-tromeres, and heterogeneous DNA regions). These recur-ring motifs can occur both within and outside genes. When inside genes, they can disrupt the function of these genes. These repetitive motifs are heritable and transmitted from generation to generation. They form another, non-Mendelian type of mechanism for SGD. Fragile X syn-drome is an example of one such non-Mendelian SGD. Fragile X syndrome is a genetic condition associated with a variety of developmental problems ranging from learning difficulties to mental retardation. The symptoms are variable and can include anxiety, hyperactive behavior (e.g., fidgeting and excessive physical movements), impul-sivity, problems with social interactions and communica-tion, and seizures. The syndrome’s incidence is 1 in 4000 boys and 1 in 8000 girls, with greater severity of symptoms in males.
eBook - PDF- D. Peter Snustad, Michael J. Simmons(Authors)
- 2016(Publication Date)
- Wiley(Publisher)
GENES ON BOTH THE X AND Y CHROMOSOMES Some genes are present on both the X and Y chromosomes, mostly near the ends of the short arms (see Figure 5.2). Alleles of these genes do not follow a distinct X- or Y-linked pattern of inheritance. Instead, they are transmitted from mothers and fathers to sons and daughters alike, mimicking the inheritance of an autosomal gene. Such genes are therefore called pseudoautosomal genes. In males, the regions that contain these genes seem to mediate pairing between the X and Y chromosomes. In the animal kingdom, sex is perhaps the most conspicu- ous phenotype. Animals with distinct males and females are sexually dimorphic. Sometimes this dimorphism is established by environmental factors. In one species of turtles, for example, sex is determined by temperature. Eggs that have been incubated above 30°C hatch Sex Chromosomes and Sex Determination In some organisms, chromosomes—in particular, the sex chromosomes—determine male and female phenotypes. In this pedigree, II-1 is affected with X-linked hemophilia. If III-1 and III-2 have a child, what is the risk that the child will have hemophilia? Calculating the Risk for Hemophilia Solve It! I II 1 2 1 2 3 2 1 4 III ▶ To see the solution to this problem, visit the Student Companion site. ◾ FIGURE 5.9 Analysis of a pedigree showing the segregation of X-linked color blindness. I II III IV V Key: 1 2 1 2 1 2 1 2 3 4 1 3 4 5 3 4 5 Color blind Known carrier Y chromosome is transmitted with probability 1 /2. Y chromosome is transmitted with probability 1 /2. Mutant allele is transmitted with probability 1 /2. If a carrier, mutant allele is transmitted with probability 1 /2. Mutant allele is transmitted with probability 1 /2. P(IV-4 is color blind) = 1 /2 x 1 /2 = 1 /4 P(V-1 is color blind) = 1 /2 x 1 /2 x 1 /2 = 1 /8 Disorders such as hemophilia and color blindness, which are caused by recessive X-linked mutations, are more common in males than in females.
eBook - PDF- Kathleen A. Ireland(Author)
- 2018(Publication Date)
- Wiley(Publisher)
Scientists are just beginning to understand the significance of the Y chromosome genes to the male. Only one, the SRY gene, codes for male anatom- ical traits. The remaining Y chromosome genes are “housekeeping” genes—genes that are active in most body cells and do not confer male characteristics. None of these genes have specific homologous counterparts on the X chromosome. This is a potential problem during nuclear division, as the Y chromosome cannot condense and pair up with the X chro- mosome in the same fashion as autosomal chromosomes. Instead, the Y chromosome includes a series of palindromes that allow it to fold back on itself during cell division. With limited ability to cross over during meiosis or to silence dysfunctional genes on either the X or the Y chromosome during development, mutations are more often retained and expressed in the developing male. In females, having two copies of the X chromosome with all of its genes doubles the chance of expressing a functional allele. The male, however, has only one X chromosome. The alleles on that single chromosome must be used even if they are slightly defective. Genes carried on one sex chromosome with no counterpart on the other sex chromosome code for sex-linked traits. Because there are so many more functional genes on the X chromosome than on the Y, these are the genes usually referred to when discussing sex- linked traits. Characteristics carried on the X chromosome include color-blindness and hemophilia (Figure 21.9). Because males have only one X chromosome, any defective gene on it can cause disease. Scientists say this explains why males have higher rates of X-chromosome diseases, such as Duchenne muscular dystrophy and hemophilia. Paradoxically, having only one X chromo- some may have accelerated evolution among males. X-linked diseases can be powerful enough to kill males in utero or before reproductive age, which helps remove the defective genes from the population.
eBook - PDF- Joyce James, Colin Baker, Helen Swain(Authors)
- 2008(Publication Date)
- Wiley-Blackwell(Publisher)
Chapter Six GENETICS patients with end stage disease, people will be advised about treatments which could prevent the disease that they are most likely to inherit. Genes All the genetic information about an organism is passed from cell to cell via DNA in the nucleus. The DNA is arranged in genes on chro-mosomes. A chromosome is a linear DNA molecule bound with proteins called histones, which keep the chromosome tightly folded so that it fits into the nucleus. Chromosomes In humans there are 23 pairs of chromosomes. The sex chromosomes (X and Y) have different structures but each of the other pairs (auto-somes) have similar structures. Normal female chromosomes are XX and normal male chro-mosomes are XY. It is possible to identify and count the number of chromosomes in a cell by special staining techniques. The results from these techniques are called the karyotype (Fig. 6.1). Introduction Genetics is the study of inherited variations and characteristics (traits). Genes are a unique sequence of deoxyribonucleic acid (DNA) which code for a particular protein. Our genes are inherited from our parents – they determine what we look like and how we react to certain conditions. Some disorders and diseases can be traced to the inheritance of a single gene. Others depend on the inheritance of a group of genes or defective chromosomes. Knowledge about the genes that cause disease has led to research on gene therapy, which aims to give copies of the normal gene to the affected person. Nurses need to understand the significance of inherited disease since many genetic diseases can be detected by genetic screening techniques in early embryos. Genetic screening is also assuming a greater importance in the assess-ment of risk for developing a disease in later life, e.g. some types of cancer and Alzheimer’s disease. Identification of the genes involved in disease will undoubtedly influence the way in which patients are treated in the future.
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