Metabolic Molecules 2. Water 3. Protein 5. Enzymes 6. Cell Respiration 9. Photosynthesis 3: Genetics 1. Genes 2. Chromosomes 3. Meiosis 4. Inheritance 5. Genetic Modification 4: Ecology 1. Energy Flow 3. Carbon Cycling 4. Climate Change 5: Evolution 1. Evolution Evidence 2. Natural Selection 3. Classification 4. Cladistics 6: Human Physiology 1. Genetic disorders are diseases, syndromes, or other conditions that are caused by mutations in one or more genes or by chromosomal alterations.
Genetic disorders are typically present at birth, but they should not be confused with congenital disorders, which are any disorders, regardless of cause, that are present at birth. Some congenital disorders are not caused by genetic mutations or chromosomal alterations. Instead, they are caused by problems that arise during embryonic or fetal development or during the process of birth.
An example of a nongenetic congenital disorder is fetal alcohol syndrome. This is a collection of birth defects, including facial anomalies and intellectual disability, caused by maternal alcohol consumption during pregnancy.
Some of the disorders are caused by mutations in autosomal genes, others by mutations in X-linked genes. Which disorders would you expect to be more common in males than females? Very few genetic disorders are controlled by dominant mutant alleles. A dominant allele is expressed in every individual who inherits even one copy of it. If it causes a serious disorder, affected people may die young and fail to reproduce.
Therefore, the mutant dominant allele is likely to die out of the population. A recessive mutant allele, such as the allele that causes sickle cell anemia or cystic fibrosis, is not expressed in people who inherit just one copy of it. These people are called carriers. They do not have the disorder themselves, but they carry the mutant allele and their offspring can inherit it.
Thus, the allele is likely to pass on to the next generation rather than die out. As we learned in the Cell Reproduction chapter , mistakes may occur during meiosis that results in nondisjunction. This is the failure of replicated chromosomes to separate properly during meiosis. Some of the resulting gametes will be missing all or part of a chromosome, while others will have an extra copy of all or part of the chromosome.
If such gametes are fertilized and form zygotes, they usually do not survive. If they do survive, the individuals are likely to have serious genetic disorders.
Most chromosomal disorders involve the X chromosome. The X and Y chromosomes are the only chromosome pair in which the two chromosomes are very different in size. This explains why nondisjunction of the sex chromosomes tends to occur more frequently than nondisjunction of autosomes. A genetic disorder that is caused by a mutation can be inherited.
Therefore, people with a genetic disorder in their family may be concerned about having children with the disorder. A genetic counselor can help them understand the risks of their children being affected.
One method of prenatal testing is amniocentesis. In this procedure, a few fetal cells are extracted from the fluid surrounding the fetus in utero , and the fetal chromosomes are examined.
Down syndrome and other chromosomal alterations can be detected in this way. The symptoms of genetic disorders can sometimes be treated or prevented.
Females that are heterozygous for an X-linked coat color gene will express one of two different coat colors over different regions of their body, corresponding to whichever X chromosome is inactivated in the embryonic cell progenitor of that region.
An individual carrying an abnormal number of X chromosomes will inactivate all but one X chromosome in each of her cells. However, even inactivated X chromosomes continue to express a few genes, and X chromosomes must reactivate for the proper maturation of female ovaries. As a result, X-chromosomal abnormalities are typically associated with mild mental and physical defects, as well as sterility.
If the X chromosome is absent altogether, the individual will not develop in utero. Several errors in sex chromosome number have been characterized.
Individuals with three X chromosomes, called triplo-X, are phenotypically female but express developmental delays and reduced fertility.
The XXY genotype, corresponding to one type of Klinefelter syndrome, corresponds to phenotypically male individuals with small testes, enlarged breasts, and reduced body hair. More complex types of Klinefelter syndrome exist in which the individual has as many as five X chromosomes. In all types, every X chromosome except one undergoes inactivation to compensate for the excess genetic dosage. This can be seen as several Barr bodies in each cell nucleus. Turner syndrome, characterized as an X0 genotype i.
In addition to the loss or gain of an entire chromosome, a chromosomal segment may be duplicated or lost. Duplications and deletions often produce offspring that survive but exhibit physical and mental abnormalities. Duplicated chromosomal segments may fuse to existing chromosomes or may be free in the nucleus. Figure 5. This individual with cri-du-chat syndrome is shown at two, four, nine, and 12 years of age. Improve this page Learn More.
Skip to main content. Module 8: Cell Division. Search for:. Common Disorders Learning Outcomes Identify common errors that can create an abnormal karyotype Identify syndromes that result from a significant change in chromosome number. Practice Question Figure 1.
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