Monogenetic disorders are caused by a mutation in a single gene. The mutation may be present on one or both chromosomes (one chromosome inherited from each parent). Examples of monogenic disorders are: sickle cell disease, cystic fibrosis, polycystic kidney disease, and Tay-Sachs disease. A genetic disorder is a condition that is caused by an abnormality in an individual's DNA. Abnormalities can be as small as a single-base mutation in just one. There is no question that the rapid advance in genetic technology is changing our viewpoint on medical practice, which is dramatically.


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In contrast to Turner syndrome, which results from the absence of human genetic disorders sex chromosome, three alternative conditions result from the presence of an extra sex chromosome: Klinefelter syndrome, trisomy X, and 47,XYY syndrome.

These conditions, each of which occurs in about 1 in 1, live births, are clinically mild, perhaps reflecting the fact that the Y chromosome carries relatively few genes, and, although the Human genetic disorders chromosome is gene-rich, most of these genes become transcriptionally silent in all but one X chromosome in each somatic cell i.

The phenomenon of X inactivation prevents a female who carries two copies human genetic disorders the X chromosome in every cell from expressing twice the amount of gene products encoded exclusively on the X chromosome, in comparison with males, who carry a single X.

What is a Human Genetic Disorder?

In brief, at some point in early development one X chromosome in human genetic disorders somatic cell of a female embryo undergoes chemical modification and is inactivated so that gene expression no longer human genetic disorders from that template.

This process is apparently random in most embryonic tissues, so that roughly half of the cells in each somatic tissue will inactivate the maternal X while the other half will inactivate the paternal X.


Cells destined to give rise to eggs do not undergo X inactivation, and cells of the extra-embryonic tissues preferentially inactivate the paternal X, although the rationale for this preference is unclear. The inactivated X chromosome typically replicates later than other chromosomes, and it physically condenses to form a Barr bodya small structure found at the rim of the nucleus in female human genetic disorders cells between divisions see photograph.

Although the process is apparently random, not every female human genetic disorders an exact 1: Indeed, studies suggest that ratios of X inactivation can vary.

Furthermore, not all genes on the X chromosome are inactivated; a small number escape modification and remain actively expressed from both X chromosomes in the cell.

Genetic Disorders

Although this class of genes has not yet been fully characterized, aberrant expression of these genes has been raised as one possible explanation for the phenotypic abnormalities experienced by human genetic disorders with too few or too many X chromosomes.

Klinefelter syndrome 47,XXY occurs in males and is associated with increased stature and infertility. Males with Klinefelter syndrome, like normal females, inactivate one of their two X chromosomes in each cell, perhaps explaining, at least in part, the human genetic disorders mild clinical outcome.


Trisomy X 47,XXX is seen in females and is generally also considered clinically benignalthough menstrual irregularities or sterility human genetic disorders been noted in some cases. Females with trisomy X inactivate two of the three X chromosomes in human genetic disorders of their cells, again perhaps explaining the clinically benign outcome.

There is some evidence of mild learning disability associated with each of the sex chromosome trisomies, although there is no evidence of mental retardation in these persons.

Human genetic disease

These individuals show clinical outcomes similar to those seen in males with Klinefelter syndrome but with slightly increased severity. Diseases associated with single-gene Mendelian inheritance The term Mendelian is often used to denote patterns of genetic inheritance similar to those described for traits in the garden pea by Gregor Mendel in the s.

Disorders associated with single-gene Mendelian inheritance are typically categorized as autosomal dominant, autosomal recessive, or sex-linked. Each category is described briefly in this section. For a full explanation of Mendelian genetics and of the concepts of dominance and recessiveness, see the article heredity.

Autosomal dominant inheritance A disease trait that is inherited in an autosomal dominant manner can occur in either sex and can be transmitted by either parent. In such a case the pedigree i. The figure illustrates the pedigree human genetic disorders a family with achondroplasiaan autosomal dominant disorder characterized by short-limbed dwarfism that results from a specific mutation in the fibroblast growth factor receptor 3 Human genetic disorders gene.

In pedigrees of this sort, circles refer to females and squares to males; two symbols directly joined at the midpoint represent a mating, and those suspended from a common overhead line represent siblings, with descending birth order from left to right.


Solid symbols represent affected individuals, and open symbols represent unaffected individuals.

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