Autosomal & Chromosomal Disorders

Human Genetic Disorders There are many genetic disorders that plague humans. Some disorders are caused by a change in just one allele in a gene while others are much more complicated. It is the hope of many scientists that one day, we will have the advanced technology to isolate the defective genes that cause these disorders and completely remove them from the human genome.

Autosomal Disorders Autosomal disorders involve dominant, recessive, or other types of traits that can produce multiple abnormalities. These traits are found on chromosomes 1-22. Cystic Fibrosis, Sickle Cell, and Huntington’s disease are just a few examples of autosomal disorders.

Cystic Fibrosis (CF) Is caused by a recessive allele in chromosome #7. Animation Is caused by a recessive allele in chromosome #7. Most cases of CF involve a gene missing 3 bases in the middle of a sequence for a protein. Because of the faulty protein, cells can not transport chloride ions across their membranes. Children with CF have serious digestive problems as well as producing a thick mucus that clogs lungs & breathing passageways.

SCA Animation Sickle Cell Disease Is caused by an incomplete/co-dominant gene on chromosome #11. The sickle cell gene has just one DNA base different from the normal one. As a result, abnormal hemoglobin is formed which can “sickle” under low levels of oxygen. These sickled cells are rigid & tend to get stuck in capillaries which can damage cells, tissues & organs. In some cases it can be fatal. Individuals with both recessive genes are said to have sickle-cell disease. Those who are heterozygous for the trait are said to have sickle-cell trait.

Inheritance of Sickle-cell Disease Incomplete Dominance

Huntington’s Disease (HD) Is caused by a dominant gene on chromosome #4 Individuals with HD have a slightly larger huntingtin gene than usual. By middle-age, these abnormal proteins then begin to attack brain cells. Affected people are irritable, depressed, lose muscle control & eventually die. HD Animation

Huntington’s Disease

Other Autosomal Disorders Recessive: Albinism (lacking skin pigment melanin) Phenylketonuria (PKU- can’t break done phenylalanine causing problems with brain development) Tay-Sachs (fatty acids accumulate in the nerve cells of the brain; usually fatal) Dominant: Achondroplasia (dwarfism) Hypertrichosis (werewolf syndrome)

Albinism

Achondroplasia

Hypertrichosis

Chromosomal Disorders Chromosomal disorders occur when homologous chromosomes fail to separate (nondisjunction) during meiosis. This creates a genome with either an EXTRA chromosome or a MISSING chromosome Nondisjunction can occur with any chromosome (1-23). Downs, Turner’s, and Klinefelter's syndromes are just a few examples of chromosomal disorders.

Downs Syndrome (DS) Is caused by an extra of chromosome #21. Most cases of DS are trisomy 21, however there are other types of DS (Mosaic and Translocation) . Often DS is associated with some impairment of cognitive ability and physical growth as well as facial appearance. The incidence of Down syndrome is estimated at 1 per 800 to 1,000 births, although these statistics are heavily influenced by the age of the mother. Down syndrome is not generally an inherited disorder, (Translocation DS may be an exception). DS Animation

Turner Syndrome Is caused by a missing X chromosome, monosomy X, in females (XO). In rarer cases a second X chromosome is present but abnormal, while others with the condition have some cells with a second X and other cells without it (mosaicism). In Turner syndrome, female sexual characteristics are present but generally underdeveloped.

Klinefelter's Is caused by an extra X chromosome in males (XXY). The principal effects are development of small testicles and reduced fertility.

XYY or Super-male Syndrome The XYY syndrome is a disorder in which a male infant is born with an extra Y chromosome. Boys with XYY syndrome tend to be tall and have difficulties with language. The IQ tends to be slightly lower than that of other family members. Most XYY males have normal testosterone levels, sexual development and usually have normal fertility. The XYY syndrome was once thought to cause aggressive or violent criminal behavior, but this theory has been disproved. Some medical geneticists question whether the term "syndrome" is appropriate for this condition because its phenotype is normal and the vast majority of 47,XYY males don’t know their karyotype.