Chromosomal Disorders Syndromes Disorders of Chromosome structure 1

Chromosomal Disorders (Syndromes) ************

Disorders of Chromosome structure:

1 -Cri-du-chat (Cat cry syndrome): n n The name of the syndrome came from a catlike mewing cry from small weak infants with the disorder. Other characteristics are small head, broad face and mental retardation. Cri-du-chat patients die in infancy or early childhood. The chromosome deficiency is in the short arm of chromosome 5.

1 -Cri-du-chat (Cat cry syndrome):

2 - Myelocytic leukemia n Another human disorder that is associated with a chromosome abnormality is chronic myelocytic leukemia. A deletion of chromosome 22 was described by P. C. Nowell and Hungerford and was called “Philadelphia” (Ph’) chromosome.

Disorders of Chromosome number:

1 - Down Syndrome (Trisomy 21) (47, +21) n n n The best known and most common chromosome related syndrome. Formerly known as “Mongolism”. In 1866, when a physician named John Langdon Down published an essay in England in which he described a set of children with common features who were distinct from other children with mental retardation he referred to as “Mongoloids. ” One child (male or female) in every 800 -1000 births has Down syndrome.

Reason of Down syndrome n n Down syndrome results if the extra chromosome with pair of homologous chromosomes number 21 (three copies) due to non-disjunction during meiosis in pair of somatic homologous chromosome number 21 hence produce egg or sperm with 24 chromosome. For example when egg (23+X) chromosomes unite with sperm (22+X or 22+Y) to give zygote with 47 chromosomes (45+XX female or 45+XY male ) (trisomy 21 or Down syndrome).

Symptoms of Down syndrome n Short in stature, broad short skulls, flat nostrils, open small mouth, large protruding tongue, folded ears, short broad hands, short neck, short fingers, Space between first and second toe, transverse palmer crease, skin folds on eye and mental retardation.

2 - Edward's syndrome, (Trisomy 18) 47, +18 n n n Is the second most common trisomy after Down's syndrome. Edward's syndrome occurs when three copies (trisomy) of chromosome 18 occur. Trisomy 18 is therefore caused by a genetic abnormality such as trisomy 21 but in chromosome 18. Trisomy 18 was discovered by John Hilton Edwards, a British geneticist, in 1960. His discovery of it led to the association of his name with the syndrome. One child (male or female) in every (8000 -11000) births has Trisomy 18.

Symptoms of Edward's syndrome n Multiple congenital malformations of many organs, malformed ears, small mouth and nose with general elfin appearance, short neck, large protruding heels, prominent sternum. 90% die in the first 6 months. Very rare.

Symptoms of Edward's syndrome

3 - Patau syndrome, ( Trisomy 13) 47, +13 n Patau syndrome occurs when three copies (trisomy) of chromosome 13 occur. Trisomy 13 is therefore caused by a genetic abnormality such as trisomy 21 and in trisomy 28 but in chromosome 13. One child (male or female) in every (15000 - 20000) births has Trisomy 13.

Symptoms of Patau syndrome n Mental retardation, deafness, cleft lip, defects in small eyes, extra fingers or toes, and large protruding heels, and malformations of brain, nervous system, and heart.

Symptoms of Patau syndrome

4 - Klinefelter Syndrome, male with (44+XXY). n n - Called the XXY syndrome - 1 : 500 -1000 male births Reason of Klinefelter Syndrome This syndrome due to nondisjunction during meiosis in pair of sex chromosomes, hence produce egg with (22 +XX) chromosomes. When this egg unites with sperm 23 chromosomes (22+Y) give male with 47 chromosomes (44+XXY).

Symptoms of Klinefelter syndrome n Long stature, incomplete sexual development, rudimentary small testes and prostate, long limbs, large hands and feet, little body hairs, some breast tissue development. Most common cause of male infertility

- Turner Syndrome, female with (44+X 0) n n n Called the XO syndrome, 1 in 2, 500 - 3000 female births 99% of affected fetuses die in uterus Reason of Turner Syndrome Due to non-disjunction during meiosis in pair of sex chromosomes, hence produce egg with 22 chromosomes. When this egg unites with sperm 23 chromosomes (22+X) give female with 45 chromosomes (44+X).

Symptoms of Turner syndrome n Short stature, broad chest, webbing of skin in neck region, cardiovascular abnormalities, incomplete sexual development (infertile), and impaired hearing. Mental retardation.

Barr Bodies n n n n 1940’s two Canadian scientists noticed a dark staining mass in the nuclei of cat brain cells Found these dark staining spots in female but not males This held for cats and humans They thought the spot was a tightly condensed X chromosome Barr bodies represent the inactive X chromosome and are normally found only in female somatic cells. A woman with the chromosome constitution 47, XXX should have 2 Barr bodies in each cell. XXY individuals are male, but have a Barr body. XO individuals are female but have no Barr bodies. Number of Barr Bodies = (number of chromosome X -1) Female with 46 chromosomes (44+XX) has one barr body. n n

Barr Bodies