Genetics: Sex Chromosome Abnormalities


Sex Chromosome Abnormalities 

f9-10b_abnormal_sex_chr_cThe majority of known types of chromosomal abnormalities involve sex chromosomes.  In frequency of occurrence, they are only slightly less common than autosomal abnormalities.  However, they are usually much less severe in their effects.  The high frequency of people with sex chromosome aberrations is partly due to the fact that they are rarely lethal conditions.  Like Down syndrome and other autosomal problems, sex chromosome gross abnormalities can be diagnosed before birth by amniocentesis and chorionic villi sampling.

Sex chromosome abnormalities are gender specific.  Normal males inherit an X and a Y chromosome while females have two X’s.  A single Y chromosome is sufficient to produce maleness while its absence is necessary for femaleness.  Female abnormalities are due to variations in the number of X chromosomes.  Male abnormalities are the result of irregular numbers of either the X or the Y chromosome or both.


Female Sex Chromosome Abnormalities

f9-10a_abnormal_sex_chr_cTurner syndrome
occurs when females inherit only one X chromosome–their genotype is X0 (i.e., monosomy X).  If they survive to birth, these girls have abnormal growth patterns.  They are short in stature, averaging 4 foot 7 inches as adults, and often have distinctive webbed necks (i.e., extra folds of skin), small jaws, and high arched palates.  They generally lack prominent female secondary sexual characteristics.  They have exceptionally small, widely spaced breasts, broad shield-shaped chests, and turned-out elbows.  Their ovaries do not develop normally and they do not ovulate.  The few oöcytes that they produce are destroyed by the time they are two.  They are in a sense postmenopausal from early childhood and are sterile.  However, they can become pregnant and give birth if fertilized eggs from a donor are implanted.  Women with Turner syndrome have a higher than average incidence of thyroid disease, vision and hearing problems, heart defects, diabetes, and other autoimmune disorders.  In a fewindividuals, there is slight mental retardation.  Turner syndrome is rare.  Current estimates of its frequency range from 1 in 2,000 to 1 in 5,000 female infants.  If diagnosed in early childhood, regular injections of human growth hormones can increase their stature by a few inches.  Beginning around the normal age of puberty, estrogen replacement therapy can result in some breast development and menstruation.  These treatments allow Turner syndrome women to appear relatively normal.

  Turner Syndrome–characteristics of Turner Syndrome and treatment for it.
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Triple X Syndrome in child and adult slide_13Triple-X syndrome occurs in women who inherit three X chromosomes–their genotype is XXX or more rarely XXXX or XXXXX.  As adults, these “super-females” or “metafemales” , as they are sometimes known, generally are an inch or so taller than average with unusually long legs and slender torsos but otherwise appear normal.  They usually have normal development of sexual characteristics and are fertilebut tend to have some ovary abnormalities that can lead to premature ovarian failure.  They may have slight learning difficulties, especially in speech and language skills, and are usually in the low range of normal intelligence (especially the XXXX and XXXXX individuals).  They frequently are very tall in childhood and tend to be emotionally immature for their size.  This sometimes results in teachers and other adults labeling them as troublemakers because they expect more maturity from bigger girls.  However, they are usually as emotionally mature as other girls of their age.  None of these traits prevent them from being socially accepted as ordinary adult women.  Individuals who are genetic mosaics (XX/XXX) have less noticeable symptoms.  Triple-X syndrome is less rare than Turner syndrome, but little is known about it.  The frequency is approximately 1 in 1,000 female infants and it occurs more commonly when the mother is older.
Male Sex Chromosome Abnormalities

Klinefelter Syndrome 3718413Klinefelter syndrome males inherit one or more extra X chromosomes–their genotype is XXY or more rarely XXXY or XY/XXY mosaic.  In severe cases, they have relatively high-pitched voices, asexual to feminine body contours as well as breast enlargement, and comparatively little facial and body hair.  They are sterile or nearly so, and their testes and prostate gland are small.  As a result, they produce relatively small amounts of testosterone.  The feminizing effects of this hormonal imbalance can be significantly diminished if Klinefelter syndrome boys are regularly given testosterone from the age of puberty on.  Like triple-X females (described above), many Klinefelter syndrome men are an inch or so above average height.  They also are likely to be overweight.  They usually have learning difficulties as children, especially with language and short-term memory.  If not given extra help in early childhood, this often leads to poor school grades and a subsequent low self esteem.

However, most men who have Klinefelter syndrome are sufficiently ordinary in appearance and mental ability to live in society without notice.  It is not unusual for Klinefelter syndrome adults with slight symptoms to be unaware that they have it until they are tested for infertility.  They are usually capable of normal sexual function, including erection and ejaculation, but many, if not most, are unable to produce sufficient amounts of sperm for conception.  Klinefelter syndrome males with more than two X chromosomes usually have extreme symptoms and are often slightly retarded mentally.  Men who are mosaic (XY/XXY) generally have the least problems.  T

here is no evidence that Klinefelter syndrome boys and men are more inclined to be homosexual, but they are more likely to be less interested in sex.  They have a higher than average risk of developingosteoporosis, diabetes, and other autoimmune disorders that are more common in women.  This may be connected to low testosterone production.  Subsequently, regular testosterone therapy is often prescribed.  The frequency of Klinefelter syndrome has been reported to be between 1 in 500 and 1 in 1000 male births.  This makes it one of the most common chromosomal abnormalities.   Males with Down syndrome sometimes also have Klinefelter syndrome.  Both syndromes are more likely to occur in babies of older mothers.

  Klinefelter Syndrome–traits described by a doctor of medical genetics

xyy syndrome ch8f13XYY syndrome males inherit an extra Y chromosome–their genotype is XYY.  As adults, these “super-males” are usually tall (above 6 feet) and generally appear and act normal.  However, they produce high levels of testosterone.  During adolescence, they often are slender, have severe facial acne, and are poorly coordinated.  They are usually fertile and lead ordinary lives as adults.  Many, if not most, are unaware that they have a chromosomal abnormality.  The frequency of XYY syndrome is not certain due to statistical differences between different studies.  It may be as common as 1 in 900 male births to as rare as 1 in 1500 or even 1 in 2,000.  XYY syndrome is also referred to as Jacobs syndrome.

Early studies of XYY syndrome done in European prisons initially led to the erroneous conclusion that these men were genetically predisposed to antisocial, aggressive behavior, below average intelligence, and homosexuality.  Contributing to the early view that XYY syndrome men have serious personality disorders was the case of Richard Speck.  In 1966, he coldly murdered 8 nurses in a Chicago dormitory.  At his trial, his lawyer claimed that he was innocent due to uncontrollable urges caused by his XYY genotype.  This novel appeal was akin to claiming insanity or severely diminished mental competence.  The jury was not convinced and found him guilty of murder.  He was sentenced to life in prison where he eventually died.  In fact, Richard Speck did not have an XYY genotype.  However, some researchers suggest that the high testosterone levels of XYY men can make them somewhat more prone to violence and that this may cause higher rates of wife beating.

xyy2 syndromeDNA genetics sex gender clip_image00437

Sex Chromosome Abnormalities
Female Genotype Syndrome Male Genotype Syndrome
XX normal XY normal
XO Turner XXY Klinefelter