Congenital chromosomal aberrations.

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 There are two basic types of abnormalities that are due to chromosomes, one hereditary and one non-hereditary that is nevertheless present from conception to death.   The second type consists of chromosomal aberrations.  Every cell of our body contains 23 chromosome pairs. However, parental chromosome pairs located in the germ cells (spermatozoa for men, ova for women) have to break up so that each parent will contribute one half of his or her genetic code to the offspring.  In men,  this break-up,  known as meiosis,  occurs during the life span of the sperm,  before it is ready to be ejaculated.  In women,  meiosis occurs just after the ovum is released from the ovary at ovulation.    When errors in meiosis occur,  the offspring has a chromosomal aberration.  If a chromosome is lost,  the offspring will have what is called a monosomy, i.e., will be missing a chromosome (as in Turner's syndrome).   If there is non-disjunction of a chromosome pair (if a pair stays fused),  the offspring will have what is called a trisomy -three copies of a chromosome instead of the usual two (such as in trisomy-21 or Down's syndrome).  There are also a huge number of less complete errors that can occur, involving partial deletions, duplications, disjunctions or conjunctions of pieces of chromosomes. The types of chromosomal accidents I have just mentioned are not the only ones which can affect a person.    When the sperm penetrates the ovum,  the genetic code from each must combine before the ovum starts dividing (mitosis) into cells which will eventually form tissues and organs.    This phase can also go wrong,  and the error can then also be duplicated indefinitely,  or variably each time a cell divides.   Errors in one or the other phase of meiosis are not under strong genetic control.  In other words,  they do not run much in families.  It is not because somebody has trisomy-21 for example,  that they are much more likely to have a chromosomally aberrant baby.  They are more at risk,  but by less than 1%, because there are subtle genetic (metabolic) factors that contribute to the risk.    Rarely,  a mutant gene can actually reliably cause chromosomal errors.  It has recently been found that this is the case for the fragile-X syndrome -a form of autism caused by breakage of one long arm of the X chromosome.   A gene known as fMRI  has the effect of splitting the X chromosome and breaking off one of its long arms.  That explains why the fragile-X syndrome runs in families more than most other chromosomal aberrations.  At any rate,  whether there is a hereditary component or not,  chromosomal aberrations are congenital disorders because they are present at birth,  and they are disorders of development.

A vignette on a case of fragile-X syndrome

In a 1992 book entitled The fragile-X child   Betty Schopmeyer and Fonda Lowe describe John,  a 7 year-old karyotype-confirmed fragile-X child.    John is a very clumsy child with several autistic traits.  He performs in the mentally deficient range. He is neophobic (has an aversion for any unusual stimulus).  He is self-stimulatory (smells things,  rocks himself), avoids eye contact,  and is extremely emotionally labile. His speech is cluttered, perseverative and impoverished. He is dysprosodic (expresses inappropriate inflexions).  He adopts strange postures.  He is interpersonally recluse.    He is variably hyperactive and has a very low attention span. 

N.B.   There exist a number of physiognomic signs characteristic of fragile-X syndrome,  not described in this particular case report.  These include macro-orchidism (large genitalia),  an elongated face,  large ears, hyper-extensible joints, arched palate, pectus excavatum (a concave chest), flat feet, myopia, mitral valve prolapse (a cardiac defect), dental malocclusion (improper alignment of the teeth), and dermal ridge (finger and hand print) anomaly.   Most of these markers are due to anomalies of development of conjunctive tissue.