2. Structural aberration Aberrations in chromosome structure result from the breakage and reunion of chromosome segments and may involve one, two, or more chromosomes. When a chromosome breaks, two unstable sticky ends are produced. Generally, repair mechanisms rejoin these two ends without delay. However, if more than one break has occurred, repair mechanisms may not be able to distinguish one sticky end from another and may rejoin the wrong ends.
Chromosome breaks can occur from: 1) spontaneous errors in replication or crossing-over 2) mutations in genes that normally repair breaks 3) environmental agents, such as ultraviolet light, radiation(exposure to ionizing radiation ), viruses, or chemicals.
chromosome structure changes 1) Deletion 2) Duplication and Insertion 3) Inversion 4) Translocation
Deletiontion
Terminal deletion (short arm)
Interstitial deletion
Terminal deletion (long arm)
Duplication—extra copies of a chromosome segment
Inversio — a reversal in the order of a n chromosome segment
An inversions occurs when a single chromosome undergoes two breaks and is reconstituted with the segment between the breaks inverted.
pericentric inversion
Including the centromere, in which there is a break in each arm.
paracentric inversion
Not including the centeomere, in which both breaks occur in one arm
insertion,ins An insertion is a nonreciprocal type of translocation that occurs when a segment removed from one chromosome is inserted into a different chromosome, either in its usual orientation or inverted.
Translocation—movement of a segment of one chromosome to another chromosome Reciprocal translocations: This type of rearrangement results from breakage of nonhomologous chromosomes,with reciprocal exchange of the broken-off segments. Usually only two chromosomes are involved , and because the exchange is reciprocal, the total chromosome number is unchanged.
(Robertsonian translocation, rob) (balanced translocation)
This type of rearrangement involves two acrocentric chromosomes that fuse near the centromere region with loss of the short arms.
ring chromosome,r
isochromosome,(i) is a chromosome in which one arm is missing and the other duplicated in a mirror-image fashion.
(unbalanced translocation)
chromosomal disorder
summarization Approximately 20% of all conceptions have a chromosomal disorder, but most of these fail to implant or are spontaneously aborted so the birth frequency is 0.6%. The frequency of chromosomal disorders in early spontaneous abortions is 60%, whereas in late spontaneous abortions and stillbirths, the frequency is 5%.
Various surveys indicate that 1 in 119-154 livebirths have a major chromosomal abnormality. Sex chromosome abnormalities occur once in every 300-400 births. In mothers over 35 ,this frequency is even greater, about 1 in 250.
Commonest chromosomal disorders seen in newborns Disorder
Birth frequency
Balanced translocation
1 in 500
Unbalanced translocation
1 in 2000
Pericentric inversion
1 in 100
Trisomy 21
1 in 700
Trisomy 18
1 in 3000
Trisomy 13
1 in 5000
47,XXY
1 in 1000 males
47,XYY
1 in 1000 males
47,XXX
1 in 1000 females
45,X
1 in 5000 females
Notice!
Notice! Not all of these chromosomal changes are associated with disease, but generally autosomal abnormalities tend to be more severe than sex chromosomal abnormalities and deletions more severe than duplications.
Common problem Congenital abnormalities 2) Ambiguous genitalia 3) Delayed development , Short stature 4) Mental retardation 5) Two or more unexplained miscarriages 6) Infertility 7) Amenorrhoea 8) Abnormal or inappropriate physical sexual development 9) Family studies known to carry cytogenetic anomalies 10) Family history of Down syndrome 1)
Down syndrome Down syndrome is the most common type of trisomy that babies may be born with. The karyotype is written 47, XX ,+ 21 or 47, XY ,+ 21.
Clinical Manifestations 1) Short stature 2) Special facial features upslanting palpebral fissures, epicanthus, brushfield spots on the iris, protruding tongue, small ears 3) Mild to severe mental retardation 4) 80% of trisomy 21 conceptions result in early miscarriage. 5) heart problems
karyotype 47,XX,+21 An extra chromosome 21 is the classic chromosomal constitution of individuals with Down Syndrome. Partial trisomies of chromosome 21 and mosaicism has also been observed associated with Down Syndrome.
trisomy 21
Mosaic
XX llll X ll X ll
ll ll
X ll
X
X l
X l
ll
ll ll
lll
l
Yl
47,XX(XY), +21
46,XX(XY)/ 47, XX(XY),+21
trisomy 13 and trisomy 18 Trisomy 13, or Patau syndrome, and trisomy 18, Edward syndrome, are very severe chromosome changes and both usually lead to early infant death, often within the first year of life. Most trisomy 13 and 18 conceptions result in early miscarriage. An extra copy of any of the other autosomes results in early miscarriage.
5P15.2
Changes in the number of sex chromosomes Changes in the number of sex chromosomes has a less severe effect on development and sometimes individuals with extra sex chromosomes can even be asymptomatic. Examples of the inheritance of an extra sex chromosome include: XXY, XXX or XYY. Sex-chromosome abnormalities may also be caused by nondisjunction of one or more sex chromosomes.
karyotype
Turner syndrome gonadal differentiation, no pubes, undevelopmental breast, amenorrhea infertile short in stature, characteristic webbing of the neck, low posterior hair line, cubitus valgus, cardiac defects, normal intelligence. (The majority of those that do survive are probably ‘mosaics’ i.e. they have a normal cell line in some tissues.)
Diagnosis
Klinefelter Syndrome 47, XXY men , enlarged breasts, sparse facial and body hair, small testes, and an inability to produce sperm. Overweight or taller. Although they are not mentally retarded, most XXY males have some degree of language impairment. Femals body shap with gynaecomastia in aolescence, breast cancer, hypogonadism Many naevi
Causes No one knows what puts a couple at risk for conceiving an XXY child. Advanced maternal age increases the risk for the XXY chromosome count, but only slightly. recent studies show that half the time, the extra chromosome comes from the father. an egg with two Xs, or a sperm having both an X and a Y chromosome..
Diagnosis
Treatment Surgery, can reduce breast size. Regular injections of the male hormone testosterone Special education for compensating for language disability.
Testosterone Treatment Ideally, XXY males should begin testosterone
treatment as they enter puberty. XXY males diagnosed in adulthood are also likely to benefit from the hormone. A regular schedule of testosterone injections will increase strength and muscle size, and promote the growth of facial and body hair. In addition to these physical changes, testosterone injections often bring on psychological changes as well. As they begin to develop a more masculine appearance, the self-confidence of XXY males tends to increase. Many become more energetic and stop having sudden, angry changes in moods.
47,XYY The abnormality has a variable phenotype that can include tall stature and acne.
49,XXXXY This karyotype shows a variant of Klinefelter's syndrome. Individuals with this syndrome are male, typically with the karyotype 47,XXY. They exhibit a characteristic phenotype including tall stature, infertility, gynecomastia and hypogonadism. Aneuploidy above one extra chromosome is usually fatal but because of X-inactivation, which "turns off" all but one X chromosome per cell, the effects of 3 extra chromosomes are reduced.
Gonadal Dysgenesis A syndrome of gonadal dysgenesis in which there is a testis on one side and a "streak gonad" on the other. The phenotype is generally male, but may be female since the individual is a mosaic. Various karyotypes have been identified, including 45,XO/47,XYY; 45,XO/46,XY; and 45,XO/46,XYo.
Prenatal detection --amniocentesis Prenatal detection of chromosomal abnormalities is accomplished chiefly by amniocentesis. A thin needle is inserted into the amniotic fluid surrounding the fetus (a term applied to an unborn baby after the first trimester). Cells are withdrawn have been sloughed off by the fetus, yet they are still fetal cells and can be used to determine the state of the fetal chromosomes such as Down's Syndrome and the sex of the baby after a karyotype has been made.