- Chromosomes refer to any of the rod-shaped or threadlike DNA-containing structures of cellular organisms that are located in the nucleus of eukaryotes, are usually ring-shaped in prokaryotes (such as bacteria), and contain all or most of the genes of the organism.
- Simply put, it is a strand of DNA that is encoded with genes.
- Chromosomes are important because they contain the entire (or at least the vast majority of) genetic information for an organism.
The Human Chromosomes
- Every normal human cell, except for sperm and egg cells, has 23 pairs of chromosomes for a total of 46 chromosomes.
- Sperm and egg cells have only one of each pair of chromosomes for a total of 23. Each chromosome contains hundreds to thousands of genes.
- The sex chromosomes are one of the 23 pairs of chromosomes. Normal people have 2 sex chromosomes, and each is either an X or a Y chromosome. Normal females have two X chromosomes (XX), and normal males have one X and one Y chromosome (XY).
Chromosomal abnormalities can fall under two categories:
- Structural abnormalities (eg, deletions or rearrangements) or
- Abnormalities of chromosomal number.
Abnormalities of chromosomal number generally arise from meiotic non- disjunction (failure of chromosome pairs to separate during cell division) or through anaphase lag (loss of chromosome during cell division).
Some common chromosomal disruptions include:
- Aneuploidy: Chromosome number that is not a multiple of 23.
- Polyploidy: Chromosome number that is 3 or 4 times the haploid number of 23.
- Deletion: Loss of part of chromosome.
- Translocation: Exchange of chromosome parts between non-homologous chromosomes.
- Balanced translocation: No genetic material lost; clinically asymptomatic.
- Robertsonian translocation: Joining of long arms of two acrocentric chromosomes with loss of short arms.
- Inversion: Reunion of separated portion.
Some Important Chromosomal Disorders
A. Cri du Chat Syndrome
Caused by partial deletion of chromosome 5p.
Patients exhibit a high-pitched cat-like cry at birth, which is usually due to structural abnormalities in the larynx; patients also have severe developmental delay and cognitive deficits and distinctive facial abnormalities (round face, low-set ears, microcephaly, and a hypoplastic nasal bridge).
Lab findings: Cytogenetic studies reveal a deletion of chromosome 5p.
B. Down Syndrome
Trisomy 21 causes 95% of cases (usually because of meiotic nondisjunction); 4% of cases are caused by Robertsonian translocation of the long arm of chromosome 21 to another chromosome (usually chromosome 14 or 22); 1% of cases are caused by mosaicism, resulting from mitotic nondisjunction of chromosome 21 during embryogenesis.
Severe mental retardation;duodenal and esophageal atresia; short hands with simian crease (single palmar crease). Specific facial features include flat face; epicanthal folds; wide-set eyes; Brushfield spots (white spots on periphery of iris).
Congenital heart defects: Endocardial cushion defects leading to ostium primum atrial septal defects, ventricular septal defects, and atrioventricular valve malformations.
C. Edward Syndrome
Most cases are caused by trisomy 18 (usually caused by meiotic nondisjunction). A few cases are caused by mosaicism, resulting from mitotic nondisjunction of chromosome 18 during embryogenesis.
Severe mental retardation; rocker-bottom feet; specific facial features include prominent occiput, micrognathia (small jaw), low-set ears; congenital heart and renal defects; overlapping third and fourth fingers.
D. Klinefelter syndrome
This disorder is characterized by two or more X chromosomes with one or more Y chromosomes (most commonly 47, XXY karyotype with a single Barr body) and is most commonly caused by maternal meiotic nondisjunction. Other causes include mosaicism or paternal meiotic nondisjunction.
Small atrophic testes; tall stature; lack of secondary male characteristics and gynecomastia.
Male infertility, often resulting from reduced spermatogenesis. Occasionally associated with mild mental retardation.
Lab findings: Decreased testosterone levels; increased FSH and LH levels.
E. Patau Syndrome
Most cases are caused by trisomy 13 (usually caused by meiotic nondisjunction). A few cases are caused by mosaicism, resulting from mitotic nondisjunction of chromosome 13 during embryogenesis or translocation between chromosomes 13 and 14.
Severe mental retardation; microcephaly and holoprosencephaly; cleft lip and palate and microphthalmia (small eyes); polydactyly; congenital heart and renal defects; umbilical hernia; rocker-bottom feet.
F. Prader-Willi syndrome and Angelman syndrome
Both diseases are due to an identical deletion on chromosome 15q and demonstrate imprinting, a phenomenon in which the same mutation results in different phenotypes depending on whether the mutated chromosome was of maternal or paternal origin.
Prader-Willi syndrome develops when the deletion is on the paternally derived chromosome, whereas Angelman syndrome develops when the deletion is on the maternally derived chromosome.
Prader-Willi syndrome: Mental retardation; hypogonadism; hypotonia; obesity leading to diabetes.
Angelman syndrome: “Happy puppet” with ataxic gait and inappropriate laughter; mental retardation; seizures.
G. Turner syndrome
Caused by partial or complete monosomy of the X chromosome (XO karyotype with no (Barr body).
Short stature with a broad chest and widely spread nipples; cystic hygroma of the neck, leading to webbed-neck appearance; lymphedema of extremities; coarctation of the aorta and other congenital heart defects.
Reproductive symptoms include primary amenorrhea, replacement of ovaries with fibrous strands (no ova or follicles), and infantile genitalia and breasts.
Lab findings: Decreased estrogen production; increased FSH and LH levels.
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