Chromosome Abnormalities

Question 1

Trisomy 21

Answer 1

Extra copy of chromosome 21
In 75% of cases, occurs as a result of maternal (oocyte) non-disjunction

Craniofacial defects: brachycephaly, upslanting palpebral fissures, medical epicanthic folds, small nose with low nasal bridge, small ears
Cardiac defects in approx. 40% of cases, usually septal defects of PDA (patent ductus arteriosus)

Question 2

Klinefelter syndrome 47, XXY

Answer 2

Nondisjunction during meiosis
Extra copy of chromosome x
Male presence of Barr bodies: inactive X chromosome gets shut off, normal for females
Sterile, testicular atrophy, gynecomastia

Question 3

Trisomy 18 (Edward syndrome)

Answer 3

Extra copy of chromosome 18 
Mental retardation
Cardiac defects 
Craniofacial: low ears, micrognathia
Limited capacity for survival, only handful of cases living to be 10

Question 4

Trisomy 13 (Patau syndrome)

Answer 4

Extra copy of chromosome 13
Mental retardation, holoprosencephaly
Cardiac defects
Deafness
Craniofacial: cleft lip and palate, eye defects
Limited survival, usually only survive 7 days

Question 5

Turner syndrome 45, X

Answer 5

One less copy of X chromosome
In 80% of cases occurs from paternal (spermatocyte) nondisjunction
Only monopsony that is compatible with life
Short stature, broad chest, short neck
Congenital lymphedema of hands and feet
Streak gonads (gonadal dysgenesis)
Lack of secondary sex characteristics development at puberty

Question 6

Cri du Chat syndrome

Answer 6

Deletion on the short arm of chromosome 5
Characteristic cry of the cat sound due to malformation of larynx
Growth retardation
Mental retardation, microcephaly
Cardiac defects

Question 7

Structural abnormalities

Answer 7

Usually occurs during genetic recombination (meiosis I)
Deletions: large portion (visible) is lost
Microdeletions: small portion (not visible) is lost
Imprinting: a gene is selectively silenced on one allele because we only need one (maternal or paternal)

Question 8

Angelman syndrome

Answer 8

Micro deletion on the long arm of chromosome 15
Microdeletion on maternal chromosome and male allele is silenced
Mental retardation
Puppet-like gait
Absent speech
Prone to unprovoked periods of uncontrolled laughter

Question 9

Prader-Willi syndrome

Answer 9

Microdeletion on the arm of chromosome 15
Microdeletion on the paternal chromosome
Mental retardation
Obesity and hypotonia
Hypogonadism

Question 10

Charcot-Marie-tooth CMT

Answer 10

Duplication on chromosome 17
The affected gene is called peripheral myelin protein 22
Weakness of lower limbs; foot drop; poor balance; sprained ankles; weak muscles of the hand
Patients may also display sensory symptoms including anesthesias or parethesias of the limbs

Question 11

Chronic myelogenous leukemia

Answer 11

Translocation results in gene protein called fused BCR-ABL protein which activates cell cycle protein causing uncontrolled mitotic division. This protein also inhibits DNA repair
Genomic instability
Results in cancer of white blood cells (leukemia)

Question 12

Robertsonian trisomy 21

Answer 12

Translocation between chromosomes 14 and 21

Occurs during meiosis

Question 13

Fragile sites

Answer 13

Parts of the chromosomes that are prone to trinucleotide amplification (typically at CGG repeats)