Cytogenetics I and II

Question 1

describe metacentric, submetacentric and acrocentric

Answer 1

• metacentric: p and q arm same length
• submetacentric: p arm < q arm
• acrocentric: presence of satellites where p arm should be

Question 2

describe trisomy 21

Answer 2

• Most common liveborn autosomal trisomy
• Risk factor: increased maternal age which increases the risk of meiotic nondisjunction during oogenesis (most common is meiosis I nondisjunction)
• FISH can be used to detect trisomy 21

Question 3

describe trisomy 18

Answer 3

• Edward syndrome
• Genetic mechanism is nondisjunction during oogenesis

Question 4

describe trisomy 13

Answer 4

• Patau syndrome
• Genetic mechanism is nondisjunction during oogenesis

Question 5

describe Turner syndrome

Answer 5

• (45, X)
  
  • X chromosome monosomy
  • many are mosaics
    
    • some cells are (45, X), others are (46,XX) and (47,XXX)
• Genetic mechanism is nondisjunction during meiosis
• Feature:
  
  • Short stature
  • Webbed neck
  • Amenorrhea (no menstruation)
  • Gonadal dysgenesis

Question 6

describe Klinefelter syndrome

Answer 6

• Nondisjunction during meiosis
• features:
  
  • Testicular dystrophy
  • Gynecomastia (increase in male breast tissue)
  • Infertility
  • Female distribution of hair

Question 7

describe nondisjunction

Answer 7

• most common in meiosis I in females
• individuals with Down syndrome usually have 2 maternal chromosome 21 and 1 paternal chromosome 21

Question 8

describe reciprocal translocations

Answer 8

• exchange of genetic material between non-homologous chromosomes
• when reciprocal translocation occurs in somatic cells:
  
  • transformation to cancer and increased growth rate of cells
  • may have no effects
• if the reciprocal translocation is present in the germ line cells, then there is a higher incidence of spontaneous abortions in the carrier

Question 9

explain reciprocal translocation and cancer

Answer 9

• Philadelphia chromosome t(9;22)
  
  • activation of ABL protooncogene in hematopoietic cells
  • results in chronic myeloid leukemia (CML)
• t(8;14)
  
  • Burkitt’s lymphoma

Question 10

describe Robertsonian translocation

Answer 10

• occurs between acrocentric chromosomes
  
  • 13, 14, 15, 21, 22
• there is loss of the short arms of the 2 chromosomes
  
  • the satellite structures in the acrocentric chromosomes contain genetic material for RNA
  • fusion of the 2 long arms of the 2 chromosomes

Question 11

contrast alternate vs adjacent segregation

Answer 11

• alternate segregation: balanced chromosomal material
• adjacent segregation: unbalanced chromosomal material (partial trisomy) on fertilization
  
  • Down syndrome due to Robertsonian translocation accounts for 2-5% of children with Down syndrome

Question 12

describe Cri-du-chat syndrome

Answer 12

• Chromosome 5p has a deletion
• features:
  
  • cat-like cry
  • intellectual disability
  • speech problems
  • microcephaly
• can use FISH to diagnose

Question 13

describe Velocardiofacial syndrome

Answer 13

• aka DiGeorge
• 22q11 deletion
• features (CATCH-22)
  
  • Congenital heart defects
  • Abnormal faces
  • Thyroid aplasia
  • Cleft palate
  • Hypocalcemia

Question 14

describe Wolf-Hirschhorn syndrome

Answer 14

• deletion of chr 4p
• Features:
  
  • widely spaced eyes
  • prominent nose
  • abnormal iris
  • cardiac anomalies
  • intellectual disability

Question 15

describe WAGR syndrome

Answer 15

• microdeltion in chr 11
• features (WAGR)
  
  • Wilms’ tumor
  • aniridia (no iris)
  • genitourinary malformations
  • delay in growth and development

Question 16

describe pericentric vs paracentric inversion

Answer 16

• pericentric: involves the centromere
• paracentric: does not involve the centromere
• usually balanced and no clinical problems in carriers

Question 17

describe isochromosomes

Answer 17

• loss of one arm of a chromosome and duplication of the other arm
• X isochromosome: long arms of the X chromosome join to form an isochromosome
• typically results in chromosomal and gene dosage imbalance