Numerical Chromosomal Abnormalities Flashcards

1
Q

What are the events in meiosis that produce genetic variability among offspring?

A
  • Crossing over
  • Assortment of alleles
  • Reduction in genetic material from diploid to haploid
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2
Q

What are the two key differences between mitosis and meiosis?

A

1 - paternally- & maternally-derived homologous chromosomes pair at the onset of Prophase I (the two homologs segregate independently in mitosis)

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3
Q

What are bivalents and why are they important?

A

Bivalents are structures in which mat & pat homologous become paired along their entire length
(brought together by the synaptonemal complex)

=> 2-3 crossovers occur on each chromosome
=> genetic reassortment b/w chromosomes

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4
Q

What is the synaptonemal complex?

A

A proteinaceous structure that promotes inter-homolog interaction (necessary for bivalent structures)

It disassembles at the end of Prophase I
=> bivalents only held together by chiasmata

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5
Q

What is the most frequent mutational mechanism in humans, and during which stage does it occur?

A

Chromosome Nondisjunction

during Meiosis I (most error-prone step)

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6
Q

What are the three classifications for chromosomes?

A
  1. Metacentric = centromere is in the middle, chromosome arms are approx same length
  2. Submetacentric = centromere slightly removed from center
  3. Acrocentric = centromere is near one end
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7
Q

How else are chromosomes classified?

A

Cytogenetically, based on banding patterns observed microscopically

Unique banding patterns => unequivocal identification of each chromosome

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8
Q

What is chromosome nondisjunction?

A

The missegregation of chromosomes at metaphase (in either mitosis or meiosis) in which daughter cells receive extra or fewer chromosomes

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9
Q

What is aneuploidy?

A

A condition in which cells contain an abnormal chromosome number

(frequently the result of chromosome nondisjunction)

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10
Q

What type of errors predominate almost all trisomies, and when do they occur?

A

maternal errors in the first meiotic division

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11
Q

Disturbances in the recombination pathway are associated with abnormalities in chromosome segregation in the first mitotic division. How does the location of chiasmata affect nondisjunction?

A

Crossvover too near/far from the centromere increase chromosome nondisjunction.

Distal exchanges are less effective in ensuring appropriate spindle attachment and separation of paired homologs in meiosis I

Proximal exchanges, or excessive number of exchanges, lead to entanglement of paired homologs in meiosis I => reductional division

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12
Q

What is the maternal age effect?

A

The notable increase of aneuploidy in the eggs of women approaching menopause

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13
Q

What is the “two hit” theory for the maternal age effect?

A

1st Hit = diminished recombination from lack of chiasma or mislocalization -> chromosome more susceptible to possible nondisjunction

2nd Hit = diminished ability of oocytes to successfully complete chromosome segregation in the presence of unfavorable recombination events

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14
Q

What is the function of the cohesion complex in meiosis?

A

It ensures cohesion b/w sister chromatids and maintains inter-homolog associations distal to the site of crossovers

Degradation of cohesion complexes over the extended meiosis I-arrest (oocytes) results in precocious separation of homologs.

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15
Q

What are the common clinical features of Trisomy 21?

A
(Down Syndrome)
Characteristic facies
Short stature
Hypotonia
Moderate intellectual disabilities

Endocardial cushion defects
Duodenal atresia and other GI anomalies
Hirschprung disease

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16
Q

What are the common clinical features of Trisomy 18?

A
(Edwards Syndrome)
Intrauterine growth retardation
Characteristic Facies
Severe intellectual  disabilities 
Characteristic hand positioning

Valvular heart disease
Posterior fossa CNS maldevelopment
Diaphragmatic hernias
Renal anomalies

17
Q

What are the common clinical features of Trisomy 13?

A

(Patau Syndrome)
Characteristic facies
Sever intellectual disabilities

Holoprosencephaly
Facial clefts
Polydactyly
Renal anomalies

18
Q

What are the common clinical features of 47, XXY?

A
(Klinefelter Syndrome)
Tall stature,
Hypogonadism
Elevated frequency of gynecomastia
High frequency of sterility
Language impairment
19
Q

What are the common clinical features of 45, X?

A
(Turner Syndrome)
Short stature
Webbed neck
Edema of hands and feet
Broad shield-like chest
Narrow hips
Renal and cardiovascular anomalies
Gonadal dysgenesis (failure of ovarian maintenance)
20
Q

What is mosaicism?

A

The presence of at least two genetically different cells in a tissue, that is derived from a single zygote.

  • Arises from mutations in single cells (prenatal or postnatal) generating clones of cells genetically different from the original zygote.
  • Mosaic phenotypes are highly variable and the effects of mosaicism are very difficult to predict.
21
Q

What is germ-line mosaicism?

A

A result of a somatic mutation that occurs early in development and generates a mutt sub-population of germ cells.

  • It is possible for a germ line mosaic individual to conceive multiple offspring with apparently new mutations
  • Germ line mosaicism has been demonstrated in nearly every Mendelian and chromosomal disorder