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Genetics PMY-3 > Variations In Chromosone Number > Flashcards

Variations In Chromosone Number Flashcards

1
Q

How are chromosomes treated with chemicals to view patterns

A

First the fixed chromosomes on a slide are treated with trypsin so that the DNA loosens up bit
-trypsin is a protease which, when used for a short time will partially digest proteins associated with the chromosome

  • Next, the fixed chromosomes are stained with a purple dye called Geimsa
    • some regions of the chromosomes pick up more dye resulting in a banding pattern

-The pattern of staining is viewed under the microscope and photographed

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2
Q

What can cause variations in two haploid sets?

A
  • Change in total number of chromosomes
  • Deletion or duplication of genes/segments of chromosome
  • Rearrangements of genetic material within or among chromosomes
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3
Q

What is aneuploidy?

A
  • Variations in chromosome number

- Organism gains or loses one or more chromosomes(not entire set)

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4
Q

What are the types of aneuploidy?

A

Monosomy

Trisomy

Polyploidy

Euploidy

Triploid

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5
Q

What is Monosomy?

A

Loss of single chromosome in diploid genome

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6
Q

What is trisomy?

A

Gain of single chromosome

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7
Q

What is euploidy?

A

Complete haploid sets of chromosomes are present

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8
Q

What is polyploidy?

A

More than two sets of chromosomes are present

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9
Q

What is triploid?

A

Three sets of chromosomes

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10
Q

How can triploidy(3n) happen?

A
  • Most common cause is dispermy (one egg and two spermatozoa)
  • Could be fertillization of a primary oocyte
  • Could be a failure to separate homologous chromosomes in Meiosis 1
  • Could be a failure to separate homologous chromosomes in Meiosis II
  • Could be retention of a polar body
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11
Q

How often does triploidy (3n) occur?

A

Occurs in 2-3% of conceptuses and ~ 20% of chromosomal abnormal first-trimester miscarriages

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12
Q

How can tetraploidy(4n) happen?

A
  • Rare occurrence, may be chromosome separation failed during meiosis 1 or II in the egg cell and then dispermy
  • Failed sister chromatid separation in an early embryo
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13
Q

What is nondisjunction ?

A

Failure to separate

  • Gives rise to chromosomal variation
  • Paired homologs fail to disjoin during segregation
  • Nondisjunction during meiosis 1 or II
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14
Q

What can result from Monosomy?

A
  • Loss of one chromosome
  • Produces 2n-1 complement
  • Haploinsufficiency: when one copy is not sufficient for organism to survive

Loss of sex chromosomes is compatible with life

46, XX loss of X= 45,X. 47,XY loss of Y= 45, X

Turner Syndrome

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15
Q

Contrast the effect of a loss of sex chromosome and loss of autosomes

A
  • Loss of any one of the autosomes is not compatible with life
  • loss of a sex chromosome IS compatible with life
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16
Q

What are the effects of trisomy of autosomes?

A

2n+1 chromosomes

Most trisomy involving the autosomal chromosomes are lethal

Three exceptions which result in live-born children

  • trisomy 21(Down syndrome)
  • trisomy 18(Edward syndrome)
  • trisomy 13 (Patau Syndrome)
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17
Q

What disease results from trisomy 21?

A

Down syndrome

18
Q

What disease results from trisomy 18?

A

Edward syndrome

19
Q

What disease results from trisomy 13?

A

Patau syndrome

20
Q

Differentiate trisomy and triploidy

A

Triploidy-extra chromosome added to each pair resulting in 69, XXX or 69, XXY or 69, XYY

Trisomy adds an extra chromosome to a single pair (e.g. chromosome 21 pair gets gets another to cause Down syndrome) 47, XX or 47, XY

21
Q

What is the frequency of Down syndrome?

A

1: 800 live births

22
Q

What is the life expectancy of a person with Down syndrome?

A

Live expectancy on average is about 60 years

23
Q

What are the phenotypic characteristics of Down syndrome?

A
  • short stature
  • depressed nasal bridge
  • upslating palpebral fissures
  • Brushfield spots on iris of the eye
  • Epicanthal folds (skin fold from the nose to upper eyelid)
  • short neck
  • low-set small, folded ears
24
Q

Aside from phenotypic characteristics, how may down syndrome affect an individual?

A
  • Intellectual disability and heart malformations

- affected individuals bear a striking resemblance to one another

25
Q

Where is the down syndrome critical region?

A
  • critical region of chromosome 21
  • Cytogenic location 21q22
  • 1.6 Mb region
  • thought to be responsible for some of the combebsck
26
Q

Aside from cytogenic location 21q22, what other genes are responsible for Down syndrome?

A

Other genes in chromosome 21 thought to be responsible for features of Down Syndrome

  • APP(amyloid precursor protein) -Alzheimer’s disease
  • AML1 (acute myeloid leukemia 1) -Leukemia
27
Q

What is the most common occurrence of nondisjunction?

A

Most common origin is nondisjunction of chromosome 21 during oogenesis at meiosis I
-estimated to account for 75% of cases

28
Q

What is the second most common origin of nondisjunction?

A

Second most common origin of nondisjunction during oogenesis at meiosis II
- about 20% of cases

29
Q

Why is there increased risk of extra 21st chromosome with maternal age?

A
  • Meiosis 1 is initiated in all the eggs of a female while she is a fetus
  • Oocyte development is arrested in meiosis 1 where synapses are formed and recombination has begun
  • At ovulation, beginning at female puberty, meiosis is reinitiated in one egg and continues to meiosis 2
  • the process is again arrested after ovulation and is not complete till fertillization.

THE OLDER THE FEMALE, THE LONGER THE OVA HAVE BEEN ARRESTED IN METAPHASE 1

30
Q

How does maternal age correlate to extra 21st chromosome?

A

Increased risk with increased maternal age

31
Q

Is Down syndrome inherited?

A

Familial Down syndrome is inherited and runs in families

Involves a process called translocation

1-5% of Down syndrome is inherited

32
Q

What are the features of Trisomy 18 (Edward syndrome)

A
  • Clenched fist, overlapping of fingers
  • Rocker bottom feet
  • Congenital heart defects
  • Low-set ears, small lower jaw (micrognathia)
  • Microcephaly (small head)
  • Intellectual disability
33
Q

What is the genetic mechanism of trisomy 18?

A

Genetic mechanism is nondisjunction during oogenesis

34
Q

What are the features of trisomy 13?

A

Features:

  • Polydactyl (extra fingers and toes)
  • Cleft lip and palate
  • Microphthalmia(small eyes)
  • Microcephaly (small head)
  • Intellectual disability
  • Cardiac anomalies
35
Q

What is the most common mechanism of trisomy 13?

A

Most common genetic mechanism is nondisjunction during oogenesis

36
Q

What is the karyotype of trisomy 13?

A

47, XX, +13

37
Q

What is Mosaicism ?

A

Less severe presentations of trisomies a than other presentations, such as some babies having 15/ 20 cells have trisomy (75% mosiacism)

38
Q

Explain the first theory of Mosaicism?(miotic nondisjunction)

A

Non-disjunction May occur as a post-zygotic event during embryonic development; that is mitotically (after fertillization of a normal ovum and normal sperm)

This will lead to 2 cell types:

  • One continues as normal karyotype
  • the other with trisomy 21
39
Q

Explain the second theory of Mosaicism? (Trisomy rescue)

A
  • Non-disjunction May occur in the ovum during oogenesis, and fertilization a normal sperm leads to trisomy 21 in the conception
  • During a post-fertilization mitosis, one of the extra chromosome 21 is evicted during cell division
  • Now the fetus has 2 cell types, one with normal karyotype, and one that is trisomy 21
40
Q

How can trisomy rescue lead to uniparental disomy ?

A

-trisomy of autosomal chromosomes (except 13, 18 and 21) are not compatible with life

  • theory that fertilizedess will survive to develop an embryo if one of the extra chromosomes is removed
    • 30% chance of giving rise to uniparental disomy

Genetics PMY-3 (27 decks)

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