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Medical Genetics > Sheet 3 > Flashcards

Sheet 3 Flashcards

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Biology 101 flashcards USMLE® Step 1 flashcards
1
Q

What are telomeres?

A

Repetitive sequences that exist at the end of all the chromosomes and do not encode for genes.

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2
Q

All 46 chromosomes carry the same telomeric pattern, which is:

A

TTAGGG

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3
Q

What is the main reason for aging?

A

Telomerase is switched off = DNA is lost from the telomere

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4
Q

What is Aneuploidy?

A

Any chromosome number that is not an exact multiple of haploid number

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5
Q

What are 2 examples of aneuploidy?

A

1) Trisomy: The presence of an extra chromosome.

2) Monosomy: The absence of a single chromosome.

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6
Q

What is the cause of aneuploidy?

A

Nondisjunction

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7
Q

Aneuploidy results from the

fertilization of which gametes?

A

The gamete in which the nondisjunction occurred

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8
Q

What is Polyploidy/Euploid?

A

Is when an organism has more than two complete sets of chromosomes (extra but exact multiples of n).

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9
Q

What are 2 examples of Polyploidy/Euploid?

A

1) Triploidy (3n) is three sets of chromosomes (3 copies per chromosome).
2) Tetraploidy (4n) is four sets of chromosomes.

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10
Q

Polyploidy is common in:

A

Plants (not animals)

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11
Q

Who is more normal in appearance: Polyploids or aneuploids?

A

Polyploids

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12
Q

What can cause Polyploidy/Euploid?

A

When two sperms fertilize an egg by mistake (lethal to the fetus).

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13
Q

What is a common karyotype for Polyploidy/Euploid?

A

XXY

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14
Q

Breakage of chromosomes can change their structures in four different ways:

A

1) Deletion: Removes a chromosomal segment
2) Duplication: Repeats a segment
3) Inversion: Reverses the orientation of a segment within a chromosome
4) Translocation: Moves a segment from one chromosome to another

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15
Q

What is the difference between recombination and translocation?

A

Recombination is the exchange of DNA between non-sister chromatids of homologous chromosomes (happens in meiosis to give diversity).
Translocation is the exchange of genetic material between non-homologous chromosomes.

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16
Q

What is Trisomy 21 (Down syndrome)?

A

An aneuploid condition that results from having three copies of chromosome 21.

17
Q

How many children born in the United Sates have Down Syndrome?

A

1 out of every 700

18
Q

What correlation is related to Down Syndrome? Is there a cause for the correlation?

A

Maternal age and the risk of down syndrome; We don’t know why they’re
related.

19
Q

What should a mother who is pregnant at an advanced age do?

A

Karyotyping for the amniotic fluid or chorionic villi to check for trisomy 21 in the fetus.

20
Q

What is the male to female ratio for Trisomy 21?

A

3:2

21
Q

What is Down Syndrome clinically characterized by?

A

1) Mental retardation (IQ 25-50)
2) Low nasal bridge (90%)
3) Hypotonia (80%): Loose muscles
4) Up slanting palpebral fissures (80%)
5) Small, low-set ears (60%): A classical clinical abnormality for chromosomal aneuploidy
6) Congenital heart disease (30%-50%)
7) Epicanthic folds in the eye
8) Protruding tongue
9) Intestinal problems
10) Gap between first and second toes
11) 15-fold increase in risk for leukemia
12) Simian line (transverse crease) (45%): One line in their hand
13) Distinct facial features

22
Q

Which clinical Features are easily recognized at birth?

A

1) Low nasal bridge (90%)
2) Hypotonia (80%): Loose muscles
3) Up slanting palpebral fissures (80%)
4) Congenital heart disease (30%-50%)
5) Epicanthic folds in the eye
6) Simian line (transverse crease) (45%): One line in their hand

23
Q

Approximately how many children with Down Syndrome and congenital heart disease die during their first year?

A

40%

24
Q

What are the congenital heart problems seen in Trisomy 21?

A

1) VSD (Ventricular Septal Defect)

2) AV canal defects

25
Q

How can we calculate the probability of down syndrome in relation to the maternal age?

A

Curve of correlation = (# of Down Syndrome patients)/( # of births)

26
Q

Why does Trisomy 21 happen?

A

Due to Nondisjunction in either meiosis I or meiosis II

27
Q

94% of patients are due to a ___(paternal/maternal) error, whereas 4.5% are due to a ___(paternal/maternal) error.

A

Maternal; paternal

28
Q

What is the percentage of nondisjunctions in the egg during meiosis 1?

A

64%

29
Q

What is the percentage of nondisjunctions in the egg during meiosis 2?

A

19%

30
Q

What is the percentage of nondisjunctions in the egg intermediately?

A

11%

31
Q

What is the percentage of nondisjunctions in the sperm during meiosis 1?

A

1%

32
Q

What is the percentage of nondisjunctions in the sperm during meiosis 2?

A

3.5%

33
Q

What is the percentage of nondisjunctions in the sperm due to an unknown error?

A

1.5%

34
Q

How do we know if the extra chrmosome in Trisomy 21 is maternal or paternal?

A

By using noncoding repetitive DNA regions as polymorphic markers.

35
Q

What is Partial Trisomy 21 (21q)?

A

No extra chromosome 21, BUT one of the chromosomes 21 has 2 fused q arms instead of 1 and
that is represented as (46,XX,21 q+)

36
Q

What are the clinical features of Trisomy 18 (Edward syndrome)?

A

1) CHD (95%) ~ Congenital heart disease
2) Failure to thrive
3) Mental retardation
4) Growth retardation
5) Hypertonia: Tight muscles
6) Prominent occiput: occipital bone
7) Low-set, malformed ears
8) Short sternum
9) Intestinal problems
10) Clenched fist (unusual hand opposition)
11) Rocker bottom feet

37
Q

What are the clinical features of Trisomy 13 (Patau syndrome)?

A

1) CHD (85%)
2) Mental retardation
3) Hyper- or hypotonic
4) Scalp defects
5) Microcephaly: Head size smaller than
average
6) Small eyes
7) Low-set malformed ears
8) Cleft lip/palate
9) Polydactyly (extra digit) & Syndactyly
(digits fused together)
10) Polycystic kidneys
11) Rocker bottom feet

Medical Genetics (7 decks)

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