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Medical Genetics > Exam 3 > Flashcards

Exam 3 Flashcards

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1
Q

What are inheritance of traits on sex chromosomes termed?

A

Nonmendelian

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2
Q

What percentage of the nuclear genome’s DNA is made up of the X chromosome?

A

5%

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3
Q

How many Mbs and genes does the X chromosome contain? Y chromosome?

A
  • X chromosome: 155 Mb, 1200 genes
  • Y chromosome: 60 Mb, few dozen genes
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4
Q

Define imprinting

A

Imprinting refers to the fact that some genes are expressed only on paternally transmitted chromosomes and others are expressed only on maternally transmitted chromosomes

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5
Q

Define anticipation

A

Anticipation refers to earlier age-of-onset of some genetic diseases in more recent generations of families

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6
Q

Describe the Lyon Hypothesis

A
  • Stated that X inactivation occurs early in the female embryonic development and that one X is inactivated
  • Once this X chromosome is inactivated it will remain so in all descendents of that cell
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7
Q

In regards to the X chromosome, females are ___ and males are ___

A

Mosaics, Hemizygous

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8
Q

What is the term for an inactivated X chromosome?

A

Barr body

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9
Q

X-linked ocular albinism demonstrates ___ of the X chromosome as a result of random X inactivation

A

Mosaicism

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10
Q

What are the 3 Lines of Evidence supporting the Lyon Hypothesis?

A
  1. Clinical phenotype: Females mosaics, males not for X-linked traits (X-linked ocular albinism)
  2. Biochemical phenotype: Glucose-6-phosphate dehydrogenase
  3. Cytogenetic phenotype: Barr bodies
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11
Q

Where is the XIST gene located?

What type of RNA is it transcribed into?

Is it located on every X chromosome?

A
  • The X inactivation center
  • long noncoding RNA (lncRNA)
  • Yes, but is “turned on” only one X chromosome
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12
Q

There is homology between which arm of the X and Y gene?

A

Short arm (p)

The tips of the short and long arm of the X chromosome do not undergo inactivation. More short arm than long arm

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13
Q

What are the 3 possibilities for females when pertaining to the X chromosome?

A
  1. Homozygous for disease allele
  2. Heterozygous (this leads to mosaicism)
  3. Homozygous for normal allele
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14
Q

What are the 2 possibilities for males when pertaining to the X chromosome?

A
  1. Disease allele
  2. Normal allele
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15
Q

What is the most common mating for X-linked recessive inheritance?

A

Normal male and carrier female

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16
Q

List the percentage of carriers/affected among boys and girls born to these parents:

Normal male and carrier female:

Affected male and homozygous normal female:

Affected male and carrier female:

A
  • Girls 50%, Boys 50%
  • Girls 100%, Boys 0%
  • Girls 100% (50% affected), Boys 50%
17
Q

What is a manifesting heterozygote?

A

A heterozygous female who expresses a dominant phenotype of the X-linked gene. They usually maintain a small percentage of active normal X chromosomes which causes them to be relatively mildly affected.

About 5% of female heterozygotes exhibit hemophilia and are manifesting heterozygotes

18
Q

Hemophilia A facts

A
  • Defective factor VIII gene
  • Hemarthroses (bleeding into the joints) is common
  • Donor plasma used to purify factor VIII. This led to an increase in infections and HIV
  • Pts w/ nonsense/frameshift mutations = severe, Pts w/ missense mutations = mild/moderate
19
Q

Duchenne Muscular Dystrophy Facts

A
  • Mutation in DMD gene
  • Weakness and atrophy of muscle, Elevated CK in the blood stream
  • DMD gene is largest in DNA at 2.5 Mb
  • DMD = frameshift, BMD = in-frame alterations
20
Q

What are 3 examples of X-linked recessive diseases?

A
  1. Hemophilia A
  2. Duchenne Muscular Dystrophy
  3. Color Vision (lose ability to see one color)
21
Q

T/F: X-linked dominant diseases are more common than recessive diseases

A

False, they are less common

22
Q

Are males or females more likely to be affected for an X-linked dominant disease?

A

Females

They have 2 chromosomes so are twice as likely to be affected but heterozygotes have a milder expression

23
Q

What are 3 examples of X-linked dominant diseases?

A
  1. Hypophosphatemic rickets
  2. Incontintia pigmenti type 1 (females only, males die)
  3. Rett Syndrome (mostly females)
24
Q

Y-linked genes are also referred to as ___

A

Holandric

25
Q

What are some examples of Y-linked genes?

A
  • HY: minor histocompatibility antigen
  • DFNY1: mutation in gene causes hearing loss
  • USPY
  • DDX3Y
  • DAZ1
26
Q

Explain a sex-limited trait

Give examples

A
  • A trait is said to be sex limited when the trait occurs in only one of two sexes
  • Inherited uterine/testicular defects, Producing breast milk
27
Q

Define heteroplasmy

A

The mixture of different mtDNA sequences

28
Q

Describe imprinting

A

The process of gene silencing. The transcriptionally silenced genes are known to be imprinted. Imprinted alleles tend to be methylated on the cytosine

(Ex. Mother has inactive copy of allele and father has active copy)

29
Q

How many human genes are imprinted? How are they organized in the genome?

A
  • 100
  • Most are located in genomic regions that contain clusters of several or more imprinted genes
30
Q

What is the molecular mechanism of imprinting?

What other phenommenon is this similar to?

A
  • Methyl group attachment to 5’ region, along with histone hypoacetylation and condensation of chromatin inhibits the binding of proteins that promote transcription
  • X-inactivation
31
Q

Describe the causative mutation of Prader-Willi and Angelman Syndrome

A
  • Caused by deletion of 4Mb of long arm of chromosome 15. Deletion from father = Prader-Willi, Deletion from mother = Angelman Syndrome
32
Q

Compare/Contrast Prader-Willi and Angleman Syndrome

A
  • Prader-Willi: short stature, hypotonia (poor muscle tone), small hands/feet, obesity, mild to moderate intellectual disability, and hypoganadism
  • Angelman Syndrome: severe intellectual disability, seizures, and an ataxic gate
33
Q

What is the incidence rate of Prader-Willi/Angelman Syndrome?

A

1 of every 15,000 people are affected and 70% of cases are caused by chromosome deletions for both diseases

34
Q

What’s the difference in mutation of Prader-Willi from Angelman Syndrome

A

Mutation of maternal chromosome 15 can effect the gene UBE3A which is only expressed on the maternal chromosome

35
Q

What other mechanism besides imprinting can cause Prader-Willi/Angelman Syndrome?

A

Uniparental disomy can lead to these syndromes due to an individual inheriting 2 copies of a chromosome from 1 parent and none from the other

2 chromosomes from mother will result in PWS because of no active paternal genes. AS when vice versa

36
Q

Describe symptoms of Beckwith Wiedemann syndrome

A
  • Large size for gestational age
  • Neonatal hypoglycemia
  • Enlarged tongue
  • Omphalocele (type of tumor)
  • Creases on ear lobe
  • Leg length discrepancy
37
Q

What is the inheritance pattern of Beckwith-Wiedemann syndrome?

A
  • 50-60% of cases are caused by loss of methylation or alterations of DMR2. This region contains several genes, including CDKN1C
  • 20-30% of cases are due to uniparental disomy of parental chromosome 11p15.5
38
Q

Describe the mechanism of Beckwith-Wiedmann syndrome involving the DMR1 region

A

Located on DMR1, of maternally transmitted chromosome 11, is a normally inactive gene that encodes for insulin-like growth factor 2 (IGF2) and an active IGF2 on the paternally transmitted chromosome. This pattern of inheritance provides each individual with one active copy of IGF2 except in the case of Beckwith-Wiedemann

39
Q

How is the DMR1 region related to Silver-Russel syndrome?

A

This disorder results from loss of methylation of DMR1 on chromosome 11, therefore down regulating IGF2 resulting in diminished growth

Medical Genetics (4 decks)

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