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Medical Genetics > Sex-linked inheritance > Flashcards

Sex-linked inheritance Flashcards

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1
Q

What are sex-linked genes?

A

Genes located on either the X or Y chromosome

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2
Q

What categories are X-linked diseases traditionally grouped into?

A

X-linked recessive and X-linked dominant

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3
Q

What is a key characteristic of X-linked recessive inheritance?

A

Males are more frequently affected than females

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4
Q

Name three well-known diseases caused by X-linked recessive genes.

A
  • Hemophilia A
  • Duchenne muscular dystrophy
  • Red–green color blindness
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5
Q

How many copies of the X chromosome do females inherit?

A

Two copies

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6
Q

What is the term for the process where one of the X chromosomes is randomly inactivated in females?

A

X inactivation

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7
Q

What happens if a male inherits a recessive disease gene on the X chromosome?

A

He will be affected by the disease

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8
Q

What is the disease frequency for females with an X-linked recessive disease allele?

A

q^2

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9
Q

What is the disease frequency for males with an X-linked recessive disease allele?

A

q

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10
Q

Why are X-linked recessive diseases more common in males than females?

A

Males are hemizygous for the X chromosome

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11
Q

Can a father transmit an X-linked gene to his son?

A

No

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12
Q

What is hemophilia A caused by?

A

Mutations in the gene that encodes clotting factor VIII

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13
Q

What is the prevalence of hemophilia A in males?

A

Approximately 1 in 5000 to 1 in 10,000

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14
Q

What are common symptoms of hemophilia A?

A
  • Prolonged bleeding
  • Hemarthroses
  • Bruising
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15
Q

What is the typical treatment for hemophilia A?

A

Administration of factor VIII

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16
Q

What is the estimated percentage of hemophilia patients infected with HIV due to donor-derived factor VIII?

A

Approximately 50%

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17
Q

What is the DMD gene responsible for?

A

Duchenne muscular dystrophy

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18
Q

What is the prevalence of Duchenne muscular dystrophy (DMD) in males?

A

Approximately 1 in 4000

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19
Q

What does pseudohypertrophy in DMD refer to?

A

Infiltration of muscle by fat and connective tissue

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20
Q

What enzyme is elevated in the blood of DMD patients?

A

Creatine kinase (CK)

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21
Q

What is the role of dystrophin in muscle cells?

A

Maintains structural integrity

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22
Q

What is the mutation rate of the DMD gene?

A

About 10^-4 per locus per generation

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23
Q

What percentage of female heterozygous carriers of the DMD mutation may experience muscle weakness?

A

8% to 10%

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24
Q

Fill in the blank: Hemophilia B is also known as ______.

A

Christmas disease

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25
True or False: Hemophilia B is more common than hemophilia A.
False
26
Who was the first known carrier of hemophilia in the British royal family?
Queen Victoria
27
What type of mutation usually leads to severe hemophilia A?
Nonsense or frameshift mutations
28
What is the advantage of recombinant factor VIII over donor-derived factor VIII?
No possibility of viral contamination
29
What does the DMD gene produce in brain cells compared to muscle cells?
A slightly altered form of the DMD gene product ## Footnote The transcription initiation site is farther downstream in the gene and a different promoter is used in brain cells.
30
What percentage of DMD patients have some degree of intellectual disability?
Approximately 25% ## Footnote This is attributed to dystrophin’s expression in the brain.
31
How does Becker muscular dystrophy (BMD) differ from Duchenne muscular dystrophy (DMD) in terms of severity?
BMD is less severe than DMD ## Footnote The progression of BMD is also slower.
32
What is the average age of onset for Becker muscular dystrophy (BMD)?
11 years of age
33
What percentage of DMD patients are confined to a wheelchair before 12 years of age?
95%
34
What percentage of BMD patients become wheelchair bound after 12 years of age?
95%
35
What is the common mutation type found in DMD and BMD?
Deletions and duplications ## Footnote 65% of DMD cases and 85% of BMD cases result from deletions.
36
What distinguishes frameshift mutations from in-frame alterations in DMD and BMD?
Frameshift mutations produce premature stop codons; in-frame alterations do not ## Footnote In-frame alterations involve a multiple of three bases being deleted or duplicated.
37
What is the dystrophin assay used for?
To distinguish between DMD and BMD ## Footnote It also helps to differentiate these diseases from other forms of muscular dystrophy.
38
What percentage of cases does genetic testing identify a disease-causing mutation in for DMD and BMD?
About 95%
39
What type of drugs can help treat muscular dystrophy by causing ribosomes to read through premature stop codons?
Small-molecule drugs
40
What are antisense oligonucleotides used for in the context of muscular dystrophy?
To induce skipping of the altered DMD exon during mRNA translation
41
What is the primary function of rod cells in the human retina?
To allow vision in dim light
42
What is the genetic basis for red-green color vision defects?
Inherited in an X-linked recessive fashion
43
What percentage of European males are affected by red-green color perception defects?
About 8%
44
What is deuteranopia?
Inability to perceive green
45
What is protanopia?
Inability to perceive red
46
What is the prevalence of true colorblindness (monochromacy)?
Approximately 1 in 100,000 persons
47
What is rod monochromacy?
An autosomal recessive condition where all visual function is carried out by rod cells
48
What does the presence of multiple copies of the green opsin gene indicate?
Has no effect on color perception ## Footnote Only the red gene and the first green gene are expressed in the retina.
49
What mechanism produces the unique aspect of deletions in the red and green opsin genes?
Unequal crossover during meiosis
50
What does unequal crossover result in?
Loss of chromosome material on one homolog and gain on the other
51
What are the two major forms of monochromatic vision?
Rod monochromacy and blue cone monochromacy
52
What is the expected inheritance pattern for X-linked recessive disorders?
Absence of father-to-son transmission
53
What is an example of an X-linked recessive disorder?
Juvenile retinoschisis ## Footnote Caused by the RS1 gene.
54
What is the mating type most commonly associated with X-linked recessive genes?
Carrier female and normal male
55
In a mating between an affected father and a homozygous unaffected mother, what will be true for their sons?
All sons must be unaffected
56
What can happen in a mating between an affected father and a carrier mother regarding their daughters?
Half will be heterozygous carriers and half may be affected
57
How complex are recurrence risks for X-linked recessive disorders compared to autosomal disorders?
More complex
58
What is the outcome of mating between an affected father and a carrier mother for X-linked recessive disorders?
Half of the daughters will be heterozygous carriers and half will be affected. Half of the sons will be unaffected and half will be affected.
59
True or False: In X-linked recessive disorders, an affected son can receive the disease allele from his father.
False
60
What factors complicate recurrence risks for X-linked recessive disorders compared to autosomal disorders?
The risk depends on the genotype of each parent and the sex of the offspring.
61
What is a manifesting heterozygote?
A female heterozygote who exhibits symptoms of an X-linked recessive disease.
62
What percentage of female heterozygotes for hemophilia A are expected to be affected?
About 5%
63
Fill in the blank: X inactivation is a ______ process.
random
64
What syndrome is characterized by having only a single X chromosome and can be associated with X-linked recessive diseases?
Turner syndrome
65
Name an example of an X-linked dominant disease.
Hypophosphatemic rickets
66
True or False: X-linked dominant diseases are more prevalent than X-linked recessive diseases.
False
67
What is Rett syndrome characterized by?
Autistic behavior, intellectual disability, seizures, and gait ataxia.
68
What gene is primarily associated with classic Rett syndrome?
MECP2
69
How is X-linked dominant inheritance typically expressed in females?
Females are about twice as commonly affected as males.
70
What is a distinctive feature of X-linked dominant inheritance regarding father-to-son transmission?
Affected fathers cannot transmit the trait to their sons.
71
What does the presence of skipped generations indicate in X-linked dominant inheritance?
Skipped generations are uncommon.
72
How many Y-linked genes have been identified?
60 Y-linked genes
73
What is the mode of transmission for Y-linked traits?
Strictly from father to son
74
What type of chromosome carries housekeeping genes that have inactivation-escaping homologs on the X chromosome?
Y chromosome

Medical Genetics (40 decks)

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