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Questions and Answers
Males are hemizygous for the X chromosome.
Males are hemizygous for the X chromosome.
True
X-chromosome inactivation in females is a random process.
X-chromosome inactivation in females is a random process.
True
Females have four X chromosomes.
Females have four X chromosomes.
False
X-linked diseases can be transmitted from father to son.
X-linked diseases can be transmitted from father to son.
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X-linked recessive diseases are often expressed clinically in females.
X-linked recessive diseases are often expressed clinically in females.
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An inactive X chromosome is known as a Barr body.
An inactive X chromosome is known as a Barr body.
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Mitochondrial DNA is inherited from the father.
Mitochondrial DNA is inherited from the father.
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X-linked dominant diseases can be expressed clinically in many females.
X-linked dominant diseases can be expressed clinically in many females.
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Fragile X syndrome is the most common form of inherited mental retardation.
Fragile X syndrome is the most common form of inherited mental retardation.
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The FMR1 gene is involved in the formation of synapses.
The FMR1 gene is involved in the formation of synapses.
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Normal CGG repeats range from 50 to 100 for Fragile X syndrome.
Normal CGG repeats range from 50 to 100 for Fragile X syndrome.
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Huntington's disease is caused by a CAG triplet repeat in the coding region.
Huntington's disease is caused by a CAG triplet repeat in the coding region.
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Expansion of the FMR1 gene is more likely to happen during paternal meiosis.
Expansion of the FMR1 gene is more likely to happen during paternal meiosis.
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Both males and females can be affected by Fragile X syndrome.
Both males and females can be affected by Fragile X syndrome.
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Anticipation in genetics refers to the increase in age of onset of genetic disorders across generations.
Anticipation in genetics refers to the increase in age of onset of genetic disorders across generations.
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In Huntington's disease, the paternal parent carries the most risk for expansion of the CAG repeat.
In Huntington's disease, the paternal parent carries the most risk for expansion of the CAG repeat.
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Haemophilia A is characterized by a deficiency of factor VII.
Haemophilia A is characterized by a deficiency of factor VII.
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X-linked dominant diseases are typically more severe in males than in females.
X-linked dominant diseases are typically more severe in males than in females.
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The mitochondrion contains a circular DNA genome that is paternally inherited.
The mitochondrion contains a circular DNA genome that is paternally inherited.
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All daughters of an affected male with X-linked dominant traits will be affected.
All daughters of an affected male with X-linked dominant traits will be affected.
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Triplet repeat disorders can result from unstable contraction in the number of repeats.
Triplet repeat disorders can result from unstable contraction in the number of repeats.
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Mitochondrial inheritance affects only females.
Mitochondrial inheritance affects only females.
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Duchenne Muscular Dystrophy typically has an onset at around 10 years of age.
Duchenne Muscular Dystrophy typically has an onset at around 10 years of age.
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Leber’s hereditary optic atrophy is an example of a disease associated with abnormal mtDNA.
Leber’s hereditary optic atrophy is an example of a disease associated with abnormal mtDNA.
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In triplet repeat expansion diseases, the base sequence of the repeat can vary between individuals.
In triplet repeat expansion diseases, the base sequence of the repeat can vary between individuals.
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Males and females are affected equally in X-linked recessive diseases.
Males and females are affected equally in X-linked recessive diseases.
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X-linked diseases can be transmitted from mother to son.
X-linked diseases can be transmitted from mother to son.
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X-chromosome inactivation is a process that occurs randomly in male cells.
X-chromosome inactivation is a process that occurs randomly in male cells.
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Female carriers of X-linked recessive diseases may show clinical symptoms.
Female carriers of X-linked recessive diseases may show clinical symptoms.
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Mitochondrial DNA disorders are inherited exclusively from the mother.
Mitochondrial DNA disorders are inherited exclusively from the mother.
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Random X inactivation in females results in all X-linked traits being fully expressed.
Random X inactivation in females results in all X-linked traits being fully expressed.
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All males and females have equal amounts of proteins encoded by X-linked genes.
All males and females have equal amounts of proteins encoded by X-linked genes.
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Fragile X syndrome is a results of a CAG repeat mutation.
Fragile X syndrome is a results of a CAG repeat mutation.
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Disease expression in heterozygous females can vary due to skewed X-inactivation.
Disease expression in heterozygous females can vary due to skewed X-inactivation.
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Expansion of the CGG repeat in Fragile X syndrome occurs primarily in males during meiosis.
Expansion of the CGG repeat in Fragile X syndrome occurs primarily in males during meiosis.
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Huntington's disease is associated with an unstable expansion of CAG repeats beyond 36-40 copies.
Huntington's disease is associated with an unstable expansion of CAG repeats beyond 36-40 copies.
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Fragile X syndrome can be transmitted through both maternal and paternal lines.
Fragile X syndrome can be transmitted through both maternal and paternal lines.
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A full mutation of the FMR1 gene in Fragile X syndrome is categorized as having between 200 to 2000 CGG repeats.
A full mutation of the FMR1 gene in Fragile X syndrome is categorized as having between 200 to 2000 CGG repeats.
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In Fragile X syndrome, males are more frequently affected than females due to the X-linked inheritance pattern.
In Fragile X syndrome, males are more frequently affected than females due to the X-linked inheritance pattern.
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The Sherman paradox refers to the increased age of onset of genetic disorders across generations.
The Sherman paradox refers to the increased age of onset of genetic disorders across generations.
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Maternally transmitted mutations in Fragile X syndrome do not exhibit instability during meiosis.
Maternally transmitted mutations in Fragile X syndrome do not exhibit instability during meiosis.
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Anticipation in genetics can manifest as increased severity of symptoms in succeeding generations.
Anticipation in genetics can manifest as increased severity of symptoms in succeeding generations.
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Haemophilia A is caused by a deficiency of factor IX.
Haemophilia A is caused by a deficiency of factor IX.
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Duchenne Muscular Dystrophy typically leads to death in early 30s due to respiratory complications.
Duchenne Muscular Dystrophy typically leads to death in early 30s due to respiratory complications.
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X-linked dominant conditions can affect both males and females, but males typically show more severe symptoms.
X-linked dominant conditions can affect both males and females, but males typically show more severe symptoms.
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The mitochondrial genome is inherited exclusively from the father.
The mitochondrial genome is inherited exclusively from the father.
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All children of an affected mother will likely inherit mitochondrial disorders.
All children of an affected mother will likely inherit mitochondrial disorders.
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Mitochondrial inheritance is characterized by both male and female transmission.
Mitochondrial inheritance is characterized by both male and female transmission.
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Triplet repeat disorders can arise from stable expansions of repeat sequences.
Triplet repeat disorders can arise from stable expansions of repeat sequences.
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Leber’s hereditary optic atrophy is associated with mutations in mtDNA.
Leber’s hereditary optic atrophy is associated with mutations in mtDNA.
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In Fragile X syndrome, the base sequence of the repeat is CAU.
In Fragile X syndrome, the base sequence of the repeat is CAU.
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X-linked recessive diseases are more commonly expressed in females than in males.
X-linked recessive diseases are more commonly expressed in females than in males.
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Study Notes
X-Chromosome Inactivation
- In female cells, one of the two X chromosomes is inactivated.
- Inactivation occurs early in embryonic life, is random, and is virtually complete and permanent.
- The inactive X chromosome appears as a dark-staining mass known as a Barr body.
X-linked Diseases
- Genes carried on the X chromosome are "X-linked."
- X-linked diseases exhibit a characteristic pattern of inheritance, with a complete lack of male-to-male transmission.
- They can be dominant or recessive.
- Males are hemizygous for X, so X-linked mutant traits are fully expressed.
- Random X inactivation occurs in females, leading to variable expression of X-linked traits:
- Balanced X-inactivation: normal expression of X-linked traits.
- Skewed X-inactivation: "Manifesting heterozygote" - the normal X chromosome is preferentially inactivated, resulting in clinical manifestation of X-linked traits.
X-linked Recessive (XLR)
- XLR diseases are rarely expressed clinically in females.
- XLR diseases are characterized by a higher proportion of affected males compared to females.
X-Linked Dominant (XLD)
- XLD diseases are expressed clinically in many females.
- XLD diseases are characterized by a higher proportion of affected females compared to males.
- Expression of XLD symptoms may be more severe and constant in males than in females.
Extra-nuclear Inheritance (Mitochondrial DNA Disorders)
- Mitochondria contain a ~16kb circular DNA genome (mtDNA) that is maternally inherited.
- Diseases resulting from abnormal mtDNA often involve skeletal/cardiac muscle impairment and neurological problems.
Triplet Repeat Disorders
- Triplet repeat disorders are caused by unstable expansion in the number of trinucleotide repeats within genes.
- These expansions can lead to various genetic disorders.
- Examples include:
- Fragile X syndrome: CGG repeat in the 5' UTR of the FMR1 gene.
- Huntington's disease: CAG repeat in the coding region of the HTT gene.
- These disorders exhibit:
- Parent-of-origin effects: expansion of repeats occurs more frequently in one parent.
- Anticipation: increased severity and reduced age of onset in subsequent generations.
X Chromosome Inactivation
- In females, one of the two X chromosomes is inactivated early in embryonic life.
- This inactivation is random meaning either the paternal or maternal X can be inactivated.
- It is virtually complete, permanent, and clonally propagated.
- The inactive X chromosome appears as a dark-staining mass called a Barr body.
- This inactivation leads to mosaicism in females, meaning different cells have different active X chromosomes.
X-Linked Diseases
- Genes carried on the X chromosome are called X-linked.
- X-linked diseases have a characteristic pattern of inheritance with no male-to-male transmission.
- They can be either dominant or recessive.
- Males are hemizygous for the X chromosome, so X-linked mutant traits are fully expressed in males.
- Females have two X chromosomes, so X-linked traits are variably expressed due to random X inactivation.
X-Linked Recessive (XLR) Diseases
- XLR diseases are rarely expressed clinically in females.
- Males who inherit the mutated X chromosome are affected.
- Haemophilia A, a deficiency of factor VIII, is an example of an XLR disease.
- Duchenne Muscular Dystrophy, a form of muscular dystrophy, is another example of an XLR disease.
X-Linked Dominant (XLD) Diseases
- XLD diseases are expressed clinically in many females.
- Vitamin-D resistant rickets (hypophosphatemic rickets) is an example of an XLD disease.
- Expression of XLD symptoms is usually more severe and constant in males than in females.
Extra-Nuclear Inheritance
- The mitochondrion contains a ~16kb circular DNA genome, mtDNA.
- It is maternally inherited since sperm mitochondria are located near the tail and not injected into the ovum.
- Abnormal mtDNA can cause human diseases, often involving skeletal/cardiac muscle impairment and neurological problems.
- Leber's hereditary optic atrophy is an example of a mitochondrial disease.
Triplet Repeat Disorders
- Sequences of three bp (base pair) repeats are found throughout the genome.
- They can be polymorphic, meaning the number of repeats varies between individuals.
- Triplet repeat disorders are due to unstable expansion in the number of repeats.
- These expansions can lead to disease.
- Examples of triplet repeat disorders include Fragile-X syndrome and Huntington's disease.
Fragile-X Syndrome
- The most common form of inherited mental retardation.
- It affects approximately 1/2000-1/4000 individuals, including females.
- This syndrome is caused by a mutation in the FMR1 gene on the X chromosome.
- The FMR1 gene has a CGG triplet repeat in the 5' UTR.
- Expansion beyond 200 copies prevents translation of the FMR1 protein.
- The repeat expansion is unstable during meiosis, particularly in females, leading to a maternal parent of origin effect.
- Premutation carriers have 50-100 repeats and are usually unaffected but can transmit the expanded allele to their children.
Huntington's Disease
- Caused by a CAG triplet repeat expansion in the protein coding region of the Huntington gene.
- The expansion leads to a polyQ region in the protein.
- The repeat is unstable during meiosis, particularly in males, resulting in a paternal parent of origin effect.
- Expansion beyond 36-40 repeats leads to the expression of a protein that causes neuronal damage.
- The disease has an autosomal dominant inheritance pattern.
Anticipation
- The Sherman paradox is sometimes seen in triplet repeat disorders.
- This is seen as a reduced age of onset or increased severity of the disease in later generations.
- This indicates that the repeat expansion is increasing with each generation.
Parent of Origin Effects
- Expansion of the repeat occurs in females for Fragile-X and in males for Huntington's disease.
- This is why these disorders are said to be “maternally transmitted” for Fragile-X and “paternally transmitted” for Huntington's disease.
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Description
This quiz explores the mechanisms of X-chromosome inactivation and its implications for X-linked diseases. It covers concepts such as random X inactivation, the role of Barr bodies, and the inheritance patterns of X-linked traits. Test your understanding of these essential genetic principles.
