15 Questions
What is the usual outcome if one allele of a gene is malfunction or deleted?
No effect on the phenotype
What is genomic imprinting?
An epigenetic phenomenon that occurs during gamatogenesis
Why do imprinting disorders occur?
Due to the transcriptional repression of one allele
What is the result of imprinting on gene expression?
Genes show different expression patterns depending on parental origin
What is the outcome if one allele is imprinted/methylated/inactivated and the other allele is deleted?
Clinically significant conditions
What is the name of the syndrome characterized by absence of speech, mild to moderate mental retardation, small hands and feet, and a dancing gait?
Angelman syndrome
What is the consequence of uniparental disomy for some specific genes?
The parent of origin determines the expression of the genes.
What is the term used when both inherited copies of a chromosome are from the same parent and have originated in the same grandparent?
Uniparental isodisomy
What is the cause of uniparental disomy?
Meiosis I nondisjunction followed by loss of genetic information
What is the possibility in a child with uniparental isodisomy?
The child may be affected by an autosomal recessive condition even if only one parent is a carrier of the mutation
What is the result of a genetic deletion on the q arm of Chromosome 15, depending on the parent of origin?
Prader-Willi Syndrome if inherited from the father, and Angelman Syndrome if inherited from the mother
What is the typical cause of Prader-Willi syndrome?
Deletion in the paternal chromosome 15 or maternal uniparental disomy
What is a characteristic of both Prader-Willi syndrome and Angelman syndrome?
Both males and females are affected
What is the result of a female with Prader-Willi syndrome having a child, if that child inherits the deletion from her?
The child will have Angelman syndrome
What is the characteristic of the paternal gene on Chromosome 15 in Prader-Willi syndrome?
It is deleted
Study Notes
Genomic Imprinting
- Each gene has 2 alleles, one inherited from each parent, but usually only one allele is sufficient for normal expression
- Imprinting: different expression of genetic material based on inheritance from mother or father, due to epigenetic changes during gametogenesis
- Genes show different expression patterns depending on parental origin
- Imprinted genes are methylated and have altered chromatin structure, while non-imprinted genes have relaxed chromatin
Imprinting Disorders
- Result from alterations in genes that are usually expressed from only one allele, depending on parental origin
- Transcriptional repression of one allele is due to imprinting, resulting in differential methylation
- Examples: Angelman and Prader-Willi syndromes, Beckwith-Wiedemann syndrome
Genomic Imprint of Chromosome #15
- Deletion on Chromosome 15q depends on parental origin
- Maternal deletion: Angelman Syndrome
- Paternal deletion: Prader-Willi Syndrome
- Both males and females are affected
- Alternate mechanism: uniparental disomy
Prader-Willi Syndrome
- Most cases caused by deletion in paternal Chromosome 15 or maternal uniparental disomy
- Characteristics: severe obesity, hyperactivity, severe mental retardation
Angelman Syndrome
- Most cases caused by deletion in maternal Chromosome 15 or paternal uniparental disomy
- Characteristics: absence of speech, mild to moderate mental retardation, small hands/feet, happy puppet syndrome
Uniparental Disomy
- Inheritance of two copies of a chromosome from one parent and none from the other
- Caused by nondisjunction followed by loss of genetic information
- Types:
- Heterodisomy: Meiosis II nondisjunction
- Isodisomy: both copies inherited from the same grandparent
- Consequence: imprinting disorders, such as Prader-Willi and Angelman syndromes
- Autosomal recessive conditions can arise in a child even if only one parent is a carrier in uniparental isodisomy
Learn about genomic imprinting, a process where genes exhibit different expression patterns based on parental origin. Understand how epigenetic changes during gametogenesis affect gene expression.
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