2E- Sex-Linked and Nontraditional Inheritance Flashcards
What is the Lyon hypothesis?
stated that X inactivation occurs early in female embryonic development and that the X chromosome contributed by the father is inactivated in some cells, whereas in other cells the X chromosome contributed by the mother is inactivated
When and where does X inactivation occur?
takes place 7-10 days after fertilization takes place at the X-inactivation center
What is the X inactivation center?
a single 1-Mb region on the X chromosome long arm that spreads along the chromosome
Is X chromosome inactivation random or selective?
Random
True or False: once an X chromosome is inactivated in a cell, it will remain inactive in all decedents of that cell
True
Is the inactivated X completely inactivated or can a small mount of genes still get transcribed?
~15% of the genome can be transcribed. This is why Turners syndrome (XO) has weird stuff
What are Barr bodies?
inactive highly condensed X chromosome found in somatic cells of normal females
What is the XIST gene?
found in the X inactivation center and is transcribed only on the inactive X chromosome
What is the role of the XIST gene products?
RNA transcript made from the XIST gene is not translated into protein it remains in the nucleus and coats the inactive X chromosome
Methylation of what dinucleotides is in the inactivated X chromosome?
CG
How many Barr bodies does a female have?
the number of Barr Bodies in somatic cells is always one less than the number of X chromosomes
Are X-linked diseases more common among males or females?
Males. They only have 1 X chromosome
Is there father-to-son transmission in X-linked disorders?
No. Papa only gives a Y to the son
Let’s say a dude has a recessive X-linked disorder. What will be the genotype of the children? (assuming the mom is a normal noncarrier)
Boys- all unaffected. XY
Girls- all carriers. XX*
Let’s say a chick is a carrier for a X linked recessive disorder. What will the children’s genotype look like? (assuming dad is normal)
Boys- 1/2 will be normal (XY) and 1/2 will be affected (X*Y)
Girls- 1/2 will be noncarriers (XX) and 1/2 will be carriers (X*X)
Let’s say there’s an affected father with an X linked disease. He fertilizes a woman who is a carrier for the same disease. What will the kids genotypes look like?
XY (x) XX
Girls- 50% affected, 50% carriers
Boys- 50% affected, 50% normal
What type of disorder is hemophilia A?
X linked recessive
Which factor is missing in hemophilia A?
VIII
What types of mutations cause severe hemophilia A?
frameshift or nonsense
What is the result of missense mutations in hemophilia A?
produce a single amino acid substitution without a dominant negative effect resulting in an altered but partially functional protein product
Where do most missense mutations on the factor VIII take place?
CG hotspots
What is the main cause of death in hemophilia A patients?
Intracranial bleeding
What % of hemophilia A patients are in the severe category factor VIII levels that are less than 1% of normal?
50%
What is Duchenne Muscular Dystrophy (DMD)?
progressive weakness and loss of muscle one of the most severe and common forms
What is the genetic cause of DMD?
X linked recessive
What protein is disfxnl in DMD?
Dystrophin
What is the role of dystrophin in the skeletal muscle cell?
Maintains the integrity
What is unique about deletions in the genome that causes color blindness?
They are caused by unequal crossover during meiosis
What is the most common X-linked mutation that causes mental retardation in males?
Fragile X syndrome
What are some clinical features of fragile X syndrome?
distinctive facial appearance with large ears and long face, hypermobile joints and macroorchidism in postpubertal males
(a big-balled horse gymnast)
What is the region called that gets expanded to cause fragile X syndrome?
FMR1 region
Premutations must pass through which parent to cause the full clinical manifestation?
The mom
What is mtDNA?
Mitochrondrial DNA
What is the size and shape of mtDNA?
mtDNA occurs in several copies per organelle and consists of 16,569 base pair arranged on a double stranded circular molecule
What makes up mtRNA’s?
2 rRNAs and 22 tRNAs transcription takes place in the mitochondria
True or False: mtDNA contains no introns.
True
remember, mtDNA is just like bacterial DNA. that’s where it evolved from
Why is mutation rates of mtDNA 10x higher than that of nuclear DNA?
no repair mechanisms from damage of free radicals
What is the term for proteins made in and out of the mitochondria to be used in the electron transport chain?
Respiratory protein subunits
What is heteroplasmy?
each cell contains a different population of mtDNA molecules, where some might have a mutation and some might not.
What is homoplasmy?
a mammalian cell whose copies of mtDNA are all identical
How does heteroplasmisity relate to the severity of a disease?
the larger the percentage of mutant mtDNA molecules, the more severe the expression of the disease
What are the clinical manifestations of Leber hereditary optic neuropathy (LHON)?
rapid loss of vision in the central visual field as a result of optic nerve death vision loss typically begins in 3rd decade of life
What typeof mutation causes LHON?
missense mutation in protein coding mtDNA genes
All mitochondrial diseases come from which lineage?
The mother.
Remember, when the sperm fertilizes the egg, the egg kills all the sperm’s mitochrondria. You only have mitochrondrial DNA from your mom. Mitochrondrial diseases spread to every kid the woman has.
What is the presence of heteroplasmy in LHON?
minimal so expression tends to be relatively uniform and pedigrees for LHON display clear pattern of mito inheritance
What mitochondiral disease is characterized by epilepsy, dementia, ataxia, and myopathy?
Myoclonic epilepsy with ragged red fiber syndrome (MERFF)
What type of mutation causes MERRF?
disease caused by single-base mutations in a tRNA gene
Is there a presence of heteroplasmy in MERRF?
yes and thus is highly variable in its expression of severity
Clinical scenario: A patient presents with stroke-like episodes beginning before age 40. These episodes often involve temporary muscle weakness on one side of the body (hemiparesis), altered consciousness, vision abnormalities, seizures, and severe headaches resembling migraines. What mitochrondrial disease might be at fault?
Mitochondrial encephalomyopathy and stroke like episodes (MELAS)
What type of mutation causes MELAS?
disease caused by single-base mutations in a tRNA gene
Is there a presence of heteroplasmy in MELAS?
yes and thus is highly variable in its expression of severity
Give me the mechanism of genomic imprinting
For instance, an allele transmitted by the mother would be inactive, and the same allele transmitted by the father would be active. The normal individual only has one transcriptionally active copy of the gene
Where does the genomic imprinting occur for Prader Willi and Angleman syndrome?
deletion of 4 mb of the long arm of chromosome 15
Which parent has the genomic imprinting on chromosome 15 to cause Prader Willi syndrome?
Father.
Remember, (P)rader willi = (P)aternal
Which parent has the genomic imprinting on chromosome 15 to cause Angleman syndrome?
Maternal 15
What are the clinical manifestations of Prader Willi syndrome?
short stature, hypotonia (poor muscle tone), small hands and feet, obesity, mild to moderate mental retardation, and hypogonadism
What are the clinical manifestations of Angleman syndrome?
severe mental retardation, seizures, uncontrollable laughing and an ataxic gait
