Genetics Flashcards
When can amniocentesis be done?
16 weeks
What is used to track intrauterine growth over time?
Fetal US
When are structural anomalies (like myelomeningocele) best detected on US?
Between 12-24 weeks gestation
What maternal problem can be a complicating factor that blunts the accuracy of US findings?
Maternal obesity
What 3 hematologic diseases can be diagnosed prenatally?
Factor 8 deficiency, factor 9 deficiency, sickle cell disease
What is monogenic inheritance?
Genetic disorders caused by a mutation in one gene
What is multifactorial inheritance?
Genetic disorders caused by mutations in multiple genes
What is epigenetics?
Genetic disorders caused by a combination of gene mutations and environmental factors
In which inheritance pattern are males affected and females carriers?
X-linked recessive
If there are any female relatives with a disorder, which inheritance pattern is eliminated?
X-linked recessive
Is male-to-male transmission possible in X-linked recessive disorders?
No
If a question casually mentions a kid who had two uncles with a similar problem, what should you think?
You know that only males are affected…pointing towards an X-linked recessive disorder
Which type of inheritance disorders frequently involve an enzyme deficiency?
X-linked recessive
Name 8 frequently tested X-linked recessive conditions
- Hemophilia A/B: Factor 8/9 enzyme deficiencies
- G6PD Deficiency: Enzyme deficiency
- CGD: Enzyme to break down bacterial cells in neutrophil is defective
- Duchenne Muscular Dystrophy
- Nephrogenic Diabetes Insipidus: Enzyme deficiency
- Retinitis Pigmentosa: Enzyme deficiency
- Androgen Insensitivity (Testicular Feminization): Enzyme defective
- Wiskott Aldrich Syndrome
When will you see a phenotypic female, but genetic male and what is the inheritance pattern of this disorder?
Androgen insensitivity (X-linked recessive)
Mom is known carrier of an X-linked recessive disorder…chances of affected male, female carrier, affected children?
Males: 50% affected, 50% not affected
Females: 50% carriers, 50% not carriers
Children: 25% carriers, 25% disease, 50% normal
For X-linked dominant disorder, Dad has gene on his X chromosome, which kids have disease?
- Only has affected X to give to daughters…all daughters will have disorder
- Only has Y to give to sons…none of his sons will have disorder
For X-linked dominant disorder, Mom has gene on one of her X chromosomes, which kids affected?
50% of her kids will have it and 50% won’t
Are there carriers for an X-linked dominant disease?
No
Which other inheritance pattern is the same as X-linked dominant?
Autosomal dominant…makes it very difficult to distinguish
How are mitochondrial disorders transmitted?
Exclusively through mothers
*Mother and mitochondria both start with an M
Which disorders have no male to male transmission?
X-linked disorders (recessive or dominant)
Name 4 X-linked dominant conditions to consider (remember, not too many of them and they will most likely not ask you to pick this up from question)
- X-linked hypophosphatemic rickets
- Pseudohyperparathyroidism
- Aicardi syndrome
- Alport syndrome
What is the only way a disease can be transmitted from father to son?
Autosomal dominant inheritance
What are 3 characteristics of autosomal dominant disorders?
- Variable expressivity
- Possible reduced penetrance
- High risk for spontaneous mutation with no prior family history
What is the mneumonic for 8 of the autosomal dominant diseases?
TAR MAN
Tuberous Sclerosis
Achondroplasia
Retinoblastoma
Marfan Syndrome
Apert, All porphyrias (occasionally Alport syndrome)
Nail Patella Syndrome, Neurofibromatosis
*Neurofibromatosis can have variable expressivity with a high spontaneous mutation rate
What 2 things are autosomal dominant disorders associated with?
- Variable penetrance
2. Variable expressivity
What is variable penetrance?
When there is variability of whether people with the gene for a given trait actually exhibit it
When can chorionic villus sampling be done?
12 weeks
What is variable expressivity?
Range of signs and symptoms that can occur in different people with the same genetic condition…basically a mixed bag of features can be seen in people with a given syndrome with wide variable expressivity.
What rules out an autosomal recessive disorder?
Vertical transmission through 3 generations
Mneumonic for common autosomal recessive conditions?
Receding because you go to get gas from a guy wearing a hat, but he gives you a wak on the hand with a pickax... Galactosemia Alpha 1-Antitrypsin Deficiency Sickle cell and Thalassemia Hurler syndrome Ataxia Telangiectasia Tay Sachs Disease Wilson Disease Adrenogenital Syndrome (and Alpers Syndrome) Kartagener Syndrome PKU
In an autosomal recessive condition, what percent of unaffected offspring are carriers?
2/3
What is genetic anticipation?
Altered gene is passed down from one generation to next…disorder may begin earlier in life and signs and symptoms become more severe in subsequent generations…in some cases, this happens because there is an increase in the length of the unstable region in the gene
Ex: Myotonic dystrophy
Webbed neck, delayed secondary sexual characteristics, left sided congenital heart defects (bicuspid aortic valve, coarctation of aorta), short 4/5th metacarpal bones, widely spaced nipples, lymphedema of hands and feet
Turner syndrome (Primary Gonadal Dysgenesis)
Is Turner Syndrome associated with advanced maternal age?
No (since it isn’t a trisomy)
What are some features of Turner Syndrome in an adolescent?
Short stature, minimal/no breast development, primary amenorrhea, wide carrying angle of elbows, chronic otitis media, hyperconvex nails, low hairline, small mandible, high arched palate, low set ears
What causes Turner Syndrome?
Full or partial X chromosome deletion (in addition to phenotypical features outlined above)
Can a male get Turner Syndrome?
No, Turner Syndrome can only be diagnosed in a female
What is the most common chromosomal defect discovered in spontaneous abortions, and what is the most common form?
Turner Syndrome, 45XO
What is necessary for definitive diagnosis of Turner Syndrome?
Karyotype…buccal smear is not correct
If you think someone has Turner Syndrome and the initial chromosome analysis is normal, what should you do?
Use a FISH study to look for mosaicism (remember, elevated FSH can also be a feature of Turner Syndrome)
What has been used to treat short stature in Turner Syndrome?
Growth Hormone
What used to be known as “Male Turner Syndrome”?
Noonan Syndrome
Who can get Noonan Syndrome?
Males and females
4 classic features of Noonan?
Pertussis excavatum
Webbed neck
Low set ears
Pulmonic stenosis
*Envision a moon man pulling a web neck and grabbing the moon and placing it on his chest, snugly on the pectus excavatum. The moon changes tide on his chest and pulls down the ears, and also exerts pressure on the heart, resulting in pul”moonic” stenosis
What is the karyotype in Noonan?
Normal…no identifiable genetic patterns, although genetics do play a role
Describe eye issues with Noonan Syndrome
Hypertelorism, downslanting, vivid blue or blue-green irides, epicanthal folds, thick/droopy lids, strabismus/amblyopia
What are the two cardiac defects with Noonan Syndrome?
Pulmonary valve stenosis (number 1) and hypertrophic cardiomyopathy
Deeply grooved philtrum, short and webbed neck, low posterior hairline, unusually-shaped chest with superior pectus carinatum and inferior excavatum, developmental delay or MR of variable degree, joint laxity, cryptorchidism, coarse/curly hair, short stature, inverted triangular facies, low set ears, posteriorly rotated ears, fleshy helices on ears, high frequency SNHL, variable coagulation defects, male: female is 1:1
Noonan Syndrome
What is the inheritance of Marfan syndrome?
Usually autosomal dominant (15% of cases are a new mutation)
*There can be great variation of the morphology among affected family members
Connective tissue disorder associated with skeletal, cardiac, and ophthalmologic manifestations.
Marfan Syndrome
Where is the mutation in Marfan?
Chromosome 15 (fibrillin gene)
What are the skeletal abnormalities with Marfan!
Tall stature, high arched palate, dental crowding, hyperextensible joints, pectus abnormalities, arachnodactyly, disproportionately long extremities compared to the trunk
Is mitral valve prolapse one of the amour criteria?
No
What murmurs will you see with Marfan Syndrome?
Aortic or mitral valve regurgitation
Tall teenager presenting with a spontaneous pneumothorax…?
Marfan Syndrome
What are patients with Marfan syndrome at risk for that can lead to death?
Sudden death associated with aortic rupture…close cardiac followup is important (referral of other family members for genetic testing would be appropriate)
What are the 5 major criteria for diagnosing Marfan?
- Dilatation or dissection of the ascending aorta/aortic root
- Lumbosacral dural ectasia
- Ectopia lentis
- Four skeletal manifestations
- Family or genetic history
How do you tell apart Marfan and Homocystinuria?
The body type is similar for both…but in homocystinuria the lens is usually displaced downwards (Marfan the lens is displaced up)
What 2 things is homocystinuria associated with?
Mental retardation and thrombosis
What is wrong with the eyes in Marfan?
Ectopia lentis (upward and temporal displacement of the lens of the eye)
What type of exams are important for Marfans (for the eyes)?
Serial slit lamp evaluations
What are the only genetic disorders that increase with advancing maternal age?
Trisomies (incidence of all trisomies increases with advancing maternal age)
From what 2 things can trisomy 21 arise?
- Nondisjunction
- Translocation
*No phenotypic differences between the two, but genetic implications quite different
In which cause of trisomy 21 are chromosomal studies of the parents crucial?
Translocation
If you find a carrier parent with a full translocation for trisomy 21, what is the chance of Down syndrome reoccurring?
100% chance (but this is extremely rare)
With a partial translocation for trisomy 21, what is the % chance for Downs recurring and which side carries a higher risk?
Close to 15%…if partial translocation is from Mom, chances of recurrence are higher than if from Dad
What is the risk for recurrence of Downs with nondisjunction?
Risk for recurrence equals overall risk for general population (1%) plus Mom age related risk
What is the age related risk for trisomy 21 with mom at age 22? At age 40?
22: 1/1500
40: 1/90
What is the only truly useful test for detecting conditions of a missing or extra chromosome? (Turner or Downs)
Karyotyping
What chromosomal abnormalities will karyotyping miss?
Small changes in a chromosome
What is the gold standard genetic test for detecting small losses or gains in chromosomes?
Array comparative genomic hybridization (array CGH)
What test is widely used in evaluation of intellectual disability or congenital malformations?
Array comparative genomic hybridization (array CGH)
What do people with balanced translocation look like, and what are their offspring like?
Appear normal, often have offspring with a genetic condition
What do people with unbalanced translocations look like and what do their offspring look like?
Have a genetic condition, always have offspring with a genetic condition
Which age group gives birth to more infants with Down Syndrome?
Mothers in their 20s (more women in their twenties have kids, period)