••• Chapter 21^ Genetics for the Obstetrician^217
❍ Certain genetic disorders occur when both chromosomes of a pair are inherited from the same parent.
What is the process called?
UPD.
❍ What percentage of couples who have had two or more SABs will be found to have a chromosome abnormality?
6% of these couples (or 3% of the individuals).
❍ What are the risks associated with advanced paternal age?
Men who are 40 to 45 years of age or older, are at increased risk of new mutations, and associated with autosomal
dominant conditions [eg, neurofibromatosis I (NF-1), achondroplasia, Marfan syndrome, and osteogenesis
imperfecta].
❍ At a preconception visit, your patient and her husband disclose that they are first cousins. What would their
offspring be at risk of?
Birth defects, autosomal recessive conditions, and conditions that are more common in their ethnic background.
❍ What genetic test should be considered in women with unexplained ovarian failure or elevated FSH prior to
40 years of age?
Fragile X carrier screening.
❍ A new patient reports that she suffers from depression and had a heart problem as a child. She seems to have
difficulty following your conversation. Her mother, who accompanies her to the visit, has a scar over her
upper lip. What genetic testing would your order on this patient?
FISH for 22q11.2 deletion (DiGeorge).
❍ A patient of yours reports that her father has Marfan syndrome. What is her risk to have inherited the
condition? What features would you look for in your patient and what consults would you recommend?
50% risk.
Tall, thin body habitus, long, curved fingers (arachnodactyly), pectus, and striae.
Patient needs cardiology evaluation with echocardiogram, ophthalmology exam, and genetics evaluation.
❍ A patient reports that her brother died of Canavan disease. What is her risk to be a carrier?
2 /3.
❍ Chromosomes on the products of conception of your 38-year-old patient reveal a karyotype of 47,XX,+18.
What is the likely etiology of this result, and what is her recurrence risk?
The risk of trisomy increases with maternal age, and most trisomic conceptions spontaneously miscarry. Her
recurrence risk after a fetus or child with autosomal trisomy ranges from 1.6 to 8.2 times the maternal age-related risk.
❍ A patient states that she was recently diagnosed with NF-1, following a diagnosis of NF-1 in her daughter.
She tells you that her daughter has numerous café-au-lait marks, several neurofibromas, and learning
difficulties. The patient herself appears clinically normal except for several café-au-lait marks. What genetic
concept can explain this?
Variable expressivity.