Genetics

Question 1

What is the name given to trisomy 13?

What is the likelihood that the meiotic error leading to trisomy 13 is maternal?

Answer 1

• Trisomy 13 is Patau’s syndrome.

- The meiotic error in Patau’s syndrome is maternal 85% of the time.

Question 2

What is the name given to trisomy 18?

What is the likelihood that the meiotic error leading to trisomy 18 is maternal?

Answer 2

• Trisomy 18 is Edward’s syndrome.

- The meiotic error in Edward’s syndrome is maternal 90% of the time.

Question 3

What is the name given to trisomy 21?

What is the likelihood that the meiotic error leading to trisomy 21 is maternal?

Answer 3

• Trisomy 21 is Down’s syndrome.

- The meiotic error in Down’s syndrome is maternal 85% of the time.

Question 4

What is the name given to 45,X?

What is the likelihood that the meiotic error leading to trisomy 45,X is maternal?

Answer 4

• 45,X is Turner’s syndrome.

- The meiotic error in Turner’s syndrome is maternal 20% of the time.

Question 5

What is the name given to 47,XXX?

What is the likelihood that the meiotic error leading to trisomy 45,X is maternal?

Answer 5

• 47,XXX is Triple X syndrome.

- The meiotic error in Triple X syndrome is maternal 95% of the time.

Question 6

What is the name given to 47,XXY?

What is the likelihood that the meiotic error leading to trisomy 45,X is maternal?

Answer 6

• 47,XXY is Klinefelter’s syndrome.

- The meiotic error in Klinefelter’s syndrome is maternal 55% of the time.

Question 7

What is the name given to 47,XYY?

What is the likelihood that the meiotic error leading to trisomy 45,X is maternal?

Answer 7

• 47,XYY is just 47,XYY syndrome.

- The meiotic error in 47,XYY syndrome is never maternal.

Question 8

What is the difference between prenatal diagnosis and preimplantation diagnosis?

Answer 8

• Prenatal diagnosis is done from samples in vivo, e.g. from the mother’s blood (NIPT), amniocentesis, chorionic villus sampling etc.
• Preimplantation diagnosis is done from a blastocyst produced from IVF or ICSI before artificial implantation.

Question 9

What is the difference between the 12-week ultrasound can and the 20-week ultrasound scan?

Answer 9

• The 12-week ‘dating’ scan is to check foetal development.

- The 20-week ‘anomaly’ scan is to check for structural anomalies.

Question 10

When is chorionic villus sampling (CVS) done?

Answer 10

• A chorionic villus sampling is done if another scan indicates that there may be a genetic defect.
• It is done from 11 weeks of gestation.

Question 11

When is an amniocentesis done?

Give one advantage and one disadvantage of using amniocentesis of chorionic villus sampling.

Answer 11

• An amniocentesis is done if another scan indicated that there may be a genetic defect.
• It is done from 15 weeks of gestation.
• Amniocentesis is safer than chorionic villus sampling, but can only be done later - this is important if the couple decide to abort the foetus.

Question 12

At which stage during gestation can non-invasive prenatal testing (NIPT - using a sample from the mother’s blood) be done?

When is non-invasive prenatal testing (NIPT - using a sample from the mother’s blood) not appropriate?

Answer 12

• Non-invasive prenatal diagnosis can be done from 8 weeks gestation.
• Non-invasive prenatal diagnosis is not appropriate when the disease shows a recessive inheritance pattern, as the mother could be a carrier.