Genetics module

Second Revision quiz - Answers

1    Each son of a man with haemophilia has a 1 in 2 risk of inheriting the condition.

False
A man with an X-linked condition passes on his Y chromosome to his sons (all of whom will therefore be unaffected) and his X chromosome to his daughters (all of whom will be carriers).

2    The risk of having another child with a neural tube defect is 1 in 200.

False
The empiric (observed) risk of having a subsequent child with a neural tube defect is about 1 in 20 in the absence of periconceptual vitamin supplementation.

3    Genetic testing in childhood should be performed only for conditions where genetic information will enable surveillance for complications.

True
There is international agreement that children should not be tested for carrier status or adult onset conditions until they are at an age where they can make an informed decision.

4    Approximately 1 in 20 of the white population in the West Midlands is a carrier for cystic fibrosis.

True
and of these 75% have the Delta F508 mutation.

5    The majority of people with Down syndrome have trisomy 21.

True
Down syndrome is most commonly caused by trisomy associated with non disjunction at maternal meiosis. 4% of people with Down syndrome have a Robertsonian translocation involving chromosome 21.

6    Prenatal diagnosis by DNA analysis can only be performed on tissue in which the gene is being expressed.

False
All somatic cells have the same nuclear DNA code with copies of all the genes (apart from reticulocytes which lose their nuclei), although not all genes are expressed in all tissues. The advantage of DNA analysis is that the gene which is not being actively expressed in a particular cell type can still be analysed by looking directly at the DNA code.

7    Not all children with an autosomal dominant condition (such as neurofibromatosis) will have an affected parent.

True
Some people with autosomal dominant conditions are the result of a new mutation occurring in the sperm or egg which went to make that person. Of course, such a person can then pass the autosomal dominant condition on to his or her children.

8    The karyotype XYY is associated with gynaecomastia, tall stature, infertility and eunuchoid habitus.

False
The description is of a patient with features of Klinefelter syndrome, XXY. Apart from an increase in height above that expected for the family, 47XYY is not associated with any physical features.

9    Children who are carriers of balanced reciprocal translocations can usually be diagnosed because of their dysmorphic features.

False
By definition a carrier of a balanced reciprocal translocation will show no effects – there has been no gain or loss of genetic material.

 10    A woman with an autosomal dominant disease has a 1 in 2 chance that a daughter will inherit the condition.

True
Each child (regardless of sex) has a 1 in 2 risk of inheriting the faulty gene.

11    The children of a man with a disease caused by a mutation in mitochondrial DNA will not inherit the disorder from him.

True
Mitochondria are inherited from one’s mother and not from one’s father.

12    The mothers of all boys with Duchenne muscular dystrophy are carriers for the condition.

False
In a family where there is an isolated boy with Duchenne muscular dystrophy, there is a 1 in 3 chance that his disease is the result of a mutation occurring for the first time in the egg which went to make him. There is a 2 out of 3 chance that his mother is a carrier. Compare this with the 100% risk that his mother would be a carrier if her brother also had Duchenne muscular dystrophy.

13    The environmental component of the majority of multifactorial conditions has been identified.

False
The environmental component for relatively few multifactorial conditions has been identified – for instance in the neural tube defects.

14    The healthy brothers and sisters of a child with congenital adrenal hyperplasia have a 2 out of 3 chance of being a carrier.

True
When we know that a sib of someone with an autosomal recessive condition is healthy, it means that they cannot be homozygote for the disease gene. Of the four possibilities in autosomal recessive segregation, one has therefore been excluded with the resulting risk of 2 out of 3 chance that the sib is a carrier. (See the animated tutorial on autosomal recessive inheritance).

15    It can be difficult to detect carriers of autosomal recessive metabolic diseases (such as congenital adrenal hyperplasia) by biochemical assay because of the overlap of levels with the normal range.

True
For an increasing number of conditions carrier status within a family can be determined once a mutation in the causative gene has been identified in the affected person.

16    The sister of two boys who have suffered from X-linked adrenoleukodystrophy has a 1 in 2 risk of being a carrier.

True
As she has two brothers who are affected, her mother must be a carrier. Therefore her mother will pass on either the X chromosome with the normal gene or the X chromosome with the faulty gene giving a 1 in 2 risk to the sister.

17    A particular characteristic of autosomal dominant inheritance is that all affected people in a family will demonstrate exactly the same degree of severity.

False
For most autosomal dominant disorders, there is considerable variability between affected people both within and between families. This is called "variation in expression" and requires detailed physical examination of people at risk within the family.

18    If the first child of two parents who are carriers has sickle cell disease the next three children will be unaffected.

False
Chance has no memory! The risk in each pregnancy is 1 in 4 of having an affected child.

19    Different physical characteristics on examination would allow one to predict whether a baby boy had the karyotype 47,XY+21 (trisomy 21) or an unbalanced 14-21 Robertsonian translocation.

False
The physical features of a person with Down syndrome caused by either trisomy 21 or an unbalanced Robertsonian translocation are the same. It is therefore important to perform karyotyping in a baby with Down syndrome to exclude the possibility of a translocation.

20    More than 40% of children who have congenital sensory neural deafness have a genetic cause.

True
Single gene disorders are the cause of about 50% of congenital sensory neural deafness (the majority being autosomal recessive). Environmental and idiopathic factors make up 30% and 20%.