X-linked Recessive Inheritance

Question 1

This condition is an example of sex-linked recessive trait, and is characterized progressive proximal muscle weakness w/ the onset in early childhood.

Answer 1

Duchenne muscular dystrophy (DMD)

Question 2

T/F. In X-linked recessive inheritance, there is a marked discrepancy in the sex ratio, w/ only males being severely affected, & the affected boys have a similar disease course.

Answer 2

True.

Question 3

This mode of inheritance is characterized by a knight’s move pedigree pattern of affected males.

Answer 3

X-linked recessive

Question 4

In marriages between a carrier female and a normal male for an X-linked recessive trait, the genotypic ratio is.

Answer 4

one - half of the daughters will be carriers and one - half of the sons will be affected.

Question 5

Test for carrier status of DMD

Answer 5

DNA testing
Creatine kinase testing

Question 6

A woman w/ an affected child & an affected brother, or a woman w/ more than one affected child, is an ____ carrier, as the alternative explanation of multiple new mutations is so unlikely.

Answer 6

obligate

Question 7

For each daughter of an obligate carrier there is, on average, a ____ risk that she too is a carrier.

Answer 7

1 in 2

Question 8

Occasionally a female is affected with this X - linked form of muscular dystrophy. This may be explained by

Answer 8

skewed X inactivation (resulting in a manifesting heterozygote)

a new mutation on the normal X chromosome of a carrier female

a carrier with Turner syndrome (45,X)

an X – autosome translocation

Question 9

What mechanism mainly explain why in some monozygotic female twins, one is affected by an X-linked recessive trait while the other is asymptomatic to the same trait.

Answer 9

skewed X inactivation

Question 10

In X – autosome translocations, the ____ is preferentially inactivated, as otherwise ____ for the involved autosome might occur.

Answer 10

karyotypically normal X; partial monosomy

Question 11

This provided an important early clue to the localisation of the gene for X - linked muscular dystrophy, as in each female w/ muscular dystrophy due to an X – autosome translocation the breakpoint was in the band ____.

Answer 11

Xp21

Question 12

In addition to the severe form of X - linked muscular dystrophy (DMD), there is a milder X - linked form of muscular dystrophy called ____.

Answer 12

Becker muscular dystrophy (BMD)

Question 13

Becker muscular dystrophy (BMD) and its more severe form are due to different mutant alleles of the extremely large 79 - exon gene (spanning 2.2 Mb) encoding the protein ____.

Answer 13

dystrophin

Question 14

In about ___% of cases of X - linked muscular dystrophy, DNA analysis reveals a deletion of variable size. This analysis is undertaken using a ____ or the more recently introduced ____ technique.

Answer 14

65; multiplex polymerase chain reaction (PCR); multiplex ligation - dependent probe amplification (MLPA)

Question 14

T/F/. Before counselling a family with muscular dystrophy, it is not important to establish the precise type as, in addition to these X - linked forms, autosomal dominant and recessive forms of muscular dystrophy are not known.

Answer 14

False. Muscular dystrophy is know for being genetically heterogeneous.

Question 15

So far, ____ X - linked recessive traits are known in humans.

Answer 15

515

Question 16

The frequencies of X-linked recessive traits vary in different ethnic groups, for example in certain groups, ____, particularly where malaria is common, is as frequent as ____is in the UK

Answer 16

glucose - 6 – phosphate dehydrogenase deficiency; colour blindness

Question 17

T/F. For some X - linked recessive disorders, affected males may reproduce, and in this event all daughters will be carriers (obligate carriers) and all sons will be normal.

Answer 17

True.

Question 18

_____ provides an important exception to the principle of consistent male severity within a family for an X - linked recessive trait.

Answer 18

Fragile X syndrome