SEX-LINKED inheritence

Question 1

An X linked genetic condition is caused by

Answer 1

a

mutation in a gene on the X chromosome

Question 2

Affects males and females differently

Answer 2

true

Question 3

Mostly X-linked

Answer 3

true

Question 4

Usually recessive

• Dominant X-linked disorders are rare

Answer 4

true

Question 5

X – linked recessive in females

Answer 5

Females have two X chromosomes (XX) and if one of the genes on an X the chromosome has a mutation

– The normal gene on the other X chromosome can compensate for the
mutated copy

– These females are usually healthy carriers of the X linked condition

– Carrier does not have the condition but carries a mutated copy of the gene

– In some cases, females show mild signs of the condition

Question 6

X – linked recessive in males

Answer 6

Males have an X and a Y chromosome (XY)
■ If one of the genes on the male’s X chromosome has a
mutation, there is no other copy to compensate for it
■ He will be affected by the condition

Question 7

How are they passed on?

Answer 7

■ X-linked disorders affect males and females differently
■ Therefore how they are passed on from parent to offspring is also different
■ Females have 2 copies of each gene and for disease to manifest both copies
must be mutated
■ Males only have 1 copy- if there is a mutation in it, they will manifest with
disease
■ Carrier mothers can pass on the mutation to both sons and daughters
■ Affected fathers can only pass on the mutation to their daughters

Question 8

Males predominantly

affected

Answer 8

true

Question 9

Females are usually

carrier

Answer 9

true

Question 10

Female to male transmission (males
never pass on the condition to their
sons but their daughters will be
carriers)

Answer 10

true

Question 11

The mother of an affected child
is usually not affected but is an
obligate carrier

Answer 11

true

Question 12

Haemophilia

Answer 12

A bleeding disorder that affects males

■ Caused by a deficiency in clotting factors
– Factor 8, Haemophilia A
– Factor 9, Haemophilia B
■ Recurrent bleeding in the joints
■ Female carriers may have some bleeding
manifestations

Question 13

Duchenne Muscular

Dystrophy (DMD)

Answer 13

■ Muscle weakness beginning in the legs and
pelvis

■ Progressive difficulty walking, most patients
confined to a wheelchair by age 12

■ Females are rarely affected but may have
symptoms of muscle weakness

Question 14

X-linked dominant

Answer 14

■ Rare
■ Female inherits one normal and one mutated copy of the gene, the
mutated gene will be enough to cause the condition
■ It is very rare for a woman to have a dominant gene mutation on
both copies of her X chromosome
■ Male inherits a mutated X chromosome the condition will manifest
■ For certain conditions, males affected with an X-linked dominant
the disorder may not survive and will spontaneously abort

Question 15

For certain conditions, males affected with an X-linked dominant disorder may
not survive and will spontaneously abort Rett syndrome

Answer 15

true

Question 16

Rett syndrome

Answer 16

■ Affects girls almost exclusively
■ Mutations in the MECP2 gene
■ Early development is normal but then slows
■ Intellectual disability
■ Inability to perform motor functions (Apraxia)
■ Problems with walking

Question 17

A single dose of the mutant will affect the phenotype of the female

Answer 17

true

Question 18

Males are usually more severely affected

– Some conditions may be embryonic lethal in males

Answer 18

true

Question 19

affected female

Answer 19

– 50% chance of affected son/lethal

– 50% chance of affected daughter

Question 20

affected male

Answer 20

– All daughters affected (100%)

– All sons normal (100%)