X-linked recessive inheritance where the mother is a carrier: the basics a tutorial to show how the...

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X-linked recessive inheritance where the mother is a carrier: the basics a tutorial to show how the genes segregate to give the typical pedigree pattern Professor P Farndon, Clinical Genetics Unit, Birmingham Women’s Hospital 13.11.06

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Page 1: X-linked recessive inheritance where the mother is a carrier: the basics a tutorial to show how the genes segregate to give the typical pedigree pattern.

X-linked recessive inheritance where the mother is a carrier:

the basics

a tutorial to show how the genes segregate to give the typical pedigree pattern

Professor P Farndon, Clinical Genetics Unit, Birmingham Women’s Hospital

13.11.06

Page 2: X-linked recessive inheritance where the mother is a carrier: the basics a tutorial to show how the genes segregate to give the typical pedigree pattern.

Question:How can one relate an X-linked recessive pedigree pattern to the segregation of genes at meiosis?

I:1John Hodgkin

I:2Celia

II:2Kathleen

II:1Peter Smith

II:3Gerald

III:2Jean

III:3Paul Smith

III:4Kevin Smith

III:1Andrew Cox

IV:1Jason

IV:2Duane

Page 3: X-linked recessive inheritance where the mother is a carrier: the basics a tutorial to show how the genes segregate to give the typical pedigree pattern.

Hemizygotes (males) with one copy of the altered gene are affected

Question:How can one relate an X-linked recessive pedigree pattern to the segregation of genes at meiosis?

Reminder:

Answer:By imagining which of the sex chromosomes of the parents have been passed on to children as shown on the next few screens

X-Chromosome

Gene

Page 4: X-linked recessive inheritance where the mother is a carrier: the basics a tutorial to show how the genes segregate to give the typical pedigree pattern.

Humans have 23 pairs of chromosomes:

22 pairs of autosomes and

1 pair of sex chromosomes

Male karyotype

Female karyotype

Page 5: X-linked recessive inheritance where the mother is a carrier: the basics a tutorial to show how the genes segregate to give the typical pedigree pattern.

The male sex chromosomes: an X and a Y

The female sex chromosomes: two X chromosomes

Page 6: X-linked recessive inheritance where the mother is a carrier: the basics a tutorial to show how the genes segregate to give the typical pedigree pattern.

Parents

X-LINKED INHERITANCE: mother a carrier

Father Mother

CarrierUnaffected

An unaffected male will have an X chromosome with a normal gene and a Y chromosome

A woman who is a carrier of an X-linked recessive disorder has one copy of a normal gene

and one copy of an altered gene of the particular pair

Page 7: X-linked recessive inheritance where the mother is a carrier: the basics a tutorial to show how the genes segregate to give the typical pedigree pattern.

Parents

Gametes

X-LINKED INHERITANCE: mother a carrier

Father Mother

X Y XX

CarrierUnaffected

The father passes on either his X chromosome

or his Y chromosome (and so determines the sex of the fetus)

The mother passes on either the X chromosome containing the normal allele

or the X chromosome containing the altered allele

Page 8: X-linked recessive inheritance where the mother is a carrier: the basics a tutorial to show how the genes segregate to give the typical pedigree pattern.

Parents

Gametes

X-LINKED INHERITANCE: mother a carrier

Father Mother

X Y XX

CarrierUnaffected

There are

four different combinations

of the two chromosomes from each parent

Page 9: X-linked recessive inheritance where the mother is a carrier: the basics a tutorial to show how the genes segregate to give the typical pedigree pattern.

Parents

Gametes

Offspring

X-LINKED INHERITANCE: mother a carrier

Father Mother

X Y XX

Daughter

CarrierUnaffected

This child has inherited the paternal X chromosome (and so is female) and the maternal X chromosome with the normal gene

Page 10: X-linked recessive inheritance where the mother is a carrier: the basics a tutorial to show how the genes segregate to give the typical pedigree pattern.

Parents

Gametes

Offspring

X-LINKED INHERITANCE: mother a carrier

Father Mother

X Y XX

Daughter Daughter

CarrierUnaffected

This child has inherited the paternal X chromosome (and so is female) and the maternal X chromosome with the altered gene

Page 11: X-linked recessive inheritance where the mother is a carrier: the basics a tutorial to show how the genes segregate to give the typical pedigree pattern.

Parents

Gametes

Offspring

X-LINKED INHERITANCE: mother a carrier

Father Mother

X Y XX

Daughter Daughter Son

CarrierUnaffected

Page 12: X-linked recessive inheritance where the mother is a carrier: the basics a tutorial to show how the genes segregate to give the typical pedigree pattern.

Parents

Gametes

Offspring

X-LINKED INHERITANCE: mother a carrier

Father Mother

X Y XX

CarrierUnaffected

This child has inherited the paternal Y chromosome (and so is male) and the maternal X chromosome with the normal gene Son

Page 13: X-linked recessive inheritance where the mother is a carrier: the basics a tutorial to show how the genes segregate to give the typical pedigree pattern.

Parents

Gametes

Offspring

X-LINKED INHERITANCE: mother a carrier

Father Mother

X Y XX

Daughter Daughter Son Son

CarrierUnaffected

Page 14: X-linked recessive inheritance where the mother is a carrier: the basics a tutorial to show how the genes segregate to give the typical pedigree pattern.

Parents

Gametes

Offspring

X-LINKED INHERITANCE: mother a carrier

Father Mother

X Y XX

Son

CarrierUnaffected

This child has inherited the paternal Y chromosome (and so is male) and the maternal X chromosome with the altered gene

Page 15: X-linked recessive inheritance where the mother is a carrier: the basics a tutorial to show how the genes segregate to give the typical pedigree pattern.

Parents

Gametes

Offspring

X-LINKED INHERITANCE: mother a carrier

Father Mother

X Y XX

Daughter Daughter Son Son

CarrierUnaffected

Which children are affected by the disease?

Page 16: X-linked recessive inheritance where the mother is a carrier: the basics a tutorial to show how the genes segregate to give the typical pedigree pattern.

Parents

Gametes

X-LINKED INHERITANCE: mother a carrier

Father Mother

X Y XX

Daughter Daughter Son Son

Unaffected carrier

Affected male

CarrierUnaffected

Unaffected female

Unaffected male

Page 17: X-linked recessive inheritance where the mother is a carrier: the basics a tutorial to show how the genes segregate to give the typical pedigree pattern.

X-LINKED INHERITANCE: mother a carrier

One has to be very careful in explaining these risks to couples where the woman is a carrier for an X-linked

disorder to avoid potential confusion.

If the sex of the fetus is unknown, there is a 1 in 4 chance (25%) that he will be both male and affected.

Page 18: X-linked recessive inheritance where the mother is a carrier: the basics a tutorial to show how the genes segregate to give the typical pedigree pattern.

X-LINKED INHERITANCE: mother a carrier

Parents will often wish to have prenatal diagnosis by fetal sexing for serious X-linked recessive disorders, and so will want to know the sex of the fetus.

“If you have a girl, she has a 1 in 2 chance (50%) of being a carrier “(and usually completely asymptomatic).

“If you have a boy, he has a 1 in 2 chance (50%) of being affected.”

However, we usually explain the risks like this:

Page 19: X-linked recessive inheritance where the mother is a carrier: the basics a tutorial to show how the genes segregate to give the typical pedigree pattern.

Examples of X-linked recessive diseases

Fragile X syndrome 5

Non-specific X-linked mental retardation 5

Duchenne muscular dystrophy 3

Becker muscular dystrophy 0.5

Haemophilia A (factor VIII) 2

Haemophilia B (factor IX) 0.3

UK frequency / 10,000 males

Page 20: X-linked recessive inheritance where the mother is a carrier: the basics a tutorial to show how the genes segregate to give the typical pedigree pattern.

They are important clinically because of

the high risks to other family members.

X-linked recessive conditions are part of the

group of single gene disorders, which also

include autosomal dominant and recessive disorders. X-Chromosome

Gene

Page 21: X-linked recessive inheritance where the mother is a carrier: the basics a tutorial to show how the genes segregate to give the typical pedigree pattern.

The end!• Thank you for completing this revision aid

• We are interested in your comments about this aid. Please email Professor Farndon. ([email protected])

© P Farndon 2001


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