Autosomal dominant inheritance Risks to children where both parents are affected: the basics a...

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Autosomal dominant inheritance Risks to children where both parents are affected: 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: Autosomal dominant inheritance Risks to children where both parents are affected: the basics a tutorial to show how the genes segregate to give the typical.

Autosomal dominant inheritance

Risks to children where both parents are affected: 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: Autosomal dominant inheritance Risks to children where both parents are affected: the basics a tutorial to show how the genes segregate to give the typical.

Question:What are the risks to children if both parents are affected by the same autosomal dominant condition?

Page 3: Autosomal dominant inheritance Risks to children where both parents are affected: the basics a tutorial to show how the genes segregate to give the typical.

Heterozygotes with one copy of the altered gene are affected

Question:What are the risks to children if both parents are affected by the same autosomal dominant condition?

Reminder:

Answer:By determining the different combinations of the genes of the parents which can be passed on to children as shown on the next few screens

Chromosome

Gene

Page 4: Autosomal dominant inheritance Risks to children where both parents are affected: the basics a tutorial to show how the genes segregate to give the typical.

Parents

AUTOSOMAL DOMINANT: Both parents affected

Parents have two copies of autosomal genes:one copy on each of a particular pair of chromosomes

Page 5: Autosomal dominant inheritance Risks to children where both parents are affected: the basics a tutorial to show how the genes segregate to give the typical.

Parents

AUTOSOMAL DOMINANT: Both parents affected

A parent with an autosomal dominant disorder has one copy of an altered gene and one normal gene of the particular pair

It is very rare that the other parent has the same autosomal dominant disease - (but can occur - for example with achondroplasia or hypercholesterolaemia)

Page 6: Autosomal dominant inheritance Risks to children where both parents are affected: the basics a tutorial to show how the genes segregate to give the typical.

Parents

Gametes

AUTOSOMAL DOMINANT: Both parents affected

The affected parent passes on either the altered gene

or the normal gene into the eggs or sperm

This affected parent passes on either the altered or the normal gene into the eggs or sperm

Page 7: Autosomal dominant inheritance Risks to children where both parents are affected: the basics a tutorial to show how the genes segregate to give the typical.

Parents

Gametes

AUTOSOMAL DOMINANT: Both parents affected

There are

four different combinations

of the two genes from each parent

Page 8: Autosomal dominant inheritance Risks to children where both parents are affected: the basics a tutorial to show how the genes segregate to give the typical.

Parents

Gametes

Offspring

AUTOSOMAL DOMINANT: Both parents affected

This child has inherited the altered gene from the first affected parent and the altered gene from the other affected parent

Page 9: Autosomal dominant inheritance Risks to children where both parents are affected: the basics a tutorial to show how the genes segregate to give the typical.

Parents

Gametes

Offspring

AUTOSOMAL DOMINANT: Both parents affected

This child has inherited the altered gene from the affected parent but the normal gene from the other affected parent

Page 10: Autosomal dominant inheritance Risks to children where both parents are affected: the basics a tutorial to show how the genes segregate to give the typical.

Parents

Gametes

Offspring

AUTOSOMAL DOMINANT: Both parents affected

Page 11: Autosomal dominant inheritance Risks to children where both parents are affected: the basics a tutorial to show how the genes segregate to give the typical.

Parents

Gametes

Offspring

AUTOSOMAL DOMINANT: Both parents affected

This child has inherited the normal gene from the

first parent and the altered gene from the

second affected parent

Page 12: Autosomal dominant inheritance Risks to children where both parents are affected: the basics a tutorial to show how the genes segregate to give the typical.

Parents

Gametes

Offspring

AUTOSOMAL DOMINANT: Both parents affected

Page 13: Autosomal dominant inheritance Risks to children where both parents are affected: the basics a tutorial to show how the genes segregate to give the typical.

Parents

Gametes

Offspring

AUTOSOMAL DOMINANT: Both parents affected

This child has inherited the normal gene from the

first affected parent and the normal gene from

the second affected parent

Page 14: Autosomal dominant inheritance Risks to children where both parents are affected: the basics a tutorial to show how the genes segregate to give the typical.

Parents

Gametes

Offspring

AUTOSOMAL DOMINANT: Both parents affected

Which children are affected by the disease?

Page 15: Autosomal dominant inheritance Risks to children where both parents are affected: the basics a tutorial to show how the genes segregate to give the typical.

Parents

Gametes

AUTOSOMAL DOMINANT: Both parents affected

Affected Affected Unaffected

Affected

Page 16: Autosomal dominant inheritance Risks to children where both parents are affected: the basics a tutorial to show how the genes segregate to give the typical.

AUTOSOMAL DOMINANT: Both parents affected

Each child of a couple who both have an autosomal dominant disorder therefore has a

1/2 chance of being a heterozygote for the disease

1/4 chance of being homozygous for the disease gene

1/4 chance of being homozygous normal

Page 17: Autosomal dominant inheritance Risks to children where both parents are affected: the basics a tutorial to show how the genes segregate to give the typical.

The clinical effects of homozygosity for an autosomal dominant disorder

Most autosomal dominant disorders occur only in the heterozygous state because the disorder is relatively rare in the population.

For some conditions, being homozygous for the dominant disorder causes the individual to be more severely affected (as in achondroplasia) or for the disorder to have an earlier age of onset (eg familial hypercholesterolaemia).

In other disorders, for instance Huntington disease, homozygotes appear to have an indistinguishable clinical course from patients who are heterozygous for the mutant gene.

The difference may depend on how the nature of the mutation affects the gene product. For instance, conditions where the heterozygote has clinical features intermediate between homozygous normal and homozygous affected seem to be associated with alleles which stop a gene product from being produced. The heterozygotes are postulated to have half the normal concentration of the gene product whilst homozygous affected individuals make none.

Page 18: Autosomal dominant inheritance Risks to children where both parents are affected: the basics a tutorial to show how the genes segregate to give the typical.

Occasionally, each parent can be affected by a different dominant disorder.

What are the risks to their children?

Page 19: Autosomal dominant inheritance Risks to children where both parents are affected: the basics a tutorial to show how the genes segregate to give the typical.

Parents

Gametes

AUTOSOMAL DOMINANT: both parents affected; genes at different loci

UnaffectedAffected by first disorder

Affected by both disorders

Affected by second disorder

Page 20: Autosomal dominant inheritance Risks to children where both parents are affected: the basics a tutorial to show how the genes segregate to give the typical.

They are important clinically because of

the high risks to other family members.

Autosomal dominant conditions are part of the

group of single gene disorders, which also

include autosomal recessive, and X-linked

conditions.

Chromosome

Gene

Page 21: Autosomal dominant inheritance Risks to children where both parents are affected: the basics a tutorial to show how the genes segregate to give the typical.

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 2003


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