1. Autosomal recessive inheritance: the basics

2. AUTOSOMAL RECESSIVE INHERITANCE
Parents
Parents have two copies of autosomal genes: one copy on each of a particular pair of chromosomes

3. AUTOSOMAL RECESSIVE INHERITANCE
Parents
A parent who is a carrier of an autosomal recessive disorder has one copy of a normal gene and one copy of an altered gene of the particular pair

4. AUTOSOMAL RECESSIVE INHERITANCE
Parents
Gametes
The other carrier parent passes on either the normal gene or the altered gene into his/her eggs or sperm
A carrier parent passes on either the normal gene
or the altered gene into the eggs or sperm

5. AUTOSOMAL RECESSIVE INHERITANCE
Parents
Gametes
There are
four different combinations
of the two genes from each parent

6. AUTOSOMAL RECESSIVE INHERITANCE
Parents
Gametes
Offspring
This child has inherited two copies of the normal gene - one from each parent

7. AUTOSOMAL RECESSIVE INHERITANCE
Parents
Gametes
Offspring
This child has inherited the normal gene from one parent but the altered gene from the other parent

8. AUTOSOMAL RECESSIVE INHERITANCE
Parents
Gametes
Offspring

9. AUTOSOMAL RECESSIVE INHERITANCE
Parents
Gametes
Offspring
This child has inherited the altered gene from the first parent and the normal gene from the second parent

10. AUTOSOMAL RECESSIVE INHERITANCE
Parents
Gametes
Offspring

11. AUTOSOMAL RECESSIVE INHERITANCE
Parents
Gametes
Offspring
This child has inherited the altered gene from each parent

12. Which children are affected by the disease?
AUTOSOMAL RECESSIVE INHERITANCE
Parents
Gametes
Which children are affected by the disease?
Offspring

13. AUTOSOMAL RECESSIVE INHERITANCE
Parents
Gametes
Unaffected
Unaffected
Unaffected
Affected

14. 1/2 (50%) chance of being a carrier but unaffected
AUTOSOMAL RECESSIVE INHERITANCE
At conception,
Each child of two parents who are carriers for the same autosomal recessive disorder therefore has a
1/4 (25%) chance of neither being affected nor a carrier of the disease
1/2 (50%) chance of being a carrier but unaffected
1/4 (25%) chance of inheriting the disease

15. But …
what is the risk to a healthy sibling of being a carrier for the recessive disease?

16. Once a child has been born and we know he or she is not affected by the disease (ie not a disease homozygote) this removes one of the four possibilities.

17. Once a child has been born and we know he or she is not affected by the disease (ie not a disease homozygote) this removes one of the four possibilities
leaving three genotype possibilities for a healthy sibling.

18. The three genotype possibilities for a healthy sibling are:
A normal homozygote
A healthy carrier (heterozygote)
A healthy carrier (heterozygote)

19. A healthy sibling of a person with an autosomal recessive disorder therefore has a 2/3 chance of being a carrier.