Autosomal recessive inheritance: the basics

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Presentation transcript:

Autosomal recessive inheritance: the basics 18.02.03

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

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

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

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

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

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

AUTOSOMAL RECESSIVE INHERITANCE Parents Gametes Offspring

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

AUTOSOMAL RECESSIVE INHERITANCE Parents Gametes Offspring

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

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

AUTOSOMAL RECESSIVE INHERITANCE Parents Gametes Unaffected Unaffected Unaffected Affected

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

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

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.

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.

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

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