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Transcript
Autosomal recessive inheritance: 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 18.02.03 Question: How can one relate an autosomal recessive pedigree pattern to the segregation of genes at meiosis? I:1 Patrick McKee II:1 Julia II:2 Edward II:3 Zoe II:4 Elizabeth I:2 Mary II:5 Charles II:6 William II:7 Lucy II:8 Laura Question: How can one relate an autosomal recessive pedigree pattern to the segregation of genes at meiosis? Answer: By imagining which of the two genes of the parents have been passed on to children as shown on the next few screens Gene Reminder: Homozygotes with two copies of the altered gene are affected Chromosome 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 A carrier parent passes on either the normal gene or the altered gene into the eggs or sperm The other carrier parent passes on either the normal gene or the altered gene into his/her 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 AUTOSOMAL RECESSIVE INHERITANCE Parents Gametes Offspring Which children are affected by the disease? AUTOSOMAL RECESSIVE INHERITANCE Parents Gametes Unaffected Unaffected Unaffected Affected 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 I:1 II:1 John Hobson But … what is the risk to a healthy sib of being a carrier for the recessive disease? III:1 Mervin Hobson II:2 Christine Hobson III:2 Alice Hobson III:3 Richard Hobson I:2 II:3 Christopher Whitehead II:4 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 sib. The three genotype possibilities for a healthy sib are: A normal homozygote A healthy carrier (heterozygote) A healthy carrier (heterozygote) A healthy sib of a person with an autosomal recessive disorder therefore has a 2/3 chance of being a carrier. Examples of Autosomal Recessive disorders Disease Freq/1000 births Cystic fibrosis 0.5 Recessive mental retardation 0.5 Congenital deafness 0.2 Phenylketonuria (PKU) 0.1 Spinal muscular atrophy 0.1 Recessive blindness 0.1 Overall freq of recessive disease ~ 2/1000 Autosomal recessive conditions are part of the group of single gene disorders, which also include autosomal dominant and X-linked disorders. They are important clinically because of the high risks to other family members. 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
