Genetic Screening and Counselling Higher Human Biology Unit 1

Use of Family Histories A pattern of human inheritance can be revealed by collecting information about a particular characteristic from the members of a family and then using it to construct a family tree (pedigree) There are 3 types of pedigree that you need to know about

Autosomal recessive inheritance The trait is rarely expressed The trait tends to skip generations The trait is expressed in some cousins Males and females are equally affected All sufferers of the trait are homozygous recessive Non-sufferers are homozygous dominant or heterozygous

Autosomal recessive inheritance e.g Cystic Fibrosis

Autonomic Dominant inheritance The trait appears in every generation Each sufferer has an affected parent When a branch of the family does not express the trait it fails to reappear in future generations of that branch Males and females are equally affected All non-sufferers are homozygous Sufferers are homozygous dominant or heterozygous

Autosomal Dominant Inheritance e.g. Huntington’s Chorea

Sex-linked recessive More males are affected than females None of the sons of an affected male show the trait Some grandsons of affected males show the trait All sufferers of the trait are homozygous recessive Non-sufferers are homozygous dominantor or heterozygous carrier females

Sex-linked recessive trait e.g. haemophilia

Pre-natal Screening Two methods of pre-natal screening depend on fetal material being obtained to allow karyotypes to be examined

Amniocentesis Amniocentesis is carried out in the 18 th week of pregnancy It slightly increases risk of miscarriage

Chorionic Villus Sampling Cells are used for karyotyping CVS can be carried out at 8 weeks of pregnancy Causes a higher risk of miscarriage then amniocentesis

Post-natal Screening At present none of the inherited disorders can be successfully treated except for PKU Phenylketonuria results from an inborn error of metabolism If not detected soon after birth the baby suffers from mental retardation All British babies are routinely tested for excess phenylalanine after birth by means of a blood test

Testing for PKU