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
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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
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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 18th
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