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Genetic Screening and Counselling
• Pedigree charts (family tree) are used to
analyse patterns of inheritance in genetic
screening
• Once the phenotype for a characteristic is
known and a family tree is constructed most
of the genotypes can be determined
• This information is used by genetic
counsellors to advise parents of the
possibility of passing on a genetic condition to
their child
Family Tree
Remember!
• Allele-
different forms of a gene
• Homozygous- individual possesses 2 identical alleles
• Heterozygous- individual possesses 2 different alleles
• Genotype• Phenotype-
the genetic make up of an individual
expression of a gene through
appearance, behaviour etc
• Dominant-
allele that is expressed in phenotype of
a heterozygote
allele that is not expressed in
phenotype of a heterozygote
• Recessive-
Different Patterns of
Inheritance
•
•
•
•
Autosomal recessive
Autosomal dominant
Autosomal incomplete dominance
Sex linked recessive trait
Autosomal Recessive Inheritance
• Trait :
– expressed relatively rarely
– May skip generations
– Males and females equally
affected
* All sufferers homozygous
recessive e.g. cc
* Non-sufferers homozygous
dominant or heterozygous e.g. CC
E.g. cystic fibrosis
Sufferer (Homozygous recessive)
In this example, neither of the parents are sufferers
of Cystic Fibrosis, but they are both carriers of the
disease (carried on the recessive gene)
Autosomal Dominant Inheritance
• 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 affected
equally
* All sufferers homozygous
dominant (HH) or heterozygous
(Hh)
* All non sufferers homozygous
recessive (hh)
E.g. Huntington’s
disease
Sufferer (heterozygous)
In this example, one parent is a sufferer of
Huntington’s Chorea, one is a non-sufferer
Autosomal Incomplete Dominance
• A condition where individuals can suffer 2 forms of a
disease
• The fully expressed form of the disorder occurs
relatively rarely
• Partially expressed form occurs more frequently
• Males and females equally affected
• Each sufferer of fully expressed form has 2 parents
who suffer the partly expressed form
• This occurs where a mutation may occur which
produces a protein very much like the desired protein,
e.g. Haemoglobin S being produced instead of
Haemoglobin
E.g. sickle cell
anaemia
e.g. Sickle Cell Anaemia
* Sufferers of the fully expressed form of the
disorder are homozygous for the other incompletely
dominant allele e.g. SS (haemoglobin S)
* Sufferers of the partly expressed form are
heterozygous for the two alleles e.g. HS (where the
normal allele does not completely mask the effects of
the abnormal allele)
* Non sufferers homozygous for one incompletely
dominant allele e.g. HH
Sex Linked Recessive
• Many more males are
affected than females
• None of the sons of an
affected male show the trait
• Some grandsons of an
affected male show the trait
*Sufferers of the trait are
homozygous recessive (normally
male XhY rarely female XhXh)
* Non sufferers are
homozygous dominant XHY
XHXH)
Or heterozygous females are
carriers XHXh
carrier
E.g. haemophilia
Pre-implantation genetic
diagnosis (PGD).
• Pre-implantation genetic diagnosis (PGD)
is a technique that enables people with a
specific inherited condition in their
family to avoid passing it on to their
children.
• It involves checking the genes of
embryos created through IVF for this
genetic condition.
PGD
• Step 1. Normal in vitro fertilisation (IVF) treatment
to collect and fertilise your eggs.
• Step 2. The embryo is grown in the laboratory for
two - three days until the cells have divided and the
embryo consists of around eight cells.
• Step 3. An embryologist removes one or two of the
cells from the embryo.
• Step 4. The cells are tested to see if the embryo
from which they were removed contains the gene that
causes the genetic condition in the family.
• Step 5. Embryos unaffected by the condition are
transferred to the womb to allow them to develop