Genetic Screening and Counselling

1. 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

2. Family Tree

3. Remember! • Allele- different forms of a gene • Homozygous- individual possesses 2 identical alleles • Heterozygous- individual possesses 2 different alleles • Genotype- the genetic make up of an individual • Phenotype- expression of a gene through appearance, behaviour etc • Dominant- allele that is expressed in phenotype of a heterozygote • Recessive- allele that is not expressed in phenotype of a heterozygote

4. Different Patterns of Inheritance • Autosomal recessive • Autosomal dominant • Autosomal incomplete dominance • Sex linked recessive trait

5. 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

6. 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)

7. 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

8. Sufferer (heterozygous) In this example, one parent is a sufferer of Huntington’s Chorea, one is a non-sufferer

9. 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

10. 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

11. 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 XhYrarely female XhXh) * Non sufferers are homozygous dominant XHY XHXH) Or heterozygous females are carriers XHXh carrier E.g. haemophilia

13. 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.

14. 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