Inherited Diseases

A pedigree showing haemophilia in Queen Victoria’s family

Pedigree of a family with cystic fibrosis (affected individuals shown in purple)

A small part of a pedigree of a family with Huntington’s disease

Another pedigree of a family with Huntington’s disease

The patterns of inheritance These three diagrams show the typical pattern of inherited diseases. The disease caused by a dominant allele is present in every generation, unless it is not passed on at all. The disease caused by a recessive allele is not common. The sex-linked disease tends to miss a generation due to the presence of a carrier in between.

Using DNA Evidence DNA profiles are used by criminal investigators to: Prove guilt - Matching DNA profiles can link a suspect to a crime or crime scene. Exonerate an innocent person - At least 10 innocent people have been freed from death row in the United States after DNA evidence from their cases was studied. DNA evidence is also useful for: Paternity testing and other cases where authorities need to prove whether or not individuals are related Identification of a body or skeletal remains. Samples are extracted from hair or bone tissue; once a DNA profile has been created, it can be compared to samples from families of missing persons. Studying the evolution of human populations - Samples extracted from skeletons and from living people around the world can show how early human populations might have migrated across the globe and diversified into so many different races. Studying inherited disorders - Scientist also study the DNA fingerprints of families with members who have inherited diseases to try and ferret out chromosomal differences between those without the disease and who are have it, in the hopes that these changes might be linked to getting the disease.

DNA Profiling In the image on the next slide, DNA from suspects 1 and 2 are compared to DNA extracted from semen evidence. You can see in this sample that suspect 1 and the sperm DNA found at the scene match. Suspect 2 has a profile totally different from the semen sample; his DNA fragments have run much farther down the gel, meaning that they are shorter. You can also tell that he is a homozygote because there is only one darker band indicating the presence of two copies of the same fragment. The other samples tested come from heterozygotes, because they have two bands of distinct sizes in each lane. DNA isolated from the victim as well as a human DNA (K562) that serves as a standard size reference are included as controls.

Paternity testing The diagrams on the next slide show two cases of disputed paternity. In the case on the left, the alleged father was not the biological father, whereas in the case on the right the man was. You can see that one of the bands from the man on the right matches that of the child (as does one of the mother’s bands, obviously).

Inherited Breast Cancer

The faulty allele (B) can be inherited from mother or father with a 50% chance of getting it

Diagnosis and treatment of inherited breast cancer A pedigree or family tree is drawn up indicating related women with early onset (<45 years) breast cancer and any men with it, too. Women with a high occurrence of this type of breast cancer in their family have a blood test and their DNA is analysed to see if they have the faulty allele. A positive result puts their risk of developing breast cancer up from 1/2525 in the general population to 1/3. If they have the faulty allele, they are offered increased monitoring of their breasts, anti-oestrogen drugs such as Tamoxifen if they are post menopausal and even double mastectomy with reconstructive surgery if desired to prevent cancer.

If cancer does develop, lumpectomy or mastectomy is carried out, followed by radiation treatment or chemotherapy. A cancer that has spread is likely to be fatal eventually.