Genetic Disorders: Causes

Mutations

– any change in a gene that is accompanied by a loss or change in the functioning of the genetic information; many mutations are harmful
– spontaneous mutations have no known cause; improper crossing over, DNA replication
– induced mutations are caused by mutagens; radiation, chemicals, viruses

Somatic Mutations

– occur in regular body cells and usually go unnoticed
– are not passed on to future generations
– not harmful; unless involves a large number of cells, for example cancer

Gamete Mutations

– occur in reproductive cells (sperm, eggs)
– mutation can be passed on to produce an entire organism with this mutation in every cell
– mutations can be passed on to offspring and continue to be passed on for many generations

Single Gene Defects

– disorders caused by a mutation in a single gene
– red/green colour blindness, hemophilia, Duschenne muscular dystrophy

Sickle Cell Anemia

– autosomal recessive trait
– red blood cells take on a “sickle” shape instead of normal disc shape; abnormal hemoglobin
– leads to blocked capillaries, damage to other red blood cells and severe anemia
– occurs most commonly in people from areas where malaria is common; people with one or two alleles of the sickle-cell disease are resistant to malaria since the sickle red blood cells are not conducive to the parasites – in areas where malaria is common there is a survival value in carrying the sickle-cell genes.

Cystic Fibrosis

– autosomal recessive trait
– mucus builds up in the lungs and makes breathing very difficult and have problems with digestion
– most common among Europeans and Ashkenazi Jews

Tay Sachs Disease

– autosomal recessive
– severe brain deterioration due to improper fat metabolism
– usually causes death by age three or four
– most common among Ashkenazi Jews

Achondroplasia

– autosomal dominant
– abnormal bone growth; individuals have short stature with an average height 4 feet

Huntington disease

– autosomal dominant
– brain tissue degeneration that begins around an individuals thirties or forties

Chromosomal Abnormalities

– having extra or missing whole chromosomes

Aneuploidy

– having an extra chromosome or missing a single chromosome
– an individual having an extra chromosome (three of one kind) is said to be trisomic
– an individual missing one or a pair of chromosomes is said to be monsomic
– nondisjunction is the cause of these types of abnormalities

Pedigrees in Human Genetics

A pedigree chart is useful in showing the patterns of transmission of a trait within a given family.

Uses:

– to identify the way a trait is transmitted
– counselling parents on the cause of a disease
– predicting the probability of having an affected child in subsequent pregnancies

Symbols and Conventions:

– symbols represent people and lines to represent genetic relationships

• · squares represent males and circles represent females
• · horizontal lines connecting a male and female represent mating
• · vertical lines extending downward from a couple represent their children
• · subsequent generations are therefore written underneath the parental generations and the oldest individuals are found at the top of the pedigree.
• · in analyzing the pattern of inheritance of a particular trait, it is customary to shade in the symbol of all individuals that possess this trait.

Most common patterns of inheritance autosomal dominant, autosomal recessive, X-lined recessive