Mendelian Inheritance and Human Diseases
- Thousands of simple Mendelian diseases (caused by mutations in one gene) have been
described in humans.
- Individually, these diseases are not frequent, normally affecting less than 1/1,000 births.
However, their global impact is huge.
- The role of genetic counselors is to give advice to individuals suffering from these
diseases and to their families.
- Transmission of diseases in families is usually depicted using pedigrees.
- Carrier only makes sense when you are referring to a recessive allele
Mendelian Diseases: Dominant Trait
- Dominant traits are which a heterozygous (with both defective and normal allele), the
individual will show the disease
- If this person with a disease mates with a normal female, he can still pass the defective
allele 50% of the time
- 50% will be homozygote for the normal allele
Autosomal Dominant Inheritance
- Usually one affected parent
o By definition these diseases are rare and infrequent
- Homozygotes are rare
- Affects either sex
o Because this is autosomal, you can pass to either female or to male offspring
- Transmitted by either sex
Some Major Autosomal Dominant Disorders - Familial hypercholesterolemia
- Familial breast cancer
o The most common form is unable to follow through families
o Familial breast cancer is where you can see in pedigree where it is passed on
from generation to generation
o BRCA1 and BRCA2 having this gene, you will have a higher risk of breast
- Familial Alzheimer’s disease
- Neurofibromatosis type 1
- Huntington’s disease
Mendelian Diseases: Recessive Trait
- Recessive disease – if you are heterozygote, you will be a carrier and phenotypically
- The individual will be a carrier, and able of passing on the affected allele to offspring
Autosomal Recessive Inheritance
- Parents usually unaffected
- Parents usually carriers. Carriers may be common
- There is a 25% chance that the offspring of two carriers will be affected
- More frequent in consanguineous matings and genetic isolates.
- Affects either sex
- Affected are always homozygous
Some Major Autosomal Recessive Disorders
- Cystic fibrosis
- Tay-Sachs disease