The Genetic Transmission OF Diseases

One of the ways for the transmission of diseases is the obvious way, the one that follows the mandelian rule of inheritance..

* Mandelian rule:

according to Mandelian theory, the characters which pass to the next generation are either dominant or recessive. The dominant means only one allele is suffecient to produce the paternal/maternal character on the offsbring. The recessive means both alleles should be of same value to produce the character. So, by applying this rule, the inherited diseases are either dominant or recessive. putting in mind that the human cell is composed of 23 pairs of chromosomes = 46 chromosomes. Of which, 22 pairs are autosomals = somatic, while the remaining pair is the XY pair.

So, now we classify the inheritance pattern into 5 groups:

a- Autosomal Dominant:

either parent can transmit the disease to 50% offsbrings. involve all generations. No carrier state.

examples:

achondroplasia

Marfan syndrome

Von Willbrand disease

congenital spherocytosis
tuberous sclerosis

osteogenesis imperfecta

polycystic kidney disease.

b- Autosomal Recessive:

both parents are carriers. 25% normal, 25% diseased, 50% carrier offsbrings. If a carrier marries a nomal person = all the offsbrings are either normal or carriers. So, offcourse, consanguinity increases the risk.

examples:

SCD = Sickle Cell Disease

Thalassemias

CF = Cystic Fibrosis

Werdnig Hoffman Disease = Spinal Muscular Atrophy (SMA)

Friedrich’s ataxia

Cngenital Adrenal Hyperplasia (CAH)

galactosemia

phenylketonuria

wilson’s disease = excess cubber

glycogen storage disease

gaucher’s disease

c- X-linked Dominant

in female twice common than in males. 50% risk for both female/male offsbring of heterozygote female. All daughters of a diseased father are affected.

examples:

Vitamin D resistant rickets

pseudo-hypo-para-thyroidism

x-linked hypo-phosphatemic rickets

d- X-linked Recessive

Males are affected, females are carriers. Male’s daughters (XX) 100% carriers. Male’s sons (XY) 100%normal. Carrier female’s daughters (XX) 100% carriers. Carrier female’s sons (XY) 50% normal, 50% diseased.

NO MALE TO MALE TRANSMISSION.

 

examples:

Douchene muscular dystrophy

Haemophilia A (factor VIII) & B (factor IX)

Nephrogenic diabetes insipidus

G6-PD (glucose 6-phosphate dehydrogenase) deficiency

colour blindness

e- Y-linked

MALE TO MALE TRANSMISSION ONLY.

Most Y-linked genes are related to sex determination (SRY region) and reproduction, So most of these diseases are associated with infertility. Familial transmission are rare.