Genetics & Molecular Biology

Some Genetic Diseases

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Turner Syndrome

45 X
normal intelligence
menstrual failure, low estrogen, high gonadotrophins
Short Stature

Klienfelter Syndrome

47 XXX
low intelligence, tall stature,
Normal Fertility

Down's Synd

Trisomy 21, Autosomal dominant
Meiotic non-dysjunction during osteogenesis
ASVD commonest cardiac abnormality

Edward's Syndrome

Trisomy 18, AD

Patau Syndrome

Trisomy 13

X-linked dominant disorders

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Vitamin D-resistant rickets

The affect both sexes but females more than males.
All children of a homozygous mother are affected.
Half the sons and half the daughters inherit the disorder
from an affected mother with the trait.

An affected father passes the disease to all his daughters
but none of his sons

Gene associated with Diseases

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Beta-myosin :dilated cardiomyopathy
CFTR :cystic fibrosis
FGFR :skeletal dysplasias
Fibrillin :Marfan syndrome
Spectrin : Hereditory Spherocytosis

Bacteria resistance to antibiotics

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gene usually found in plasmids

Protein synthesis

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Two phases

1)Transcription is where one strand of the DNA 
double helix is used as a template by RNA polymerase to 
synthesize messenger RNA from RNA nucleotides. 

2)The mRNA then migrates into the cytoplasm maturing -
splicing of non-coding sequences. 
Translation occurs when the ribosome binds to mRNA and 
transfer RNA brings amino acids into position along the mRNA template.
The ribosome moves from codon to codon along the mRNA producing a 
polypeptide sequence.

Hemocystinuria

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mental retardation. 
Fair skin with coarse hair, 
osteoporosis,
seizure disorder, 
marfanoid habitus, 
thromboembolic risk.

Ectopia lentis

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subluxation of the lens 
Ehlers Danlos syndrome, 
Marfan’s syndrome, 
Weil-Marchesani syndrome 
Refsum’s disease
homocystinuria

Polygenic Disorder

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Amyotrophic lateral sclerosis
Ankylosing Spondylosis

Anticipation

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means increased severity/earlier age of onset of disease 
with successive generations

myotonia dystrophica
spinocerebellar ataxia type 1
Huntingdon's disease
Fragile X syndrome

G6PD Deficiency

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X-linked recessive
African, Mediterranean, Iraqi Jew, South East Asian and Chinese
predisposes to a haemolytic anaemia reaction with drugs or infection. 
aspirin, sulphonamides, antimalarials, and quinidine. 
The haemolytic anaemia is non-immune (DAT -ve). 

Pyruvate Kinase Deficiency is autosomal recessive 
chronic haemolytic anaemia exacerbated by viral infections.

X-linked recessive disorders

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red/green colour blindness,
Duchenne and Becker muscular dystrophies,
Fragile X Syndrome,
G6PD deficiency,
haemophillias A
Hunter's Syndrome.

The abnormal gene is carried on he X chromosome,
and in the carrier female, the normal allele on her other
X chromosome protects her from the disease.
Since the male does not have this protection,
he manifests the disease.

In X-linked inheritance therefore:

Males are all affected.

Females only occasionally show mild sign of disease.

Each son of a female carrier has a 1:2 chance of being affected.

Each daughter of a female carrier has a 1:2 risk of being a carrier.

Daughters of affected males will all be carriers,

but sons of affected males will not be affected since the Y chromosome is derived from father.

Dystrophia myotonica

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autosomal dominant condition with variable penetrance.  
weakness and myotonia. 
Cataracts, Ptosis, Frontal baldness, gynaecomastia, diabetes, 
reduced reflexes with myotonia are features. 
Progressive external ophthalmoplegia is a feature of Ocular muscular dystrophy