genetics 2

Question 1

What are the two fundamental laws of mendels work?

Answer 1

The law of segregation which states that genes exist in pairs and alleles which segregate from each other during gamete formation into equal numbers of gametes with the progeny obtaining one determinant from each parent
The law of independent assortment which states that members of one pair of genes segregate independently to members of other pairs

Question 2

What is achondroplasia?

Answer 2

This condition occurs in 1/10000 live births and involves individuals who have short limbs a normal sized head and body and normal intelligence
This is due to mutations in the FGFR3 gene which causes the a receptor which inhibits bone growth to signal in the absence of its ligand

Question 3

What can complicate pedigree patterns?

Answer 3

Variable expressivity
Penetrance
Variation in the age of onset
New mutations with advanced parental age

Question 4

What is variable expressivity?

Answer 4

This is variation in the severity of problems arising from a particular genetic defect
Individuals will however always exhibit some symptoms though manifestation of the phenotype will be different even for individuals with the same genotype

Question 5

What is marfan syndrome?

Answer 5

This occurs in 1/5000 individuals who typically have a tall thin stature with long arms,
long double joined fingers,
short torso, very long legs
curvature of the spine with a breast bone that curves inward or outward
flat feet and hammer toes
extreme near sightedness
unexplained stretch marks on the skin
unseen aspects which are life threatening such as problems with the heart valves and blood vessels
however this disease has a large amount of variable expressivity with not all individuals exhibiting these features

Question 6

What is incomplete penetrance?

Answer 6

Penetrance rate is the probability of a heterozygote presenting with disease where reduced penetrance is often observed as skipping a generation
This may be due to amodifiy genes and is important for determining genetic risk in famalies

Question 7

What are examples of conditions that exhibit different levels of penetrance?

Answer 7

Neurofibromatosis type 1 is a condition which exhibits complete penetrance
A non-penetrant condition is familial breast cancer due to BRAC1

Question 8

What is variation in the age of onset?

Answer 8

Not all genetic conditions are congenital as even though genotype is fixed at conception the phenotype may not manifest until adulthood allowing penetrance to be refferet to in terms of a time scale

Question 9

Why do phenotypes show differences in penetrance and expressivity?

Answer 9

Variable phenotype can eb a result of
Modifier genes
Environmental factors
Allelic variation
Complex genetic and environmental interactions

Question 10

What is the effect of random X-inactivation?

Answer 10

The disease is milder in heterozygous women

Question 11

What is a condition that shows the effect of random X inactivation?

Answer 11

Duchenne mucular dystrophy/Becker muscular dystrophy
This condition affects 1/3500 males and is the most sever and common form of muscular dystrophy where there is progressive wasting of muscles with death often occurring by age 30
The gene for this condition is located on Xp21
This means that the disease is less severe in females leading to a later age of onset and milder symptoms
Female carriers transmit the disorder to half theirs sons

Question 12

What is the gene responsible for Duchenne muscular dystrophy?

Answer 12

This gene encodes for dystrophin which provides strength to muscle cells by linking the internal cytoskeleton to the surface membrane
Without this the cell membrane becomes permeable with internal pressure leading to the cell bursting and dying
Normally this damage can be repaired by stem cells however in muscular dystrophy the damage is so severe that these stem cells become exhausted