What is the retinoblastoma gene?

Answer
The retinoblastoma gene is a tumour suppressor gene termed Rb. 
If defective or absence can promote the development of retinoblastoma. It has been localized to the long arm of chromosome 13.

Is retinoblastoma a recessive or a dominant condition?

Answer
The mutant (either defective or absent) gene is inherited as an autosomal dominant trait but at the molecular level, it behaves in a recessive fashion. 
 

What is Knudson's two hit hypothesis?

Answer
In a child with the inherited form of retinoblastoma, there is a germline 
mutation (either defective or absent)  in one allele of the Rb gene 
located in the long arm of chromosome 13. A second somatic mutation 
in the retina leads ot inactivation of functioning Rb allele and subsequently 
development of retinoblastoma. In sporadic cases, the child is born with
two normal Rb alleles. It requires two independent somatic mutation to
inactive the Rb gene function and a appearance of a neoplastic clone. 
This two-hits hypothesis explains why retinoblastoma is recessive at the molecular level ie. it requires two abnormal alleles.
 

The above is the CT scan of a chile with retinoblastoma.
What is the risk of the parents having another child with retinoblastoma if:

a. Both parents are normal

Answer
Because the tumour in this child is bilateral. The risk of another baby with retinoblastoma is 6%. 
If the tumour is unilateral and there is no family history the risk of another baby with retinoblastoma is 1%.

b. One parent has retinoblastoma?

AnswerThe risk is 40%.

What is the risk of this patient having an offspring with retinoblastoma?

Answer
Because the tumour is bilateral the condition is likely to be caused
by a germline mutation rather than somatic mutation. The risk to the offspring is 40%.

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