A protein, called pRb (protein for retinoblastoma) is synthesized by a gene (Rb) on chromosome 13 in humans. When mutant or absent in the homozygous state in a retinal cell, there is a high likelihood of developing a cancerous condition of the eye called retinoblastoma. Nonmutant pRb binds with and temporarily inactivates E2F, a transcription factor that stimulates genes to produce proteins that initiate S phase of the mitotic cell cycle (thus, Rb is a negative regulator of cell division). When phosphorylated in response to a variety of normal stimuli, pRb normally releases E2F and cell division is stimulated. Thus, Rb is a tumor suppressor gene. (For a recent review, see H. Liu et al. (2004), \"Novel functions of the RB tumor suppressor.\" Current Opinions in Genetics and Development 14: 55–64. When retinoblastoma occurs in families, the disease tends to have an early onset and express bilaterally (both eyes). When sporadic (spontaneous, not inherited), it tends to show later onset and be unilateral (one eye only). Individuals who inherit a predisposition for the disease are Rb+/Rb– and may get retinoblastoma by having a mutation in the Rb+ allele.Assume that two heterozygous parents have children. One would expect the Rb–/Rb– genotypic combination in a zygote (one-cell stage of an individual) to be characterized as: a.lethal (inviable or lasting a relatively short time) b.completely normal c.having a very low likelihood of developing retinoblastoma d.having unilateral retinoblastoma only Solution A. the Rb-/Rb- genotype B. the Rb+/Rb- genotype C. the Rb+/Rb+ genotype So answer is A.

1. A protein, called pRb (protein for retinoblastoma) is synthesized by a gene (Rb) on chromosome
13 in humans. When mutant or absent in the homozygous state in a retinal cell, there is a high
likelihood of developing a cancerous condition of the eye called retinoblastoma. Nonmutant pRb
binds with and temporarily inactivates E2F, a transcription factor that stimulates genes to
produce proteins that initiate S phase of the mitotic cell cycle (thus, Rb is a negative regulator of
cell division). When phosphorylated in response to a variety of normal stimuli, pRb normally
releases E2F and cell division is stimulated. Thus, Rb is a tumor suppressor gene. (For a recent
review, see H. Liu et al. (2004), "Novel functions of the RB tumor suppressor." Current
Opinions in Genetics and Development 14: 55–64. When retinoblastoma occurs in families, the
disease tends to have an early onset and express bilaterally (both eyes). When sporadic
(spontaneous, not inherited), it tends to show later onset and be unilateral (one eye only).
Individuals who inherit a predisposition for the disease are Rb+/Rb– and may get retinoblastoma
by having a mutation in the Rb+ allele.Assume that two heterozygous parents have children. One
would expect the Rb–/Rb– genotypic combination in a zygote (one-cell stage of an individual) to
be characterized as:
a.lethal (inviable or lasting a relatively short time)
b.completely normal
c.having a very low likelihood of developing retinoblastoma
d.having unilateral retinoblastoma only
Solution
A. the Rb-/Rb- genotype
B. the Rb+/Rb- genotype
C. the Rb+/Rb+ genotype
So answer is A