| Date | May 2017 | Marks available | 2 | Reference code | 17M.2.SL.TZ2.3 |
| Level | Standard level | Paper | Paper 2 | Time zone | Time zone 2 |
| Command term | Outline | Question number | 3 | Adapted from | N/A |
Question
Outline the use of human embryonic stem cells (hESC) to treat Stargardt’s disease.
The most common form of Stargardt’s disease is known to be autosomal recessive. Using a Punnett grid, deduce the probability of a child inheriting Stargardt’s disease, if both of the parents are carriers of the disease but do not have the disease themselves.
Markscheme
a. «an inherited form of» degeneration of retinal layer/photoreceptor cells/blindness
OR
eye genetic disorder
OWTTE
b. «hESC/stem cells» can provide/differentiate into healthy retinal cells
c. injecting «hESC/stem cells» into the retina/eye can restore vision in animal/human trials
a. correct allele identification
«eg: S=dominant/normal; s=recessive/disease»
b. correct Punnett grid
c. correct phenotypic ratio/outcome
example:
s =recessive, disease-causing form of gene,
S =dominant,
normal form parents =Ss.
Any letter can be used as capital and lower case but a legend/key is not required if correct notation is used.
Do not award mp b if the gametes do not show heterozygous organisms
phenotypic ratio:
normal : with disease
OR
3 normal : 1 with disease
OR
«75 % normal :» 25 % disease
