Question 1
Multiple choiceMedium1 markA signal forms a gradient across four adjacent regions of an early embryo. Gene G is expressed only where the signal concentration exceeds 6 arbitrary units. Cells expressing gene G differentiate into specialized cell type P; cells not expressing it differentiate into another type. Which outcome is predicted? [1 mark]
| Embryo region | Signal concentration / arbitrary units |
|---|---|
| W | 8 |
| X | 7 |
| Y | 5 |
| Z | 3 |
Show answer and explanation
Answer: C. Gene G is expressed only where the signal concentration exceeds 6 arbitrary units. Regions W and X have concentrations of 8 and 7, so their cells express gene G and differentiate into type P. Regions Y and Z are below the threshold and differentiate into another type.
A: considers only the region with the highest concentration, overlooking that X also exceeds the threshold.
B: reverses the effect of the signal by assigning type P to regions below the threshold.
D: assumes all regions have the same gene expression and fate despite their different signal concentrations.
Remember: a gradient can lead to different gene-expression patterns and cell fates in an early embryo.