Sex-linked traits (like the red-green color-blindness gene on the X chromosome) felt tricky at first. I drew male (XY) and female (XX) karyotypes and labeled the X-linked allele (Xᴺ = normal, Xⁿ = color-blind). Crossing a carrier mother (XᴺXⁿ) with a normal father (XᴺY) and mapping gametes in a Punnett square showed why sons can express the trait while daughters often remain carriers.
I then looked at real pedigree charts with circles and squares shaded to show affected individuals. Annotating those charts by hand—drawing arrows for inheritance paths—helped me predict risk in future generations.
Sample SPM Question
“A woman carrier for hemophilia (XᴴXʰ) marries a normal man (XᴴY). Construct a Punnett square to show the expected genotypic and phenotypic ratios of their children.”
My method:
- Gametes: Xᴴ, Xʰ for mother; Xᴴ, Y for father.
- Fill 2×2 square: XᴴXᴴ, XᴴY, XʰXᴴ, XʰY.
- State ratios: 1 carrier daughter : 1 normal son : 1 hemophiliac son : 1 normal daughter.
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