You can compute these for codominant and recessive diseases. Get some sick folks. Look at their kids. Test if observed numbers are consistent with the hypothesis. Binomial p-value is exact.
Normal approximation is just that.
Approximation is hard to justify when we can compute the exact probability. Page 18. introduces Likelihood ratio tests. It's simple to write down the likelihood, maximize it and plot the likelihood ratio. But here the likelihood ratio test is a simple function of the observed proportion and so an exact confidence interval could be computed. Why not do it right.