What’s this one… relative probabilities

For the recessive disorders, we’d require the unaffected mother to be a carrier AND the mother on the right-hand side to also be a carrier. This is possible and, even for rare disorders, happens. To consider probabilities, we’d have to think about the background carrier frequency in our calculations. Note that where geography, culture, religion, tradition etc., lead to a smaller effective population, that carrier frequency could be higher than the global rate.

However, for Y-linked we don’t need to think about carrier frequency:

For Y-linked, any males who has inherited an Y chromosome from an affected male have the disease, while females are never affected (which fits here; male-to-male inheritance, with all females unaffected).

Let’s move on