It is about the probabilities of Hemophilia (or Haemophilia, or Hæmophilia) inheritance. For those who don't know, Hemophilia is an X-linked recessive genetic mutation; Females (XX) who have one defective X-chromosome will be carriers (and have a 50% chance of passing the chromosome to their children, sons or daughters), Females (XX) who have two defective X-chromosomes will have hemophilia (and have a 100% chance of passing the chromosome to their children, but I don't know if this has ever been recorded), and Males (XY) who have their one X-chromosome defective will have hemophilia (and will automatically pass the disease to their daughters as carriers, but never to their sons.)
Queen Victoria was a carrier for hemophilia. That means that one of her two X-chromosomes was affected, and presumably her husband Albert's X chromosome was unaffected, because then he would had suffered from the disease. She had 9 children. 1 of her 4 sons was affected (Leopold, who had hemophilia and passed it to his daughter), and at least 2 of her 5 daughters (Alice and Beatrice) were carriers (known because some of their sons and their daughters' sons had hemophilia.)
Let's take the case of one of her other daughters, Helena. Based on her ancestry, we can say that there was a 50% chance she was a carrier, right?
But hang on. Helena married and had children - three sons and two daughters. As far as I can tell, It was never recorded that any of her sons suffered from a bleeding disorder like hemophilia (even though one of them died at one week old), so I will assume they each had a normal X chromosome. Helena's two daughters never had children, so we cannot say if they were carriers or not; they give us no new information.
But based on her three sons, I want to say it was less likely that she was a carrier. But how likely? I mean, if she were a carrier, then there would only be a (.5)^3 = 0.125 chance that her 3 sons would not inherit her faulty X-chromosome. Those three events (sons) can be assumed to be independent, but they're not independent of the probability that Helena herself was a carrier.
Wikipedia wrote:The gene can be passed down the female line without a haemophiliac son being born, but as the family line continues and no haemophiliac sons are born, it becomes less likely that a certain ancestor had the gene and passed it on through the female line.
Or is this just faulty logic and I should just keep saying that Helena had a 50% chance of being a carrier, and her daughters had 25% chances?