I find it fascinating that intelligence is linked to the X chromosome. This could mean that people with three X chromosomes would be very bright indeed. Of course they are not. They are usually of very low intelligence.

Perhaps a better explanation of the study's findings is based on environment. Given this is an old study, most women in that era would have spent a great deal of time with their children. Men would have little contact with their young as they tended to work outside the home. The women would have been central to the learning development of their children. It would also be more pronounced with the female children, as they tended to be encouraged to remain home with mother. I would not be surprised that daughters and mothers scored the same and siblings tended to also be similar and that there would be less of a correlation between the males (father and son).

Also, the IQ test DOES have a cultural / environmental bias. It does not measure 'intelligence' (whatever that means) the same way for everyone or every creature taking the test. It measures how well you take that test. And how well you take that test depends on whether you learned in the same environment as the writer of the test.

By the way, if test scores were similar for mother-daughter and for father-daughter, would not the scores for mother-father be similar? And if mother-father are similar then father-son has to be similar since mother-son are.