Question: "How come your son Jimmie isn't colorblind when his father is"
Wrong answer: "It some times skips a generations, maybe his kids will inherent it"
Right Answer: "it's on the X chromosome, he got his fathers Y chromosome, so his kids will not inherent colorblindness from his side of the family!"
Question: "How come your daughter Susie isn't colorblind when her farther is"
Confused answer: "It some times skips a generations, maybe her kids will inherent it"
Right Answer: "The confused answer is less wrong than with the son, it's on the X chromosome, she got her fathers X chromosome, so half heir sons will be color blind"
Simple genetics, your Mother has XX your father has XY, let's use little x to denote colorblindness, you get a random chromosome from each parent.
Colorblind father -> sons have normal vision, daughters are carriers:
xY + XX - > 50% XY + 50% xX
Colorblind mother -> sons are colorblind, daughters are carriers
XY + xx - > 50% xY + 50% xX
Carrier mother -> half sons are colorblind, half of daughters are carriers
XY + xX - > 25% xY + 25% XY + 25% xX + 25% XX
Finally if colorblindness runs in both families:
xY + xX -> 25% xY + 25% XY + 25% xx + 25% xX
both parents colorblind -> all children colorblind
Question: "Have you ever wondered, how does the xX females know not to use the color blind gene?"
Answer: "They do not, half the relevant cells in their eyes will have a bad photo receptor... but the brain is pretty smart, it quickly learns to boost the signal from the other half of the cells"
Warning: The above is strictly only true for X-linked dominant diseases such as colorblindness, a similar approach can be used to elucidate the inheritance pattern of Classical Mendelian traits, but does not work for complex trait such as height which is influenced by multiple genes.
My reaction to the title is to think, "Hmm. Who has been going around claiming that "it sometimes skips a generation" was an explanation?"
I would say that "it sometimes skips a generation" is an observation. The same observation can be made about many traits, so it may also be called a categorization, or a recognition of a pattern. It's not a causal explanation, although it might be taken to hint at one: "maybe a thing that causes these other skips-a-generation phenomena also causes this one". If someone doesn't know that a trait is X-linked, or doesn't know what chromosomes are, then "it sometimes skips a generation" may be the most they can say.
(Incidentally, a non-X-linked recessive trait may also be seen to "skip a generation" if it's common enough that one carrier has a decent chance of marrying another carrier. If you have enough good data, you can distinguish X-linked recessive from autosomal recessive.)
I totally agree, technically it's a correct observation, but it's also what I was taught by adults when I asked as a kid, and therefore I wanted to correct it as the real explanation is very short and concise.
Ah, that explains it. Adults are often not very good at explaining science to kids. And I'd guess the adults in question might not have known that colorblindness was X-linked, even if they were paid to teach science; I think I'd only be surprised by that ignorance in K-12 education if a teacher chose to present the subject of colorblind genetics to the class.
I once had a doctor (I'd guess in her early thirties) who, in a discussion of male-pattern baldness, mentioned the mother's father as the best data point—which means it must be X-linked, because otherwise the father's father would be an equally good data point (not to mention the father, if old enough). I said, "So, it's X-linked, then." She said, "No, it's not X-linked". I stated the above logic. She didn't comment on it, but consulted her computer system, and reported that there were five genes found to be associated with male-pattern baldness, some on the X chromosome and some not.
I think this was by parents, so they are forgiven :), your story is pretty crazy, but there is so much to know as a doctor that most becomes rules of thumbs (maps vs buttons) untill called out like you did
It gets more plausible when there are more genes involved. LIke if the family has 60/80 tallness genes and ambient population has 20/80 genes then having a "off-beat" offspring that is only 40/80 of the trait genes, the trait is way more likely to "snap back" rather than solidifying out of the ambient population.
I am not sure I follow, I am confused about whether the 60/80 family refers to both parents, and what is meant by "off-beat" and "snap-back", I am also confused about what the numbers mean is it 60/80 of the genes or 60/80 of the coding region (so only 40 genes)
60 trait supporting genes out of 80 locations that could support it. I am worried that the main finding is misleading because it is an improper application of spherical cow thinking to a concept that oriented to dealing with messiness.
fair point. I think my target audience is people like me who heard this saying about colorblindness (or other classical Mendelian diseases that runs in families)
I have added a disclaimer towards the end :)
Nice. No, it did not wonder, to be honest. Reminds me of a more general phenomenon that I often only notice that I was confused about something in retrospect.
Me to, I learned about this from another disease and taught, that's probably how it works for colorblindness as well.