From Whom Did Peter Get His Color Blindness?

My son Peter has red-green color blindness.  We discovered this when he was about 7.  No one in my family has color blindness, so I was quite surprised.

I happened to come across one of those visual tests for color blindness in some book or magazine, and for fun, asked the boys to come over and see if they could see the number in the circle.  Chris could, but Peter couldn’t.  His mother reminded me that her Father was color blind.  I thought, “Oh, that must be it.”  I confirmed it by getting him tested at the optometrist office at Springfield Mall.  Yep, he had red-green color blindness.  We don’t know which type of red-green color blindness he has, but Peter guesses it is Deuteranomaly.

The color blindness hasn’t really hampered him much in life, although I know it came up when he obtained his pilot’s license in San Diego about 10 years ago.  There are special rules for color blind folks, at least for night flying.  Peter did qualify for night flying in single-engine airplanes but had to pass a special in-person test with an FAA inspector.

It is interesting to watch him decorate his house.  He is excellent at design and even considered becoming an architect at one point.  But he tends to rely on brown, gray, or blue hues to decorate.  Christmassy colors are in short supply in his home!

Determining the origin of one’s color blindness is somewhat simpler than most human traits.  That’s because color-blindness is a sex-linked trait.  It is caused by mutations in a couple genes on the X-Chromosome, one of the sex chromosomes.

Color blindness is specifically derived from two genes, OPN1LW and OPN1MW, situated next to each other at one end of the X-Chromosome.^[1]The X-chromosome is 155 million base pairs long and OPN1LW and OPN1MW are located between 154.144 and 154.197 million base points, per DNAPainter.com.

Females have two X chromosomes and males have one X and one Y-Chromosome.  Mothers always pass an X-Chromosome to their babies regardless of gender, while fathers pass an X-Chromosome to their daughters and a Y-Chromosome to their sons.  To put it another way, if the father contributes an X-Chromosome, the child will be female.  If he contributes a Y-Chromosome, the child will be male.

X-Chromosome inheritance patterns are unique.  All 22 of the other chromosomes, called Autosomes, can be derived from any ancestor in any line, depending on which segment you’re looking at.  But X-Chromosome DNA can only be derived from a certain subset of ancestors.  This is because males do not pass an X-Chromosome to their sons.  Anytime a male-to-male (father-to-son) link occurs in your family tree, you know this is an X-Chromosome dead-end.

Genetic genealogists have even developed charts to illustrate how this works.  There are two charts, one for females and one for males.  Each chart highlights those ancestors who potentially could contribute X-DNA segments to one’s genome. The actual contributors would be an even smaller subset.

The first chart is an X-Chromosome inheritance pattern for females.  The female subject is indicated by the pink circle in the center and the blue and pink segments radiating out from her represent male and female ancestors, respectively.  The left side indicates her paternal and the right her maternal ancestors.  The defining characteristic of X-Chromosome inheritance, once again, is that male-to-male inheritance road-blocks the X-Chromosome.  For example, the female subject’s father (defined by the largest blue segment left of the circle) cannot receive an X-chromosome from his father, so it is impossible for a female to inherit any X-Chromosome traits from her paternal grandfather (or his ancestors).

The second chart is an X-Chromosome inheritance pattern for males.  It is exactly the same as the female chart on the mother’s side but is absent any genetic contribution from the father’s side.

Let’s return to Peter and consult the male chart.  We know that no one in my family has color blindness and that Peter’s maternal grandfather did, so he must have received his color blindness from his mother’s side.  But even without that knowledge, we would still be able to conclude from the chart alone that he inherited his color blindness from his mother.  She’s the only one that gave him an X-Chromosome.  Indeed, all color-blind men inherit that trait from their mother.

Let’s pencil in some names to make this clearer.  Peter’s mother is Shalon Hu.  Peter’s maternal grandparents are Hu Shi-liang and Huang Chih-ping, a Chinese couple.  (Mr. Hu, by the way, died a few years ago at age 106!)

Note that men inherit color blindness far more than women.  That’s because they receive only one X-chromosome.  For a woman to have color blindness – a recessive trait – she must possess the mutated genes for color-blindness in both of her X chromosomes.  Women are often carriers but rarely experience the condition themselves.

Theoretically, Peter’s color blindness could have come from either of his two maternal grandparents.  His mother, Shalon Hu, is a carrier, certainly.  But his maternal grandmother, Huang Chih-ping, could also have been a carrier.  The left side of the illustration above shows that a carrier mother can produce a carrier daughter.  Color-blindness can skip more than one generation before it is manifested again.

However, we know that Peter’s maternal grandfather, Hu Shi-liang, was color blind, so that narrows the options considerably.  Peter’s color blindness had to come from Hu Shi-liang’s mother.  She is indicated in the male inheritance chart with an X.  She is one of Peter’s great-grandmothers.

I don’t remember the lady’s name, but I actually met her once in Hong Kong at a family dinner over 40 years ago.  She was in her 80s, as I recall, and extremely stern-looking.  I don’t think she smiled once.  She also had the smoothest skin I’ve ever seen on someone that age.  I remember her vividly because I kept looking at her face.  I couldn’t believe how unwrinkled it was.

The right side of the illustration above shows how Peter’s children could be affected by his color-blindness, in a scenario where the affected father represents Peter and his wife is not a carrier.  You can see that Peter’s sons will never be affected.  They receive a Y-Chromosome from him.  However, all his daughters will receive the carrier gene for color-blindness, and it is possible that grandsons born to his daughters could manifest the condition and granddaughters born to his daughters could also be carriers.

Hu Shi-liang, by the way, was Hakka Chinese (客 家 人) whose family roots were in Meixian County, Guangdong Province.  My recollection is that he was born there.  The region identified by “A” in the map below indicates where Meixian County is located.  That’s where Peter’s color blindness originated.

The Hakka are an interesting story.  There are scattered Hakka communities throughout southern China, particularly in Guangdong, Jiangxi, and Fujian provinces, as well as Taiwan.  Meixian County in Guangdong is one of the main population centers for Hakka and the homeland of Standard Hakka.  The Hakka language is not just a dialect of Chinese but a separate language group, mutually unintelligible with Mandarin, Cantonese, Shanghainese (Wu), etc.  Their language fascinates linguists because it has retained certain archaic features of ancient Chinese that are not present in the other Sinic languages.  The Hakka are believed to have migrated from the central plains of Northern China over a thousand years ago, probably in response to some upheaval.  There are many theories about when and why they migrated.