Picture by Ann Marquez
One of the great things about being both a geneticist and a grandmother is I get to see how genes get transferred around. That shuffling of genes at meiosis produces some really fascinating results. This week I spent time with my youngest grandchild. He is two. He is in that delightful stage when the whole world is a wonder and a joy. His particular fascination now is colors and flowers. While dropping his big brother off at school, we walked past some flower beds of petunias and the little guy was completely taken with flowers.
“Flower, yellow!” (Except it sounded like lellow)
“Flower, Red!”(Pronounced wed)
We wanted some quality time so since the kid was so taken with flowers we decided to visit the Assiniboine Park’s Old English Garden in Winnipeg. This really is an old fashioned English garden with more varieties and colors of flowers in one spot than I have ever seen anywhere else. My grandson was positively in heaven.
This picture is from the Assiniboine Park website.
And we saw some giant trumpet shaped pink blossoms.
“Flower, pink, BIG!”
He had a wonderful time. In fact we wore him out. All the extra stimulation of the color varieties in the English Garden meant he fell asleep on the drive home and took a longer nap than usual as his developing brain processed all that input.
Watching the little guy it seemed to me that he was seeing a lot more color in these flowers than I was. He is much more fascinated by color than either his sibling or any of his cousins. From testing, I know that I can’t see shades of red as well as I can see shades of blue. I inherited my father’s X chromosome complete with his gene for red/green colour blindness. Fortunately due to X inactivation I am not red/green color blind. My color vision cells are a blend with some cells having the X chromosome I inherited from my father inactivated and in the rest it is my mother’s X chromosome that is inactivated. The result is that I do have an ability to distinguish reds and greens because of the mosaic pattern in my eye but I do have some near color deficits. Due to random meiotic shuffling, none of my sons inherited the red/green color blindness. My sons see color just fine. My brother, on the other hand, was tested in the military and his color vision far exceeds normal. He has such excellent color vision that after he left the military he actually made his living as a color matcher for a kitchen cabinet making company. Being male, he didn’t get an X chromosome from our father. Instead he got our mother’s X and likely something more.
The evolution of color vision in mammals is not a simple progression. Each vertebrate has up to 5 functional visual photopigment opsin genes that clump into two gene families that can be traced back up to 540 million years ago. Four of these genes are expressed in the cone cells in the retina. The other is expressed in the rod cells, which are generally associated with black and white vision but in low light can also have a role in color vision. Mammals started 200 million years ago with one opsin gene on the X chromosome, which itself originated from our tetrapod ancestors about 300 million years ago. For the past 35 million years the catarrhine primates, which include humans, have had two cone opsin genes side by side on the X chromosome, probably as a result of a gene duplication followed by evolutionary divergence. Another opsin gene is located on chromosome 7. With two X chromosomes, human females have two pairs of genes for color vision on them. Males have only one pair. Color blindness is therefore much more common among human males than among human females, my father and I being an excellent example of this when it comes to red.
One of the opsin genes on the X chromosome is called OPN1LW and it codes for protein pigment used by cone cells that respond to the yellow/orange part of the visible spectrum (longer-wavelengths of light). The other gene on the X chromosome is called OPN1MW and it codes for a protein pigment used to detect light at middle wavelengths (yellow/green light). The gene on chromosome 7 that codes for the third opsin gene type is called OPN1SW and it codes for a protein pigment that is used to see the blue/violet part of the visible spectrum (short-wavelength light) Thus there are three different kinds of cone cells, heavily concentrated in the foveal region of the retina, yet each individual cell expresses only one of the three possible opsins. Somehow, there is regulation that ensures one opsin gene, and only one opsin gene, is expressed. These cells are arranged in a patchy manner, including some large patches of one type or another.
With the combination of the three cone cell types we humans have trichromatic vision. A few people have tetrachromatic vision meaning we have four different types of cones and that gives those individuals even more ability to distinguish color. Males can have tetrachromatic vision only if they have two different variations (alleles) of the gene on chromosome 7. They can express one of the two genes on their single X in each cone cell and the two different alleles they have on chromosome 7. I suspect my brother, with his superior color perception has tetrachromatic vision. Females can have tetrachromatic vision in two ways. They can have two alleles on chromosome 7 and they can also have tetrachromatic vision if they have a mosaic pattern in their retina due to X inactivation. In theory, a rare women could even have hexachromatic vision but there is no evidence of that having ever occurred because we simply don’t test beyond tetrachromacy. It would seem that the X inactivation patches are usually too big for tetrachromacy for except perhaps one woman found in an investigated group of 24 women. Even so, it is well established that on average, women have much better color vision than men.
It’s far too soon to tell for certain but I suspect my color obsessed youngest grandchild also has tetrachromatic vision. If that is true, then his visual apparatus is very finely attuned to see all the wonderful colors in the Old English Garden. He will go through life seeing far more beauty than I ever will. And he will wince more often as the less gifted among us wear what appears to his eye as bizarro and clashing combinations.
My husband has green/brown color deficits. He was fascinated by the idea of prosthetic color vision glasses so he could see the world the way we did. He had an undergraduate student, Andrea Wsiaki, who did an undergraduate thesis on “Design of a computer graphics viewer for colour deficiency correction” in 2000. Others have brought this line of research to practical fruition. You can see one of the results here:
This young man’s reaction was very similar to watching my grandson in the English Garden
“That’s purple?!! Oh my God!!!”
(This post is based, in part, on material in the epigenetics chapter of our upcoming book, Embryogenesis Explained.)