Date
by Mike
Image Text: Well, the kid's definitely getting the biology geek phenotype.
Punnett squares are simple diagrams used to determine the odds of a particular offspring genotype (genetic make up) given the genotypes of the parents. The phenotype is the physical manifestation of the genes in the organism and doesn't always correlated directly to strict genotypes. Some genetic characteristics (like color blindness and eye color) can be easily traced using Punnett squares because they are the result of one clearly dominant gene and one clearly recessive. Other traits like height are a blend of multiple genes.
An interesting thing to note:
The male's parents were both colorblind but that is a recessive gene on the X chromosome. For his mother to be colorblind she must have had two copies of the gene. This man now carries the gene on his X chromosome so he will only pass it on to his daughter. She will be a carrier but will have normal color vision so long as this woman is not colorblind.
September 9th, 2009
And I didn’t not know that colorblindness was carried on the X chromosome. Learn something new every day.
October 14th, 2009
Slight correction/clarification:
A daughter would definitely be at least a carrier. If the mother is colorblind, the daughter will definitely be colorblind. If the mother is a carrier, she has a 50% chance of being colorblind. Of course, a son can only inherit colorblindness from his mother.
December 16th, 2009
Also worth noting that they’re evidently postulating/hoping for a trifecta (or quadfecta), where their red-haired, green-eyed daughter will have red-green colorblindness and be unable to recognize a difference in hue between two of her major defining characteristics. Messing with the kids’ heads seems to be a popular theme.
August 7th, 2010
Actually, if a woman is a carrier for two different types of colorblindness (red-green and purple-yellow, iirc), then her eyes will actually produce a fourth pigment and allow her to see a broader range of colors than most of us can. Her sons would all be colorblind, and her daughters could be colorblind, normal sighted carriers of colorblindness, or have the superior color vision of their mother.
December 15th, 2010
Leapfrogging off of Jon, I found a fascinating article about both colorblindness and the possibility of tetrachromacy:
http://www.rmki.kfki.hu/~lukacs/TETRACH.htm
Jon, I am curious about the situation you describe. It seems to be different from the 3 types of tetrachromacy listed in the article, none of which seem likely to me to have the results in the offspring that Jon was indicating.
According to this article, the genes producing the red pigment and the green pigment are on the X chromosome, so defects in either of those can be overcome by a female (or XXY male) who has normal genes on the other X chromosome.
The gene producing the blue pigment is on chromosome 7, so a male could also be a carrier of blue-yellow colorblindness – this type of colorblindness is naturally less common.
This article describes one type of tetrachromacy as occurring when someone has one normal gene for a pigment, but the other gene that would have been for that pigment detects a significantly different wavelength, between the wavelengths detected by typical human cones. The case studies described are fascinating to me.
February 17th, 2011
i think one point is missing: if both his parents are colorblind, that means he is also colorblind (he got an X from the mother whose alleles are both recessive). If he’s colorblind, he wouldn’t be able to see that his child’s hair is red and the eyes are green. This in itself makes their entire conversation obsolete. Punnett squares give only the probability of a child having a specific phenotype but one thing is for certain: the child will be 100% of a nerd.
April 22nd, 2011
Actually, according to the March 2007 Discover magazine, eye color is determined by the interaction of at least 3 genes, despite the simple Punnett squares we did in high school biology. I suppose you could do a Punnett square for the known genes on eye color, but a Punnett square for at least 3 genes is no longer very simple, not to mention the difficulty in being confident which variants of each gene each parent had.
http://discovermagazine.com/2007/mar/eye-color-explained/?searchterm=Hair%20color