Expert: John Locke - 12/19/2007

Question
can you tell me how would i form a punnet square showing a disease with sex linked traits? Thank You

Answer
Thanks for using AllExperts, Byrd. The answer to this question requires a visual presentation, so I'm including links at the end that show how Punnet squares are formed for sex-linked traits.

First, recall that sex-linked traits are those carried on the sex chromosomes, X and Y in humans. Disease transmission for traits carried on the X differs from those carried on the Y, so let me cover both separately. Normal inheritance problems depend on you being able to tell the difference between dominant and recessive traits; sex-linked inheritance problems add the complication of which sex the offspring will be. First, let's look at the X chromosome. In this context, a "diseased" chromosome means one that carries the allele for a genetic disease.

Men can contribute X or Y chromosomes to their offspring; woman can contribute only X chromosomes. This happens, of course, because men have an XY genotype and women an XX genotype. Thus, any diseases carried on a particular X chromosome will be expressed in men with that particular X chromosome--there is no other X-chromosome DNA available to read. If a man receives a diseased X chromosome from his mother, he has that genetic disease, regardless of whether it is dominant or recessive. Women with diseased X chromosomes have the more familiar dominant/recessive inheritance pattern: if the disease is recessive, then two affected X chromosomes are required for the woman to have the disease. One affected X chromosome makes her a carrier for that disease. If the disease is dominant, then having just one diseased chromosome will cause the woman to have that disease.

To create the Punnett square, list the mother's genotype (XX, noting which chromosomes are diseased) in the horizontal spaces across the top and the father's genotype (XY, noting if the father carries a diseased X chromosome) in the vertical spaces along the side. Then you simply pair up the chromosome combinations as you would with a regular Punnett square. You may also switch the position of the father and mother's genotypes, because you'll get exactly the same result.

Let's take an example. Consider a father with a sex-linked genetic disease (XcY) and a mother with one diseased chromosome (XcX). The disease is recessive, so the mother is a carrier without symptoms. What would be the possible genders and disease states of their children? (Try this for yourself before reading on). You will find that the parents could have a girl with the disease (XcXc), a girl who is a carrier (XcX), a boy with the disease (XcY), and a boy without the disease (XY). To become more facile with this, you may want to consider drawing the Punnett squares for other possible combinations (a recessive disease with an unaffected father and a carrier mother, a recessive disease with an affected father and unaffected mother, a dominant disease with an affected father and unaffected mother, etc.--you get the idea).

Determining inheritance for diseases carried on the Y chromosome is simpler. Only men are affected by diseases of the Y chromosome (I leave it to you to determine why). If a man has a diseased Y chromosome, he'll have that genetic disease--like with the X chromosome, there's no other Y chromosome DNA available to read. Additionally, a father with a diseased Y chromosome will have sons who are all diseased as well. He passes on his diseased Y chromosome to his sons.

You may find the additional information at the links below to be useful. Good luck!

http://biology.clc.uc.edu/Courses/bio105/sex-link.htm
http://homepages.ius.edu/GKIRCHNE/Mendel.htm
http://www.brooklyn.cuny.edu/bc/ahp/SDMG/SD.MG.Punnett.03.html