Struggling with probability and punnett square

tbird88 · Feb 18, 2015

I met to put this in students, pre nursing, can someone move this for me

KelRN215, BSN, RN · Feb 18, 2015

What are you struggling with? This was one of my favorite parts of biology. Draw a square. Divide it in to four sections. On the top, write the mother's phenotype and on the side, write the father's. Match up and viola! You have your punnet square. Autosomal recessive diseases will always have a 25% chance of offspring being unaffected, 50% chance of being a carrier and 25% chance of having the disease.

Let's look at an example. Cystic fibrosis. You have 2 parents who are carriers. Let's call the autosomal dominant (normal) gene C and the autosomal recessive (disease) gene c. Both parents are Cc. Neither has the disease but they both carry the recessive gene. For every pregnancy, there is a 25% chance that the child will have the disease, a 50% chance that a child will be a carrier and a 25% chance that the child will be unaffected. Other diseases that follow this pattern are sickle cell anemia, Fanconi anemia, Tay Sach's

[TABLE]

[TR]

[TD]Cystic Fibrosis[/TD]

[TD]C[/TD]

[TD]c[/TD]

[/TR]

[TR]

[TD]C[/TD]

[TD]CC[/TD]

[TD]Cc[/TD]

[/TR]

[TR]

[TD]c[/TD]

[TD]Cc[/TD]

[TD]cc[/TD]

[/TR]

[/TABLE]

Now let's look at another example. Huntington's disease. Autosomal dominant. One copy of the gene and you get the disease. You have 1 parent who has the Hungtington's gene, let's call him Hh where H is the dominant (disease) and h is the normal (recessive) gene. The other parent is unaffected hh- both recessive genes.

[TABLE]

[TR]

[TD]Huntington's disease[/TD]

[TD]H[/TD]

[TD]h[/TD]

[/TR]

[TR]

[TD]h[/TD]

[TD]Hh[/TD]

[TD]hh[/TD]

[/TR]

[TR]

[TD]h[/TD]

[TD]Hh[/TD]

[TD]hh[/TD]

[/TR]

[/TABLE]

50% chance with each pregnancy that the child will have the gene and eventually go on to develop the disease and 50% chance that they will be unaffected. Other disease that follow this pattern are Neurofibromatosis (though this can also be caused by a spontaneous mutation), Marfan syndrome.

Then you have X linked diseases which by and large only affect boys because girls have 2 copies of the X gene.

KelRN215, BSN, RN · Feb 18, 2015

Here's an example of an X-linked disease- Duchenne muscular dystrophy. Here you have a carrier mother XDXd where D is the normal gene and d is the recessive/MD gene. Because the mother has a second X that has the dominant/normal gene, she doesn't have the disease. The father is unaffected XDY.

[TABLE]

[TR]

[TD]Duchenne Muscular Dystrophy

[/TD]

[TD]XD

[/TD]

[TD]Xd

[/TD]

[/TR]

[TR]

[TD]XD

[/TD]

[TD]XDXD

[/TD]

[TD]XDXd

[/TD]

[/TR]

[TR]

[TD]Y

[/TD]

[TD]XDY

[/TD]

[TD]XdY

[/TD]

[/TR]

[/TABLE]

As you can see, with each pregnancy you have 25% chance of an unaffected female, 25% chance of an unaffected male, 25% chance of a carrier female and 25% chance of a male with the disease. Other conditions that follow this pattern are Adrenoleukodystrophy, color blindness and hemophilia A and B.

tbird88 · Feb 18, 2015

How do I figure out a punnett square when it is two traits?

sidraali · Feb 18, 2015

I seriously hated the punnet square.

It use to give me headaches.

R you in nursing school now or upgrading for nursing?

I dont want any punnet square in nursing school

NicuGal, MSN, RN · Feb 18, 2015

The squares are used in peds and OB for genetics. Our genetics people plot them out for parents.

tbird88 · Feb 18, 2015

I am in prenursing. I am taking Biology right now. So I don't think you have to worry about seeing the punnett square soon.

tbird88 · Feb 18, 2015

Oh I was wrong, I misspoke about not seeing the punnett square soon.

KelRN215, BSN, RN · Feb 18, 2015

I seriously hated the punnet square.
It use to give me headaches.
R you in nursing school now or upgrading for nursing?
I dont want any punnet square in nursing school

Whether or not you need to take basic biology in your nursing program, you will need to understand genetics and the punnett squares for your OB and peds classes.

KelRN215, BSN, RN · Feb 18, 2015

How do I figure out a punnett square when it is two traits?

The same way, you just have more possible outcomes.

Vita108 · Feb 18, 2015

You may find these videos helpful. I have not watched them myself, but I have watched both the teachers' videos on other topics and thought their explanations were very clear.