# level of difficulty of punnett square problems?

#### thebillsfan

##### Unseasoned Veteran
10+ Year Member
5+ Year Member
would that get any harder than a cross w/ two alleles?

#### RogueUnicorn

##### rawr.
7+ Year Member
no. even that would not be very common; obviously, it's still a good idea to know how to do more than two alleles, or better yet, just figure it out mathematically

#### Omni

10+ Year Member
5+ Year Member
You could get even 4 alleles.
Like, color of eyes can be one allele, and height can be the second allele.
But when you're doing problems like that which require you to find frequency of occurrence with two traits like that, then you can just solve for the frequency of the each independent allele, and then just multiply the chances.

For example, lets say we are given three traits: color, height, length of hair
C= Brown
c= red
H= short
h= tall
L= long
l = small
You have two individuals with this genotype: CC Hh ll crossed with cc Hh Ll. What are the chances that the off spring will have the following phenotype: Brown, short, and small hair.

Do the punnet square for each trait individually
CC x cc = 1/1 Brown, 0/1 red
Hh x Hh = 3/4 tall, 1/4 short
Ll x ll = 1/2 Long, 1/2 small

Since we're looking for brown, short, and small hair, just take the respective fractions and multiply them together. so we will have 1/1 x 1/4 x 1/2= 1/8

12.5% chance of getting that phenotype.

#### RogueUnicorn

##### rawr.
7+ Year Member
but this would no longer be a punnett square question (which i take to mean population ratios of a whole generation)

OP
T

#### thebillsfan

##### Unseasoned Veteran
10+ Year Member
5+ Year Member
but this would no longer be a punnett square question (which i take to mean population ratios of a whole generation)
so would there really be no need to do punnet squares at all on the mcat, theoretically, if we could just apply omni's method to any number of alleles? punnett squares are easier for me, but if i have extra time i always like to find another method to verify my initial result.

C

#### CaliforniaKid

would that get any harder than a cross w/ two alleles?
I doubt it will be more than 2, but just learn how to use the rule of probability and you can deal with 3 or 4 alleles easily.

#### RogueUnicorn

##### rawr.
7+ Year Member
so would there really be no need to do punnet squares at all on the mcat, theoretically, if we could just apply omni's method to any number of alleles? punnett squares are easier for me, but if i have extra time i always like to find another method to verify my initial result.
you should only do punnett squares to find population ratios like 9:3:3:1.. otherwise the statistical method is faster and better.

C

#### CaliforniaKid

you should only do punnett squares to find population ratios like 9:3:3:1.. otherwise the statistical method is faster and better.
I think you meant Phenotypic ratio.

#### RogueUnicorn

##### rawr.
7+ Year Member
i do =). thanks. i couldn't think of the word.