Dihybrid Problems

Looking at 2 traits at the same time!

Remember Mendel?

• Worked with 7 traits on pea plants- Ex. Height, flower color, pod shape, etc.

• Worked with ONLY 1 trait at a time• What if he wanted to look at 2 traits at the

same time? • Would it change things?

Law of Independent Assortment

• A principle that states that genes on different chromosomes behave independently– Two traits are inherited separately– The two traits are NOT influence by one

another– Example: Green/yellow peas and

purple/white flower color

Dihybrid Cross

• A cross involving two pairs of contrasting traits (ex. seed shape and pod color)R = round seeds r = wrinkled seedsG = green pods g = yellow pods

• Each individual has 4 alleles (letters) this time, having 2 alleles per trait

Round, Green x Wrinkled, YellowRRGG x rrgg

Determining what the parents can pass on:

• We going to use the FOIL method (just like in math class)F = FirstO = Outer

I = Inner L = Last

• When using FOIL, you should get one of each type of letter (ex. R & G)

Using the FOIL method

• Let’s take the first parent: RRGG• To make this easier, pretend (R+R) (G+G)

– First = RG– Outer = RG– Inner = RG– Last = RG

*All allele possibilities for this parent are the same

*Notice that each possibility has 1 R/r and 1 G/g…you NEVER see 2 R/r’s or 2 G/g’s together. Ex. RR or gg or Gg

Second parent

• FOIL rrgg– First = rg– Outer = rg– Inner = rg– Last = rg

* Again we see that this parent only has 1 combo of alleles to pass on

Then make a Punnett Square

• We only need 1 box for this P. SquareRG

rgRrGg