Dihybrid Crosses

Predicting TWO traits at once!

Mendel’s Law ofIndependent Assortment

What is the Law of Independent Assortment?

Genes for different traits can segregate independently during the formation of gametes.

What does that really mean?

The inheritance of one gene does not influence the inheritance of another gene.

Just because you got your dad’s “tall” gene doesn’t mean you will also necessarily get his “blue eye” gene –

the two aren’t connected!

Just because you got your dad’s “tall” gene doesn’t mean you will also necessarily get his “brown eye” gene –

the two aren’t connected!

We said…

But what if we want to know the chances that you DO get both genes?

Or what if we want to know the chances of not one particular gene, but TWO particular genes?

Would a regular Punnett Square work?

In order to predict TWO traits at once,

you will need to do a...

Four possible combinations of genes on the top and side (parental gametes)

Sixteen total possible gene combinations (offspring)

Now lets try our own example….

People like to eat green, round peas – not wrinkled and not yellow! So a farmer would

want to know, if he was breeding plants, what the chances are that he would get the

ideal crop (and make the most money!).

R = roundr = wrinkled

Y = yellowy = green

Both plants he is starting with are RrYy

So we need to make a GIANT Punnett Square to cross RrYy x RrYy…and then figure out the “chances” that the farmer will

get round, green offspring!

Parent 1: Parent 2:

When you are predicting only one trait, each parent has two possible genes to give. When you are predicting two

traits, each parent has FOUR possible genes to give!

So Parent 1 could give the following four combinations of genes to their offspring…

And Parent 2 could give the following four combinations of genes to their offspring…

Parent 1:

Pare

nt

2:

Parent 1:

Pare

nt

2:

Round, Yellow Round, Yellow Round, Yellow Round, Yellow

Round, Yellow Round, green Round, Yellow Round, green

Round, Yellow Round, Yellow wrinkled, Yellow wrinkled, Yellow

Round, Yellow Round, green wrinkled, Yellow wrinkled, green

Parent 1:

Pare

nt

2:

Round, Yellow Round, Yellow Round, Yellow Round, Yellow

Round, Yellow Round, green Round, Yellow Round, green

Round, Yellow Round, Yellow wrinkled, Yellow wrinkled, Yellow

Round, Yellow Round, green wrinkled, Yellow wrinkled, green

NOW LET’S MAKE SOME

PREDICTIONS!

Round,Yellow

Round, Yellow

Round,Yellow

Round,Yellow

Round,Yellow

Round,green

Round,Yellow

Round,green

Round,Yellow

Round,Yellow

wrinkled,Yellow

wrinkled,Yellow

Round,Yellow

Round,green

wrinkled,Yellow

wrinkled,green

How many of the farmer’s plants will produce the desired Round, green peas?

How many will produce the yellow peas, which no one will want to buy?

So would it be wise for the farmer tocross these two RrYy plants?

Ready for another example?!?

This husband and wife want the ultimate circus freak kid

– one with elephant skin and a hairy face!

If they have a baby, what are the chances that they’ll have the ideal circus freak? And what are the chances that they will just have a

normal kid?!?!

Dog-faced Man

http://2.bp.blogspot.com/_Fzq94YVbHHM/TKJks7TOePI/AAAAAAAA5mI/CKCyJNhBQNo/s1600/circus_freaks_05.jpghttp://2.bp.blogspot.com/_Fzq94YVbHHM/TKJks7TOePI/AAAAAAAA5mI/CKCyJNhBQNo/s1600/circus_freaks_05.jpghttp://1.bp.blogspot.com/_Fzq94YVbHHM/TKJjxMLk_XI/AAAAAAAA5jk/FUwSV_iexv8/s1600/circus_freaks_23.jpghttp://1.bp.blogspot.com/_Fzq94YVbHHM/TKJjxMLk_XI/AAAAAAAA5jk/FUwSV_iexv8/s1600/circus_freaks_23.jpg

Dog-faced Man Normal Face = FHairy Face = f

Normal Skin = QElephant Skin = q

Dad must be ff for face.Genetic tests show he is Qq for skin.

Mom must be qq for skin.Her dad was also “dog-faced”, so we

know that she must be Ff for face.

http://2.bp.blogspot.com/_Fzq94YVbHHM/TKJks7TOePI/AAAAAAAA5mI/CKCyJNhBQNo/s1600/circus_freaks_05.jpghttp://2.bp.blogspot.com/_Fzq94YVbHHM/TKJks7TOePI/AAAAAAAA5mI/CKCyJNhBQNo/s1600/circus_freaks_05.jpghttp://1.bp.blogspot.com/_Fzq94YVbHHM/TKJjxMLk_XI/AAAAAAAA5jk/FUwSV_iexv8/s1600/circus_freaks_23.jpghttp://1.bp.blogspot.com/_Fzq94YVbHHM/TKJjxMLk_XI/AAAAAAAA5jk/FUwSV_iexv8/s1600/circus_freaks_23.jpg

So Mom is FfqqAnd Dad is ffQq

Which means that Mom and Dad each can give FOUR possible combination of the two genes….

Mom could give .

Dad could give .

Fq or Fq or fq or fq

fQ or fQ or fq or fq

If they each can give FOUR possible combination of genes, how many boxes is our PunnettSquare going to have?

Mom = Ffqq

Dad

=ff

Qq

Fq Fq fq fq

fQ

fq

fQ

fq

FfQq

Ffqq

FfQq ffQqffQq

Ffqq

FfQq

FfqqFfqq

FfQq

ffqq

ffQq

ffqq ffqq

ffQq

ffqq

Fq Fq fq fq

fQ

fq

fQ

fq

FfQq

Ffqq

FfQq ffQqffQq

Ffqq

FfQq

FfqqFfqq

FfQq

ffqq

ffQq

ffqq ffqq

ffQq

ffqq

Normal face, normal skin

Normal face, normal skin

Normal face, normal skin

Normal face, normal skin

Normal face, elephant skin

Normal face, elephant skin

Normal face, elephant skin

Normal face, elephant skin

Hairy face, normal skin

Hairy face, normal skin

Hairy face, normal skin

Hairy face, normal skin

Hairy face, elephant skin

Hairy face, elephant skin

Hairy face, elephant skin

Hairy face, elephant skin

Mom = Ffqq

Dad

=ff

Qq

NOW LET’S MAKE SOME

PREDICTIONS!

Fq Fq fq fq

fQ

fq

fQ

fq

FfQq

Ffqq

FfQq ffQqffQq

Ffqq

FfQq

FfqqFfqq

FfQq

ffqq

ffQq

ffqq ffqq

ffQq

ffqq

Normal face, normal skin

Normal face, normal skin

Normal face, normal skin

Normal face, normal skin

Normal face, elephant skin

Normal face, elephant skin

Normal face, elephant skin

Normal face, elephant skin

Hairy face, normal skin

Hairy face, normal skin

Hairy face, normal skin

Hairy face, normal skin

Hairy face, elephant skin

Hairy face, elephant skin

Hairy face, elephant skin

Hairy face, elephant skin

How many of the circus freaks children will be “ideal” circus freaks themselves, with elephant skin and a hairy face?

How many of their kids will be perfectly normal?

How many will have just one circus freak trait?

How many will carry a recessive gene that they may pass on to their children?