Dihybrid Crosses
Homework
Dihybrid Crosses

Popcorn reading: The first two pages of
the handout, telling the story of the
Morgan fruit fly experiments.
Dihybrid Crosses

So you know how to figure out the likelihood
of having a blue-eyed baby. And you know
how to figure out the likelihood of having a
baby with a hitchhiker’s thumb.
 But what if you want to know the likelihood of
having a brown-eyed baby with a hitchhiker’s
thumb? A blue-eyed baby with a normal
thumb?
Dihybrid Cross


For two genes that are on different
chromosomes, we can do a dihybrid cross.
(Di=two) (Scientists can do very large
complicated crosses, but two is the largest
one we’ll do.)
The essential difference is that figuring out
gametes has a bit of a trick to it.
 At each table, please tear a piece of paper
out of a notebook, and then tear it into four
strips of roughly equal size.
Dihybrid Crosses

Suppose that a person is heterozygous for
eye color (Bb). One one chromosome #1,
they have the B allele. On the other
chromosome #1, they have the b allele.
– Write B on one strip, and b on another.

This person is also heterozygous for
hitchhiker’s thumb.
– Write H on the third strip, and h on the fourth.

These are their chromosomes in a diploid
cell.
Dihybrid Crosses

When they make gametes, each gamete will
get just one from each pair of homologous
chromosomes.
– Make different gametes. Each gamete should
have just one B/b chromosome, and just one H/h
chromosome. Be prepared to answer:
• How many possible combinations are there?
• What are the genotypes of those possible alleles?
Dihybrid Crosses
This person’s genotype is

BbHh


A gamete from a person who is Bb will
have either B or b. And a gamete from
a person who is Hh will have either H or
h. This is the same situation.
Dihybrid Crosses
This person’s genotype is

BbHh


BH
Dihybrid Crosses
This person’s genotype is

BbHh


BH
Bh
Dihybrid Crosses
This person’s genotype is

BbHh


BH
Bh
bH
Dihybrid Crosses
This person’s genotype is

BbHh


BH
Bh
bH
bh

Of the four gametes, how many have
B? How many have b? H? h?
Dihybrid Crosses
This person’s genotype is

BbHh


BH
Bh
bH
bh

Notice that we keep the genes in the
same order, regardless of capitalization.
For example, bH.
Dihybrid Crosses

Now practice figuring out the four
possible gametes from this genotype:
JJYy
– Note, some of them may be the same as
each other.
Dihybrid Crosses

JJYy =
– JY

JY
JY
Jy
Another practice problem, figure out the
four gametes for:
– ttGg
Dihybrid Crosses

ttGg =
– tG
tG
tG
tg
You set a dihybrid cross up exactly like
a monohybrid cross, only now there are
four gametes per parent rather than just
two.
 Example: A cross between IiLL and iill.
