Download Dihybrid Crosses

Dihybrid Crosses

What are the four possible gametes that the
individuals with these genotypes could
produce? (Remember to keep the genes in this order, so I/i
before L/l, F/f before J/j, etc.)
–
–
–
–
–
IiLL
iiLL
FFjj
ffJJ
YyHh
Dihybrid Crosses
Homework
Objectives

Be able to perform a dihybrid cross
– Be able to identify what each box in the
cross represents
– Be able to explain what Mendel’s three
Laws are, and where you can see each
one “in action” in a dihybrid cross
Dihybrid Crosses

Popcorn reading: The first two pages of
the handout, telling the story of the
Morgan fruit fly experiments.
Gregor Mendel

All of these basic
principles were first
identified by Gregor
Mendel, an Austrian
monk in the mid1800s.
– Through studying pea
plant breeding, he
described three laws
of inheritance.
QuickTime™ and a
decompressor
are needed to see this picture.
Mendel’s Laws of Inheritance

Law of Dominance: In a cross of
parents that are pure (homozygous) for
contrasting traits, offspring will only
have one of those traits.
– What is this describing that you’ve already
learned?
– Could you rephrase it to make more sense
to you?
Mendel’s Laws of Inheritance

Law of Segregation: A parent’s allele
pair separates during gamete formation,
and alleles randomly unite in
fertilization.
– Let’s figure out how we actually already
learned this when we studied meiosis…
Mendel’s Laws of Inheritance

Law of Independent Assortment: Alleles
of different genes separate
independently during gamete formation.
Therefore, traits pass to offspring
independently of each other.
– Again, let’s think back to meiosis, the
candy lab, the Baby Lab…
Laws

Law of Segregation: A parent’s allele pair separates
during gamete formation, and alleles randomly unite in
fertilization.
– = Each gamete contains only one of each kind of allele.

Law of Independent Assortment: Alleles of different
genes separate independently during gamete formation.
Therefore, traits pass to offspring independently of each
other.
– = The parent makes an equal number of each of the four kind of
gametes. A gamete with an allele from one gene does not force
it to have a certain allele from the other gene.
• For instance, for a parent who is BbHh, half their gametes have the
H allele. Half of those will have the B allele, and the other half will
have the b allele. The B/b alleles assorted independently of H. If
they had been dependent, then it could be that gametes with H
would only have b.
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.
– Baby Lab, the sequel!
Dihybrid Crosses

Suppose that a person is heterozygous for
eye color (Bb). On 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.
– Write #1 at the top of both, just like in the baby lab.

This person is also heterozygous for
hitchhiker’s thumb.
– Write H on the third strip, and h on the fourth.
– Write #2 at the top of both, just like in the baby lab.

These are their chromosomes in a diploid
cell.
Dihybrid Crosses
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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
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
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
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Of the four gametes, what % have B?
b? H? h?
Dihybrid Crosses
This person’s genotype is

BbHh


BH
Bh
bH
bh
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We keep the genes in the same order,
regardless of capitalization, like they’re a
first and last name.
Dihybrid Crosses
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Everyone get a whiteboard…
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What are the gametes that can be made
from:
JJYy
– Note, some gametes may be the same as
each other. Write all four of them anyways.
Dihybrid Crosses
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JJYy =
– JY

JY
JY
Jy
Another practice problem, figure out the
four gametes for:
– ttGg
Dihybrid Crosses
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ttGg =
– tG
tG
tG
tg
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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.
– Let’s find genotype and phenotype ratios &
probabilities
Dihybrid Cross
Try this cross, find genotype and
phenotype ratios & probabilities.
 FFjj x ffJJ

Dihybrid Cross
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Now try this cross: YyHh x YyHh.
Y = jagged, y = normal. H = square, h =
round.
 Find genotype and phenotype ratios &
probabilities. (Helpful hint: 1/16 =
6.25%, 2/16 = 12.5%.)

• YyHh x YyHh.
•
•
•
•
YH
Yh
yH
yh
YH
YYHH
YYHh
YyHH
YyHh
Yh
YYHh
YYhh
YyHh
Yyhh
yH
YyHH
YyHh
yyHH
yyHh
yh
YyHh
Yyhh
yyHh
yyhh
Genotype Probabilities: 6.25% YYHH, 12.5% YYHh, 12.5% YyHH, 25% YyHh, 6.25% YYhh,
12.5% Yyhh, 6.25% yyHH, 12.5% yyHh, 6.25% yyhh
Genotype Ratios: 1 YYHH : 2 YYHh : 2 YyHH : 4 YyHh : 1 YYhh : 2 Yyhh : 1 yyHH : 2 yyHh : 1
yyhh
Phenotype Probabilities: 56.25% jagged/square, 18.75% jagged/round, 18.75% normal/square,
6.25% normal/round
Phenotype Ratios: 9 jagged/square : 3 jagged/round : 3 normal/square : 1 normal/round