Dihybrid Crosses
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Transcript 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
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, what % have B?
b? H? h?
Dihybrid Crosses
This person’s genotype is
BbHh
BH
Bh
bH
bh
We keep the genes in the same order,
regardless of capitalization, like they’re a
first and last name.
Dihybrid Crosses
Everyone get a whiteboard…
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
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.
– Let’s find genotype and phenotype ratios &
probabilities
Dihybrid Cross
Try this cross, find genotype and
phenotype ratios & probabilities.
FFjj x ffJJ
Dihybrid Cross
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