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CHAPTER
35
CHROMOSOMES AND DIHYBRID
CROSSES
35-1 Meiosis: Sex Cell Formation with Two Traits
35-2 Laboratory lnvestigation
Dihybrid Gross Lab Activity
35-3 Dihybrid Cross Problem Set
Ch 35 Chromosomes and Dihybrid Crosses
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Page 56
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Cfropter 35
C fr.romos oftLes
an[ Diflg 6nd Cross es
Wfun raegirte it our aII,
ue can [irte utitfi ourse{au - regard[ess of tfu ruu{ts.
Mary Kay Ash
35-1 Meiosis: Sex Gell Formation with Two Traits
1.
How does the purpose of mitosis differ from the purpose of meiosis?
Objective
You will be expected to be able to describe how chromosomes and genes
segregate during sex cell formation. Be able to write all of the possible genes
present in sex cells produced by a parent of known genotype.
ln the last chapter you learned that the genes are on the chromosomes. You also
learned that wherever the chromosomes go during cell division, the attached genes go
with them. You followed one pair of chromosomes and the genes for tallness (T & t),
through meiosis to see how each sex cell is formed. Half of the sex cells ended up with
a T and half with a
lf we follow the same cell division process and also keep track of
process
another trait, the
becomes a little more complex.
t.
We will start with
a parent cell containing two pairs of
chromosomes. One pair has genes for height. lt is heterozygous
and tall. The other pair has genes for the color of the pea pods.
The plant has green pods (G) and carries a gene for yellow (g).
The plant is, therefore, tall and has green pods and
heterozygous for both traits.
is
$f' "[ L:
When this cell divides by meiosis, the cell can divide in two
different possible ways. Carefully examine the diagrams showing sex cell formation on
the next page. Be sure to study the two different ways this cell can divide. lt will only
divide one way or the other. lf a hundred cells are dividing, about 50 will divide one
way and 50 will divide the other way.
]
[
Ch 35 Chromosomes and Dihybrid Crosses
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Meiosis: Sex Cell Formation lnvolving Two Traits
ff'"[
_r_- FffW',
s$
,
The chrornosomes in the cell d the left
can position themselves in one of two
ways on the spindle as division begins.
I the chromosomes with the dominani
'
genes line up on the left, lhen lhe
sequence on the left will result. lf
chromosomes with one dominant and
""Hq'
'/
"w
/
\'/
/
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@ @@ @ @
4 Egg Cells: Two possible types
4 Egg Cel ls: Two possible types
Gh 35 Chromosomes and Dihybrid Crosses
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ln the last division, the doubled chromosomes divide by a mitosis-like division creating
two identical egg or sperm cells from each of the dividing cells. The final result, if the
parent cell was a female, will. be either two eggs containing genes TG and two eggs
containing genes tg or two eggs with genes Tg and two with genes tG. ln summary, if
many cells of the geneotype TtGg are dividing, there can be four possible sex cells, TG,
tg, Tg and tG.
2.
Describe what changes took place in step 1 (arrow 1).
3. What determines whether
the chromosomes will line up resulting in step 2 or 3?
4. Describe the changes that take place in step 4.
5.
Describe the changes that take place in steps 5 and 6.
6.
When many parent cells of genotype TtGg divide by meiosis, what are all of the
possible types of sex cells that can be obtained?
7.
When many parent cells of genotype AaBb divide by meiosis, what are all of the
possible types of sex cells that can be obtained?
Microscopic views of various stages of animal cell meiosis
Ch 35 Chromosomes and Dihybrid Crosses
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35-2
Laboratory lnvestigation
Dihybrid Cross Lab Activity
Objectlve
You will be expected to be able to describe how chromosomes and genes
segregate during sex cell formation. Be able to write all of the possible genes
present in sex cells produced by a parent of known genotype and solve dihybrid
crosses based upon this principle.
You now have a good understanding of how parents pass on genes for one trait. What
happens when we keep track of TWO traits for each parent? When two traits are
involved in a cross, it is called a DIHYBRID CROSS. The following lab activity will
help you visualize how genes are segregated when sex cells are formed.
Cover the answers with your answer paper as you work through the program.
Make use of the following information for the following crosses: Brown eyes are
dominant to blue eye color. Curly hair is dominant to the recessive straight hair.
1. How would you symbolize a person who had brown eyes and curly hair and was
heterozygous for both traits? How would you write the genotype foi a person with
blue eyes and straight hair?
1. BbCc, bbcc
2. Let's assume that a brown-eyed, curly haired father, and his wife (also brown eyedcurly haired) plan to have children. They are bdth heterozygous and carry genei for
blue eyes and straight hair. The cross would appear as follows:
BbCc
?
X
BbCc
The next step involves determining what the sex cells look like. ln a dihybrid cross,
understanding what genes can end up in sex cells is essential in understanding how to
determine the offspring. To help you understand this, toothpicks will be used. Locate 4
red toothpicks and 4 plain toothpicks. Be sure two pairs of red toothpicks have a tape
flag, onewith aC andonewith ac. The plain pairshould have a B flag andabflag.
Your toothpicks should appear as those on the next page.
Ch 35 Chromosomes and Dihybrid Crosses
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X
ilB
curly-haired
Brown-eyed,
BbCc
af
X
Brown-eyed, cu rly-hai red
BbCc
When sex cells are formed, its important to remember the following principle: WHEN
TWO GENES OF A PAIR SEPARATE DURING SEX CELL FORMATION, ONE MEMBER
OF THE PAIR GOES TO ONE SEX CELL AND THE OTHER GOES TO THE OTHER SEX
CELL. Place your toothpicks over the appropriate pictured toothpicks at the top of this
page. Each toothpick represents a chromosome and each flag represents the gene on
that chromosome. Using the principle stated above, move the toothpicks into the sex
cells drawn below: I I
SPERM
EGGS
Notice that there are two different possibilities for both sperm and egg cells.
Ch 35 Chromosomes and Dihybrid Grosses
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2.
Write the gene's symbol that would be found in each possible sex cell in the circles
shown previously. This should also be based upon toothpick placements from the
previous page.
\v"
2. BC, bc, or Bc, bC
(same for the egg cells)
you
lf
did not obtain these results, go back and
rearrange your toothpicks at the top of page two
and try to verify that these are :th€ four
possibilities. lf you need help, ask others at
your table. lf you still need help, ask your
teacher.
3.
The next step is to arrange the 4 sex cells for the male and the female in an expanded
Punnett square like the one shown below. Copy the pattern below and write the
appropriate sex cells in the appropriate spaces in the top and side of the chart that is
on your own paper.
Ch 35 Chromosomes and Dihybrid Crosses
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3.
N
Bc
BC
bc
bc
BC
BC
bc
bc
(the order ls not lmportant)
4.
What would be the next step in figuring out the various possible offspring?
Now fill in the squares with the genotype for each offspring by filling the squares in
your chart.
4.
Your final product should look tike this.
N
BC
Bc
bc
bc
BC
BBCC
BBCc
Bbcc
BbCc
Bc
BBCc
BBcc
BbCc
B
bc
Bbcc
B
bCc
bbcc
bbCc
bc
BbCc
B
bcc
bbCc
b
bcc
bcc
Compare and correct any mistakes. You should note that in filling in the offspring, the
capital letter for a given pair is always written first for the pair. Bb, Cc and not bB, cC.
Ch 35 Chromosomes and Dihybrid
Crosses
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5.
How many offspring were obtained?
You should now be able to write the phenotype for each genotype in the squares.
Examine your Punnett square and determine how many of each of the following
phenotypes are present in the offspring. f
1
Copy and fill-in the blanks:
Brown eyed and curly haired?
Brown eyed and straight haired?_
Blue eyed and curly hdred?_
Blue eyed and straight haired?_
_
How many of the total phenotypes are there for each of the above?
5.
16 total offspring,
9, 3, 3, 1 ratio for the 4 PhenotYPes
lf you did not get a 9:3:3:1 ratio for the
phenotypes, go back over your answer and
find where you made the error.
6.
How would you express this ratio in fractions?
6.
9/16, 3/16, 3/16, 1/16
7. See if you can determine the offspring for the following 2 dihybrid
AaBb X
a Tinn
x
crosses:
AA'bb
,fiRr
Be sure to show all of the possible genotypes and give the fractions for each.
lf there is not enough time in class you can finish these next crosses at home.
I
I
ASsume that in the last crOSS, that T = tall, t = short, R = [oU[d seeds, r = wrinkled
seeds. ln the last cross, what are the phenotypes of the two parents? What are the
phenotypes of their offspring and the fractions of each?
Ch 35 Chromosomes and Dihybrid Crosses
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I
35-3 Dihybrid Cross Problem Set
Objectlve
You are expected to apply the principles learned in this chapter to solve the
dihybrid cross problems that follow:
Solve the following cross problems and show your work or explain how you arrived at
the answer.
1.
tall pea plants with round seeds (TtRr) are crossed. Work out the cross and
give all possible genotypes that can be obtained in the offspring. Also state the
possible phenotypes in the offspring and the fractions of each. T=tall, t=short,
Two
R=roufld, r=wrinkled
2.
tall pea plant with round seeds (TTRR) is crossed with a short
plant with
possible
give
genotypes
all
that can
wrinkled peas (ttrr). Work out the cross and
be obtained in the offspring. Also state the possible phenotypes in the offspring and
the fractions of each.
A
with a tall plant with wrinkled
Work out the cross and give all possible genotypes that can be
obtained in the offspring. Also state the possible phenotypes in the offspring and the
3. A tall pea plant with round seeds (TtRr) is crossed
peas
(Ttrr).
fractions of each.
4. ln peas, round peas and yellow peas are dominant. Wrinkled and green peas are
recessive traits. A pea plant with round yellow peas (heterozygous for both traits)
is crossed with a plant with wrinkled green peas. What fraction of the offspring will
have wrinkled green peas?
Ch 35 Chromosomes and Dihybrid Crosses
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