Transcript Genetic Crosses

Higher Biology
Genetic Crosses
Dihybrid & Test Crosses
Genetic Crosses
By the end of this lesson you should be
able to:
Know what a dihybrid cross is
Know how many alleles for each gene there is in a
diploid organism
Know how to use a Punnett Square for a dihybrid
cross
Know what a testcross is
Know how to carry out a testcross
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Introduction
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The foundations of genetics were laid by
an Austrian monk called Gregor Mendel.
He published a paper in 1866, called
“Experiments with Plant Hybrids”- but his
work was largely ignored until the early
1900s.
Mendel carried out a large number of
crosses involving pea plants.
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Introduction
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Mendel investigated the following
characteristics in pea plants:
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Flower colour (purple & white)
Pod shape (inflated & constricted)
Seed shape (round & wrinkled)
Seed colour (yellow & green)
Stem length (tall & dwarf)
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Monohybrid Crosses- a reminder
1.
What do the following terms refer to?
a)
b)
c)
d)
e)
f)
g)
h)
i)
F1
F2
Genotype
Phenotype
Allele
Homozygous
Heterozygous
Dominant
Recessive
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Monohybrid Crosses- a reminder
2.
3.
4.
5.
6.
How many traits are involved in a monohybrid
cross?
If both parents are true-breeding, one dominant
and one recessive, what would be the phenotype of
the F1?
If two of the F1 generation from Q3 were crossed,
what would be the phenotypic ratio of the F2?
If two of the F1 generation from Q3 were crossed,
what would be the genotypic ratio of the F2?
What is a Punnett Square?
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Dihybrid Crosses
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A dihybrid cross is a cross involving two
characteristics at the same time.
E.g. in pea plants
Purple flowers
Short stem
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X
White flowers
Long stem
This means that four alleles are involved,
two for each gene.
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Dihybrid Crosses
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Mendel carried out these type of crosses.
E.g. in pea plants
Round yellow
seeded plant
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1.
2.
X
Wrinkled green
seeded plant
In the F1 he found that all the plants had round
yellow seeds.
Which two characteristics do you think are
dominant?
What term would be used to describe the other
two characteristics?
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Dihybrid Crosses
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Complete the dihybrid cross from your
diagram pack.
Black hair
Green eyes
X
Ginger hair
Blue eyes
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Back Crosses
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Sometimes called a testcross
This procedure is used to determine the
genotype of an individual with the dominant
phenotype.
When an organism exhibits a dominant
trait (characteristic), it is not obvious
whether its genotype is homozygous (true
breeding) or heterozygous for that trait.
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Back Crosses
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Example: We have a black mouse (shown
below), it could be Bb or BB.
How do we know which genotype it has?
B=Black
b=white
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Back Crosses
B=Black
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b=white
Is this mouse BB (homozygous) or Bb
(heterozygous)?
Cross it with a homozygous recessive (bb)
and check out the offspring produced.
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Back Crosses
TEST CROSS 1
X
??
bb
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Back Crosses
?? must be BB
Why?
BB
x
bb
B
B
b
Bb
Bb
b
Bb
Bb
All the offspring
have the same
phenotype as each
other & the
dominant parent.
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Back Crosses
TEST CROSS 2
X
??
bb
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Back Crosses
?? must be Bb
Why?
Bb
x
bb
B
b
b
Bb
bb
b
Bb
bb
Half the offspring
have the same
phenotype as
the dominant
parent & half have
the same phenotype
as the recessive
parent.
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Practice Questions
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1.
2.
3.
Complete the “Unlinked Backcross” sheet
from your diagram pack. Don’t fill in the
boxes in the middle of the page at this
stage!
Torrance
TYK page 93 Q2
AYK page 89 Q3
AYK pages 101 Q2.
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Genetic Crosses
Can you do it?
Know what a dihybrid cross is
Know how many alleles for each gene there is in a
diploid organism
Know how to use a Punnett Square for a dihybrid
cross
Know what a testcross is
Know how to carry out a testcross
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