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)
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Seed colour ( yellow & green )
Stem length (tall & dwarf)
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Monohybrid Crosses- a reminder
1.
i)
What do the following terms refer to?
a) b) c)
F
1
F
2
Genotype d) e) f) g) h)
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 F
1
?
If two of the F
1 generation from Q3 were crossed, what would be the phenotypic ratio of the F
2
?
If two of the F
1 generation from Q3 were crossed, what would be the genotypic ratio of the F
2
?
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 X
White flowers
Long stem
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This means that four alleles are involved, two for each gene.
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Dihybrid Crosses



1.
2.
Mendel carried out these type of crosses.
E.g. in pea plants
Round yellow seeded plant
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

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 b=white

Is this mouse BB ( homozygous ) or Bb
( heterozygous )?
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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

1.
2.
3.
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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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