Biology 12. These are crosses that involve genes for two different characteristics.
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Transcript of Biology 12. These are crosses that involve genes for two different characteristics.
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Genetics: Dihybrid Crosses
Biology 12
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Joke of the day…
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What is a dihybrid cross?• These are crosses that involve genes for two
different characteristics
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Back to Mendel and his peas…
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Mendel: Dihybrid Crosses• The natural progression for Mendel was to study
two characteristics at the same time.
• Thus, the study of 2 pairs of contrasting traits at the same time = a dihybrid cross
• ex.. round yellow seeds X wrinkled green seeds
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Review: Mendel:• Principle of Independent Assortment• When more then one trait is studied in the same cross,
the genes for each trait sort into gametes independently of the genes of the other traits
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Mendel’s peas:
Character Trait Allele
Seed shape Round A
Wrinkled a
Seed colour Yellow B
Green b
Let’s cross a homozygous round/yellow (AABB) with a homozygous wrinkled/green (aabb) plant….
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Steps for Dihybrid Crosses:
1. Determine Genotypes of Parents2. Determine Genotypes of Gametes
(remember gametes only have 1 copy of each gene) and think FOIL (from math class!)
3. Punnett Square: need 16 squares now, 4 gametes from each parent
4. Identify Phenotype Ratios in Offspring
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Example:o In snapdragons…
o Tallness (T) is dominant to dwarfness(t)o Red color is due to gene (R) and white to its
corresponding allele (r). o The heterozygous condition results in pink (Rr)
flower color.
o A dwarf pink snapdragon is crossed with a plant homozygous for tallness and red flowers. Give the possible genotypes and corresponding phenotypes for all of the possible F1 generation.
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Step #1: Determine Genotypes of Parents
“A dwarf pink snapdragon is crossed with a plant homozygous for tallness and red flowers. “
Parent 1:Dwarf pink = ttRr
Parent 2:Homozygous tall, red = TTRR
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Step #2: Determine Genotypes of Gametes
Dwarf pink = ttRr
Homozygous tall, red = TTRR
tR tr tR tr
TRTR TR TR
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TR TR TR TR
Gametes from TALL red parent
tR
tr
tR
tr
Gametes from dwarf pink parent
Step #3: Punnett SquareThe Punnett square determines the genotypes of the offspring
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TtRR
TtRr TtRr
TtRR
TtRr
TtRR TtRR
TtRrTtRr TtRr
TtRR TtRR
TtRr
TtRr
TtRR
TtRR
Step #4: Identify Phenotype Ratios in Offspring
Tall, red = 8/16
Tall, pink = 8/16
Tall, white = 0/16
Short, red = 0/16
Short, pink = 0/16
Short, white = 0/16
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Try it :• Page 154 questions 1, 3 (and then try #2)