Transcript Unit 6C - NC Biology Resources

Unit 6C
Test Crosses
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People often want to know what genes an
organism has:
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Genes in horses or dogs for breeding
Genes that they may have (for diseases)
Genes in plants for breeding successful crop
plants.
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So…how do you know what genes an organism
has if you do not know what traits the parent had?
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Test crosses are used:
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A test cross is the crossing (mating) of the organism with
another organism that you know the genotype of.
You then see what percentages of offspring you have.
Ex: I cross a black cat with a cat that I know is homozygous
black (BB).
x
Example
WW or Ww
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Ww
I have a plant with White Flowers.
White is dominant (W). Red is recessive
(w).
I want to know what the genotype of my
plant is.
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=
What two genotypes could it be?
WW or Ww
I will cross the flower with one that I know
the genotype of: Ww
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If all of the offspring are White, what was the
genotype of the original flower?
If any offspring are Red (ww), then I know
that the original flower was heterozygous.
WW
WW
Ww
Ww
W
w
W
WW
Ww
w
Ww
ww