TEST CROSS
How to tell the difference between
homozygous and heterozygous
individuals that have dominant
phenotypes
© 2007 Paul Billiet ODWS
Selfing or test cross?
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Plants can be selfed
Homozygous plants (AA) are pure breeding
Heterozygotes (Aa) will give 75% dominant
(AA and Aa) and 25% recessive (aa)
phenotypes in their offspring
Animals are not usually hermaphroditic so
they cannot be selfed
Animals require a test cross to be carried
out (though these days a gene probe is
more likely to be used).
© 2007 Paul Billiet ODWS
Coat colour in mice
Traits
Grey
White
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Alleles Genotypes Phenotypes
G
g
GG
Gg
Grey
Grey
gg
White
Grey mice could have one of two different
genotypes, GG or Gg.
If they are crossed with a white mouse
(gg) these genotypes will give two different
results
© 2007 Paul Billiet ODWS
Phenotypes
Homozygous
Grey
Genotypes
GG
Gametes
Offspring
© 2007 Paul Billiet ODWS
G
White
x
gg
G
g
g
g
G
Gg
Gg
G
Gg
Gg
Grey
White
100%
0%
g
Phenotypes
Heterozygous
Grey
Genotypes
Gg
Gametes
Offspring
© 2007 Paul Billiet ODWS
G
White
x
gg
g
g
g
g
G
Gg
Gg
g
gg
gg
Grey
White
50%
50%
g