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Chapter 9 2015
9.4,9.5, 9.6
9.4 Homologous chromosomes bear the alleles for each
character
Gene loci
P
a
B
P
a
b
Dominant
allele
Homologous
chromosomes
Genotype: PP
Homozygous
for the
dominant
allele
aa
Homozygous
for the
recessive
allele
Recessive
allele
Bb
Heterozygous,
with one dominant
and one recessive
allele
9.5 The law of independent assortment is revealed by
tracking two characters at once
• What would happen if plants were crossed
that differed in two characteristics.
– Dihybrid cross
• Two monohybrid crosses occurring simultaneously.
• Would the characteristics assort
independently or travel together?
9.5 The law of independent assortment is revealed by
tracking two characters at once
 Mendel performed the following dihybrid cross with the
following results:
• P generation: round yellow seeds  wrinkled green seeds
• F1 generation: all plants with round yellow seeds
• F2 generation:
– 9/16 had round yellow seeds
– 3/16 had wrinkled yellow seeds
– 3/16 had round green seeds
– 1/16 had wrinkled green seeds
© 2012 Pearson Education, Inc.
Figure 9.5A_1
rryy
P generation RRYY
Gametes RY
F1 generation

ry
RrYy
9.5 The law of independent assortment is revealed by
tracking two characters at once
 Mendel needed to explain why the F2 offspring
• had new nonparental combinations of traits and
• a 9:3:3:1 phenotypic ratio.
 Mendel
• suggested that the inheritance of one character has no effect on
the inheritance of another,
• called this the law of independent assortment.
© 2012 Pearson Education, Inc.
9.5B
• In Labrador retrievers Black color (B) is dominant
over chocolate (b)
• Normal vision (N) is dominant over Blindness by
progressive retinal atrophy
• Cross two heterozygous Labs for each of these
two traits.
• What are the genotypes of each dog?
• What are the possible gametes for each dog?
• What is the phenotypic ratio of this cross?
Predict the phenotypes of offspring obtained by
mating a black lab homozygous for both coat
color and normal eyes with a chocolate Lab that
is blind from PRA
What are the genotypes of each dog?
What are the possible gametes for each dog?
What are the genotypes of the offspring?
9.6 Geneticists can use the testcross to determine
unknown genotypes
 How would you determine the genotype of a black Lab
of an unknown genotype?
 What are the possible genotypes?
 What would you do to determine the genotype of this
dog?
© 2012 Pearson Education, Inc.