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BIOLOGY 12
Two-Trait Inheritance
Dihybrid Cross
Remember Mendel’s Peas…
Character
Seed shape
Seed colour
Trait
Allele
Round
R
Wrinkled
r
Yellow
Y
Green
y
 Mendel’s hypothesis was that each trait was
independent of the other
 this means that one allele does not have an
effect on another allele
 independent assortment – the chromosomes will
randomly align and divide
 there is no connection between the two different
chromosomes
Law of Independent Assortment
 it appears that the inheritance of seed shape
has no influence over the inheritance of seed
colour
 the two characters are inherited
INDEPENDENTLY
 the pairs of alleles that control these two
characters assort themselves independently
Determining Gametes
 when dealing with two homozygous genotypes,
the gametes will be the same
 example: RRYY
x
rryy
 gametes: RY, RY, ry, ry
 note: the gametes do not need the capital first
Determining Gametes
 when dealing with heterozygous genotypes, you
use FOIL (think math!)
 example: RrYy
R r Y y
First and Outside
RY and Ry
Inside and Last
rY and ry
Possible Gametes:
RY, Ry, rY and ry
Practice
 complete the following Punnett square using the
following cross: AaBb x AaBb
AB
AB
Ab
aB
ab
Ab
aB
ab
How Good Were Mendel’s Results
We could
expect
What Mendel
observed
556 x 9/16 round yellow
313
315
556 x 3/16 round green
104
108
556 x 3/16 wrinkled yellow
104
101
556 x 1/16 wrinkled green
35
32
Practice
 In watermelons the genes for green color and
for short shape are dominant over their alleles
for striped color and for long shape. Suppose a
plant with long striped fruit is crossed with a
plant heterozygous for both these characters.
What phenotypes would this cross produce and
in what ratios?
Practice
 A heterozygous tall, homozygous green pea
plant is crossed with a heterozygous tall,
heterozygous yellow plant.
 What is the probability that the offspring will
be short and green?
Single Trait Practice
 In dogs, wire hair is due to a dominant gene
(W), and smooth hair is due to its recessive
allele.
 If a homozygous wire-haired dog is mated
with a smooth-haired dog, what would be
possible phenotypes of the offspring?
Single Trait Practice
 Two wire-haired dogs (W?) are mated, and
among the offspring of their first litter is a
smooth-haired pup.
 If these two wire-haired dogs mate again, what
is the chance that they will produce another
smooth-hair pup? What are the chances that
the pup will be wire-haired?
Two Trait Practice
 A female dog is heterozygous for both fur length
and colour. She shows long, brown fur. Her mate
shows short, white fur.
 Determine the gametes of each dog.
 What is the probability that the pup will be just
like its mother?
Two Trait Practice
 In humans, the presence of freckles is due to the
dominant gene (F), and the non-freckled condition is due
to its recessive allele (f). Dimpled cheeks (D) is
dominant to non-dimpled cheeks (d).
 Two persons with freckles and dimpled cheeks have two
children. One has freckles but no dimples (F?dd), one
has dimples but no freckles (ffD?).
 What are the genotypes and phenotypes of the parents?