Test Crosses &
Dihybrid Crosses
• Y=yellow, y=green, R=round, r=wrinkled
• According to the Law of Independent Assortment, a plant
that is hybrid for 2 traits will form FOUR different
gametes
YyRr
YR
Yr
yR
yr
Independent assortment
and Segregation..
• Possible gametes
Dihybrid Cross
• EXAMPLE: #1
• What are the possible gametes for the following parents
(B=black coat, b= white, R= regular eyes and r= bulging
eyes);
• a) BBRr
• b) BbRr
• c) bbRR
Mendel used Peas again:
Character
Traits
Alleles
Seed coat
Round
R
RRYY
Round Yellow
Wrinkled
r
RRYy
Round Yellow
RRyy
Round Green
Cotyledon colour
Genotypes
Phenotypes
Yellow
Y
RrYY
Round Yellow
Green
y
RrYy
Round Yellow
Rryy
Round Green
rrYY
Wrinkled Yellow
rrYy
Wrinkled Yellow
rryy
Wrinkled Green
The expected probability of each type of seed can be calculated:
Probability of an F2 seed being round
= 75% or ¾
Probability of an F2 seed being wrinkled
= 25% or ¼
Probability of an F2 seed being yellow
= 75% or ¾
Probability of an F2 seed being green
= 25% or ¼
Therefore,
Probability of an F2 seed being round
and yellow
=
=
= 56.25%
Probability of an F2 seed being round
and green
=
=
= 18.75%
Probability of an F2 seed being
wrinkled and yellow
=
=
= 18.75%
Probability of an F2 seed being
wrinkled and green
=
=
= 6.25%
Probabilities…
From this we can predict how many seeds we could expect to get in a sample:
In 556 seeds we could expect:
556 x
round yellow
556 x
round green
556 x
wrinkled yellow
556 x
wrinkled green
= 313
= 104
= 104
= 35
NOTICE: HETEROZYGOUS CROSS HAS RATIO OF
9:3:3:1
• Was the inheritance of one
character affected by the inheritance
of another?
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 = Mendel's Second
Law, THE LAW OF INDEPENDENT
ASSORTMENT.
EXAMPLE #2:
• A pea plant that heterozygous for the traits tall (T) and
round seeds (R), is crossed with a dwarf with wrinkled
seeds. Determine the phenotype and genotype ratios of
the F1 generation
•
•
•
•
P: _________________________
P phenotype: ______________________
Gametes: __________________________
F1:
TR
tr
Tr
tR
tr
EXAMPLE #3:
• Yellow seeds (Y) are dominant over green seeds (y).
Round seeds (R) are dominant over wrinkled seeds (r). If
two plants that are heterozygous for BOTH traits
(dihybrid) are crossed, what will be the phenotype and
genotype ratios of the F1 generation?
•
•
•
•
P: _________________________
P phenotypes: _______________________________
Gametes: ____________________________________
F1
YR
YR
Yr
yR
yr
Yr
yR
yr
• A cross between an individual exhibiting the dominant
phenotype of a trait and an individual that is homozygous
recessive for that trait in order to determine the genotype
of the dominant individual.
• Recall: A dominant phenotype can be either homozygous
dominant (HH) or heterozygous (Hh)
ONE PARENT IS ALWAYS
HOMOZYGOUS RECESSIVE!!!!!!
Test crosses
• When a dominant phenotype is crossed with a
homozygous recessive any hidden traits can be revealed
• We can determine the genotype of the unknown parent by
looking at the ratios expressed in the offspring
• If the parent is PP then the phenotype of all the offspring;
• Will be ALL purple
Possible outcomes…Mono
• If the parent is Pp then the phenotype of all the offspring;
• Will be purple and white
Possible outcomes…Mono
Example
• Character: Coat colour in mice
Traits
Alleles Genotypes
Phenotypes
Grey
G
GG
Grey
White
g
Gg
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.
Homozygous
Phenotypes
Grey
Genotypes
GG
Gametes
G
v
Heterozygous
White
Grey
gg
G
g
v
White
Gg
g
G
gg
g
g
Genotypes
Phenotypes
Grey
Proportions
100%
White
0%
Grey
50%
White
50%
g
• In monohybrid crosses, to know if a dominant trait is
homozygous (RR) or heterozygous (Rr) it is necessary to
carry out a test cross. This is done with a homozygous
recessive (rr) individual. The same is true for a dihybrid
cross where the test cross is made with an individual
which is homozygous recessive for both characters
(rryy).
Dihybrid test cross…
• Possible outcomes
•
•
•
•
Parent YYRR all offspring are yellow and round
Parent YyRR  offspring will be yellow or green AND all round
Parent YYRr  offspring will be all yellow AND round or wrinkled
Parent YyRr  offspring will be yellow or green AND round or
wrinkled
• PARENT YYRR
Offspring all yellow/round
• PARENT YyRR
yr
YR
yR
YyRr
yyRr
Offspring: yellow/round
or green/round
• PARENT YYRr
yr
YR
Yr
YyRr
Yyrr
Offspring: yellow/round
or yellow wrinkled
• PARENT YyRr x yyrr
OFFSPRING: Yellow/round,
yellow/wrinkled, green round,
green wrinkled
• When doing an example test cross;
1. State your variable designation
2. Write down your predicted genotype
3. Do a Punnet square to prove your prediction
4. Write a statement
• Ex. In the “chocolate moose” brown fur is dominant to
yellow fur and round ears are dominant to pointed ears. If
a brown and round eared moose is test crossed and the
offspring are as follows, what were the genotypes of the
original parent?
5 brown round
2 yellow round
2 brown pointed
1yellow pointed
• In a test cross to determine whether a fruit fly is
homozygous (WW) or heterozygous (Ww) for long
wings, 7 long-winged flies & 1 short winged-fly are
produced. Which is a valid conclusion?
•
•
•
•
A. the unknown fruit fly is WW
B. the unknown fruit fly is Ww
C. the unknown fruit fly is ww
D. more offspring are needed
• In hamsters, long tails (L) are dominant to short tails (l). A
student wishes to perform a test cross to determine whether a
female long-tailed hamster is homozygous or heterozygous for
long tail length. She mates the hamster with a male long-tailed
hamster & studies the offspring, which are 100% long-tailed.
She conclude that the female hamster's genotype is "LL".
What mistake(s) did the student make?
•
A. she should have mated the female hamster with a male
that was known to be hybrid
•
B. she should have mated the female hamster with a shorttailed male hamster
•
C. she should have mated the female hamster with another
long-tailed female
•
D. she has to mate members of the litter before she can
make a conclusion
In Border Collies, black coat (B) is dominant to red coat (b). A breeder has a black male that has won
numerous awards. The breeder would like to use the dog for breeding if he is purebred or BB. To learn this
information, she testcrosses him with a red female (bb). Answer the following questions A, B, C, and D.
A. If the black male is BB, what kind of gamete (sperm)
can he produce?
Only _________
B. If the red female is bb, what kind of gamete (eggs)
can she produce?
Only __________
C.
If the black male is Bb, what kind(s) of gametes (sperm) can he produce?
Either _______ or ______
D.
If any of the puppies are red, what is the father's genotype?
Only __________