Monohybrid cross : hybrid of two individuals with homozygous
genotypes
Examples: Apply Scientific Knowledge to Analyze and Solve Scientific
Problems
1. In dogs, there is a hereditary type of deafness caused by a recessive gene. Two
dogs that carry the gene for deafness but have normal hearing are mated. What
are the possible genotypes and phenotypes of their offspring, the ratios and the
percent chance of each?
Gametes H h
H HH Hh
h Hh hh
Genotype: 1 hh 1 HH 2 Hh (1:1:2) (25 % hh, 25% HH, 50% Hh)
Phenotype: 3 Hearing 1 Deaf (75% Hearing, 25% Deaf)
1. Can you curl your tongue up on the sides? Tongue- curling in humans is a
domiant genetic trait. Suppose a man who is Tt for tongue-curling marries a
woman who is also Tt for this trait. What are the possible genotypes and
phenotypes of their children and the percent chance each?
Gametes T t
T TT Tt
t Tt tt
Genotype Phenotype Chance
1 TT Tongue curling 25%
1 tt None 25%
2 Tt Tongue curling 50%
1. In rabbits, black fur is dominant to white fur. If you cross a BB male with a Bb
female, what are the possible genotypes and phenotypes of the offspring and
their ratios? What is the percent chance for each type?
Gametes B B
B BB BB
b Bb Bb
Genotype: 2 BB, 2 Bb (50% BB, 50% Bb)
Phenotype: 4 Black fur (100% Black fur)
In plants and animals, organisms exhibiting the dormaint trait may be either
hybrid [heterozygous] or homozygous dominant [purebred]. Since we can’t see
an organism’s gene from the outside, and in breeding selectively it may be
important to know the exact genotype. Hence, a genetic cross known as Test
Cross is employed.
What is Test Cross?
A test cross is a genetic cross that involves using a homozygous recessive
organism to determine whether an individual is homozygous dominant.
To make a test cross, the organism whose genotype is being tested is crossed
with an organism that is homozygous recessive for trait. Every offspring from
such a mating would receive one recessive allele from a homozygous recessive
parent. If an offspring received a recessive allele from the other parent as well,
then it would show the recessive phenotype. From these results, we would know
that the other parent was hybrid and not pure for the dominant characteristic.
Rules of Test Cross:
1. Always cross the organism with the unknown genotype with an organism that is
homozygous recessive [2 lowercase alleles].
2. If possible, examine large numbers of offspring to ensure accuracy in determining
the unknown genotype.
Example: In fruit flies, red eyes are dominant over brown eyes. You mange to capture a
red-eyed fruit fly and desire to learn more about its genotype.
a. Give phenotypes [physical eye color] of the flies involved in your test cross.
Red and Brown
a. If the two flies have 400 offspring that all turn out to be red-eyed, what would be
the genotypes of flies used in your test cross [Use R and r]
Homozygous
Gametes R R
r Rr Rr
r Rr Rr
Heterzygous
Gametes R r
r Rr rr
r Rr rr
The answer is Homozygous (RR and rr)
a. Use a Punnett square to complete the cross. What percentage is heterozygous?
What percentage shows the dominant trait?
Homozygous
Gametes R R
r Rr Rr
r Rr Rr
Answer is 100% heterozygous and 100% dominant
a. If you performed the same experiment but this time 200 of 400 flies had brown
eyes, what would be the genotype of the red-eyed fly?
Rr and rr
Example: Discuss how a farmer could develop a group of sheep that are pure breeding
for white wool. Use W to represent the dominant allele for common white wool, and w
for the recessive allele for black wool.
In your answer you should:
1. State the genotype of the male and female sheep the farmer should use to breed
them.
Homozygous
1. Explain how the animal breeder can determine the genotypes of the male and
female to produce sheep that all have white wool.
The animal breeder can tell the genotypes of the male and female sheep to have
either one dominant allele (Ww) or two dominant allele (WW) to have white wool.
1. You should include at least two Punnett square with your explanation
Homozygous
Gametes W W
w Ww Ww
w Ww Ww
Heterzygous
Gametes W w
W WW Ww
w Ww ww
1. Explain how the animal breeder could make sure that the offspring would always
be pure breeding.
Animal breeder would use male and female sheep who are homozygous (WW or
ww) for the best outcome which is white fur.
