Download 3U 1.10 The Test Cross

Grade 11 Biology
The Test Cross: A Useful Tool
Name _____________
Date ______________
It is impossible to tell by appearance alone whether an individual showing a dominant trait is pure for the
trait (______________) or hybrid (________________). Breeders of plants and animals often need this
information about a parent stock. A test cross is used to determine the unknown genotype of an individual.
The unknown genotype is crossed with a pure recessive and by examining the offspring the genotype can
be determined. Always show the cross using ____ to represent mating.
Example 1 – A tall pea plant of unknown
genotype (T__) is crossed with a short pea plant
(tt) and they produce all tall pea plants. What is
the unknown genotype of the tall parent plant?
Parents?
T___
x
tt
Example 2 – A tall pea plant of unknown
genotype (T_) is crossed with a short pea plant
(tt) and they produce tall and short pea plants in a
1:1 ratio. What is the unknown genotype of the
parent plant?
Parents?
Conclusion : the unknown
genotype is ______.
T___ x
tt
Conclusion : the unknown
genotype is ______.
Overall Conclusion: By crossing an individual of unknown genotype with a ____________ individual and
examining the offspring, the unknown genotype can be determined to be _________________ or
_______________.
Homework Word Problems
Characteristics and their Traits
In pea plants:
Long stem (L) is dominant to short stem (l)
Yellow colour (Y) is dominant to green colour (y)
In fruit flies:
Winged flies (F) are dominant to wingless flies (f)
In humans :
Tongue curlers (T) are dominant to non-curlers (t)
Brown eye colour (B) is dominant over blue eyes (b)
Curly hair (H) is dominant to straight hair (h)
Normal skin colour (N) is dominant to no skin colour – albinism (n)
Dimples (D) is dominant to no dimples (d)
1. A brown-eyed man marries a blue-eyed woman and they have ten children, all brown-eyed. What are the
genotypes for all the individuals in the family?
2. A mother and father are heterozygous for dimples. What proportion of the children would
a) have dimples
b) be homozygous for the dominant gene?
3. A straight haired man marries a hybrid curly-haired woman. What are their chances of having a curly haired
child?
4. A mother is albino. The father is hybrid for this trait. What proportion of the children would be:
a) albino? b) carriers of the gene for albinism (a hybrid)
c) have normal skin colour?
5. What are the possible phenotypes produced from crossing a heterozygous pea plant with a
homozygous long stemmed pea plant?
6. a. Lisa, who cannot curl her tongue, is married to Brent who is heterozygous for this trait. They have eight
children. What is the probable number of children who can curl their tongues?
b. What is your genotype for this trait?
7. If a pure breeding winged fly mates with a pure breeding wingless fly, what are the genotypes of the F1
generation?
8. If an F1 fly mates with a wingless fly, what is the genotypic ratio of the offspring (winged: wingless)
9. What will be the genotypes and colours of the offspring of the following pea plant crosses?
a. Homozygous yellow x green
b. Heterozygous yellow x green
c. Heterozygous yellow x heterozygous yellow