Genetics
Notes 3
Test Cross
How do we know if an organism that shows a
dominant trait has a two dominant alleles or one
dominant and one recessive allele?
• A _________________ is a method for determining
whether an organism that ___________________
__________________________________________
_________________________________________.
Test Cross
• In a test cross, _________________________
_____________________________________.
– If the test organism is heterozygous, ___________
________________________________________.
– If the test organism is homozygous dominant,
________________________________________.
– Large numbers of offspring are necessary for valid
results.
Test Cross
Example:
• In guinea pigs, black coat color is dominant
over white. How would you find out whether
a black guinea pig is homozygous for coat
color (BB) of heterozygous (Bb)?
– _________________________________________
_________________________________________
________________________________________.
Test Cross
• Using the Punnett Squares below, show the
test cross described above:
Multiple Alleles
• So far, we have discussed traits that have only
two possible alleles (Ex. Yellow/Green;
Tall/Short; Round/Wrinkled), but for many
genes, several alleles exist in the population.
– This is called _______________________
– This expands the number of possible
_____________________________
Multiple Alleles
Example: Multiple alleles control the character of
____________________ in humans. A person may be
______, ______, ______, or ______.
• The letters refer to _______________________, called
A and B, which are _____________________________
_______________________
– A persons red blood cells may be coated with _____
_____________ (A or B), ___________ (AB), or
_____________ (O)
• The alleles are written as
• IA = _____
• IB = _____
• i = ______
Multiple Alleles
• Each person inherits ___________________________.
• There are 6 possible ways to inherit the alleles:
–
–
–
–
–
_______
_______
_______
_______
_______
• O (i allele) is _________________
• A and B (IA and IB alleles) are called ________________:
______________________________
– Both carbohydrate A and B are present on red blood cells
Multiple Alleles
• How is this different from incomplete
dominance?
– ___________________________________
___________________________________
• Ex. ______________________________
– ___________________________________
Genotype
ii
IAIA
IAi
IBIB
IBi
IAIB
Phenotype
Review: Dihybrid Cross
• Using the Punnett square below, work out the
following cross:
TTYY x TtYy
Review: Dihybrid Cross
• Using the Punnett square, work out the
following cross:
rrYy x RRyy
Review: Blended Inheritance
(Incomplete Dominance)
Example:
• Crossing pure red (RR) and pure white (WW)
Japanese four-o’clock flowers results in an F1
generation in which all of the flowers are pink
(RW). Show a cross between a pure red
flower (RR) and a pure white flower (WW).
Phenotypes:_________
Review: Blended Inheritance
(Incomplete Dominance)
Show a cross between two members of the F1
generation (RW x RW).
Phenotypes:_________
Review:
• What is Mendel’s law of independent assortment?
• What is incomplete dominance?
• What is codominance?
• When do you use a test cross?
• When do you use a dihybrid cross?
• When do you use a monohybrid cross?