Genetic Exceptions Test crosses, Incomplete Dominance, and Codominance
Test Cross• Used to determine an unknown dominant
genotype.
• Offspring that show a dominant phenotype could have either a homozygous dominant (RR) or heterozygous (Rr) genotype…
How can the second allele be determined?
Test cross example
• In mice, black (R) is dominant to white (r)• Take a black mouse with an unknown dominant genotype
and cross it with a homozygous recessive (rr) white mouse, then observe the color of the offspring.
If any offspring are white, the unknown allele is ______.
No white offspring then unknown allele is _____.
Rr r r
Rr r r
Rr R r
Rr R r
R ?
r
r
r
R ?
r
r
R
Incomplete Dominance• According to Mendel’s Law of Dominance; if
dominant allele is present then the dominant trait will be expressed.
However...there are always exceptions to the rules!!!• In some organisms; both the dominant(R) and
recessive(r)determine phenotype• Phenotype that results is a “blending” of proteins.• Alleles themselves do not combine, only the
characteristics(proteins) they code for do.
Example: Flower Color
Red (R) Blue (r)
All F1= Heterozygous (Rr) F2 = 1 RR : 2 Rr : 1 rr
All Heterozygous genotypes will show the blending of phenotypes. NOT MIXING ALLELES, MIXING COLORS OF PROTEINS !
R R
r
r
Rr Rr
Rr Rr
R r
R
r
RR
rr
Rr
Rr
FLASHLIGHT
GENERALLY;MOST ‘BLENDED’FLOWER COLORSILLUSTRATEINCOMPLETEDOMINANCE!
SNAPDRAGONSILLUSTRATE INCOMPLETEDOMINANCE
CO-DOMINANCE
• aka: Shared Dominance. • Two dominant alleles code two
different proteins.• R’ = the codominant allele• No blending of characteristics.• All dominant proteins are coded.
Example 1: Coat color
R= brownR’= black
R R’
R
R’
RR
R’R’
RR’
RR’
How are they similar?
How are they different?
They are both codominant for black and white….
The panda has a gene that codes for color placement.
Example 2: Sickle Cell
R= RoundR’= Sickle
Persons with RR’ suffer from milder symptoms than R’R’ because they have some normal cells
R R’
R
R’
RR RR’
RR’ R’R’
NORMAL RR
SICKLE CELL RR’
Example 3: Blood Types • 3 alleles code for blood type: IA IB and i(type O)
• A is codominant to B and both are expressed: Type AB• A is dominant to O : Type AA or AO• B is dominant to O : Type BB or BO• O is recessive: Type OO
Taste Receptor: Phenylthiocarbamide (PTC)
• The “taster” gene is incompletely dominant to the “non-taster” gene.
• People with two copies of the taster gene taste the chemical compound more intensely than those with only one copy.
• Chemicals similar to PTC are found in plants of the mustard family. (Broccoli, mustard, etc.)
• This gene is found among many other taster genes on chromosome 7.