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.