Other Types of Genetic
Crosses
And their Punnett Squares
Multiple Alleles
Wild Type Red Eyes
Mutant Apricot Eyes
Example: Fruit
fly eye colour
Mutant White Eyes
Mutant Honey Eyes
Multiple Alleles
More than 2 alleles for a gene.
 Dominance hierarchy. (One will dominate
over others.)
 Show this in solution using the first letter
of the gene name, then a superscript to
reflect how dominant it is (1 is most
dominant)

Multiple Alleles

Example: Fruit Fly eye colour (memorize
this for test)
– 4 alleles:
 Red, Apricot, Honey, White
– Each allele shows some dominance, except
White (E4):
 Red (E1): Dominant to all (E1E1, E1E2, E1E3, E1E4)
 Apricot (E2): Dominant over honey & white (E2E2,
E2E3, E2E4)
 Honey (E3): Dominant over white (E3E3, E3E4)
Codominance
Example: Coat colour in horses
Codominance

Both alleles show up in hybrid.
– Example: Some hairs are brown, some are
white.
There is NO dominant allele.
 Show this in Punnett Squares using the
first letter of the gene word, and
Superscripts – the first letter of the trait
word.

Codominance

Example: A chestnut (red) horse and a
white horse are crossed. All the offspring
are roan (both red and white hairs).
Let CR = Red
Red horse: CRCR
CW = White
White horse: CWCW
CR
CR
CW
CRCW CRCW
CW
CRCW CRCW
Incomplete Dominance
Example:
Colour of
flowers in
snapdragons
Incomplete Dominance

The alleles blend to form a different
heterozygous phenotype.
– Example: Having a red allele and a white
allele will make the flower pink.

We show this in a Punnett Square the
same way as with Codominance.
Incomplete Dominance

Example: Purebred red snapdragons
crossed with purebred white snapdragons
produced pink snapdragons.
Let CR = Red
Red snapdragon: CRCR
CW = White
White snapdragon: CWCW
CR
CR
CW
CRCW CRCW
CW
CRCW CRCW
ABO Blood Groups
Four possible blood types: A, B, AB, and O.
 Both multiple alleles and codominance.

– “A” allele (IA) codes for galactosamine on blood cell
membrane (allows your body to recognize it as its
own)
– “B” allele (IB) codes for galactose
– “O” allele (i) codes for neither

Both IA and IB are dominant over i, but neither is
dominant over the other
– IAIB: both galactosamine and galactose are expressed
Phenotypes: Genotypes
A blood type: IAIA, IAi
 B blood type: IBIB, IBi
 O blood type: ii
 AB blood type: IAIB

Donating & Receiving Blood

Giving blood:
–
–
–
–
O gives to everyone
A can give to A and AB
B can give to B and AB
AB can give to AB

Getting blood:
–
–
–
–
AB can get blood from anyone
A can get blood from A or O
B can get blood from B or O
O can only get blood from O