Download Special Crosses: Incomplete Dominance and

Special Crosses: Incomplete
Dominance and Codominance
Some alleles are neither dominant nor recessive.
Many traits are controlled by more than one gene
(polygenic traits)
What is Incomplete Dominance?
neither allele is dominant or shows up,
“blending”
New phenotype is a mix of each allele
(neither one, somewhere in middle)
Plant: Snapdragons
Humans: Hair Type
In snapdragons, red flowers are dominant over white
flowers. The heterozygous genotype has pink flowers.
Cross a homozygous red-flowered plant with a pink
flowered plant. What are the chances of producing
white flowers? (R = red and r = white)
If a yellow guinea pig is crossed with a white one, the
offspring are cream-colored. What kinds of offspring
are expected if two cream-colored guinea pig mate?
(Y = yellow and y = white)
What is Codominance?
two alleles both appear in the
phenotype, “sharing dominance”
Usually signified using superscripts.
Cat: Fur Color
Chickens: Feather Color
Cattle: Fur Color
crcr = red hairs
cwcw = white hairs
crcw = roan coat
crcw = roan coat
In certain cattle, hair color can be red (homozygous
RR), white (homozygous rr), or roan (a mixture of red
and white hairs, heterozygous Rr). When a red bull is
mated to a white cow, what genotypes and
phenotypes of offspring could be obtained?
If one of the offspring found above were mated to a
white cow, what genotypes and phenotypes of
offspring could be produced? In what proportion?
How is Codominance different
from INCcomplete dominance?
CO=sharing dominance, speckled, spotted,
both traits clearly visible (speckled fur)
INC=mixing or blending, old traits gone, new
trait is in middle (flower: red + white = pink)