Incomplete Dominance:
If Mendel were given a mommy black mouse & a daddy white
mouse & asked what their offspring would look like, he would've said
that a certain percent would be black & the others would be white. He
would never have even considered that a white mouse & a black
mouse could produce a GREY mouse! For Mendel, the phenotype of
the offspring from parents with different phenotypes always resembled
the phenotype of at least one of the parents. In other words, Mendel
was unaware of the phenomenon of INCOMPLETE DOMINANCE.
I remember Incomplete Dominance in the
form of an example like so:
RED Flower x WHITE Flower ---> PINK Flower
With incomplete dominance, a cross between
organisms with two different phenotypes
produces offspring with a third phenotype that
is a blending of the parental traits.
Example: Snap Dragon Cross
Red = R
White = W
Cross a pure red with a pure white
Co-dominance:
First let me point out that the meaning of the prefix "co-" is
"together". Cooperate = work together. Coexist = exist together.
Cohabitat = habitat together.
The genetic gist to co-dominance is pretty much the same as
incomplete dominance. A hybrid organism shows a third phenotype --not the usual "dominant" one & not the "recessive" one ... but a third,
different phenotype. With incomplete dominance we get a blending of
the dominant & recessive traits so that the third phenotype is
something in the middle (red x white = pink).
In Co-dominance, the "recessive" & "dominant" traits appear together
in the phenotype of hybrid organisms.
I remember co-dominance in the form of an
example like so:
red x white ---> red & white spotted
Look at problem 3 in 50 genetics packet. Let’s do that cross. White Cow x Roan Bull.
WW = white, Roan = RW
Blood Type is also considered to be a type of co-dominance.
Codominance and Blood Types
Two alleles in a gene pair are each associated with different substances. When both
substances appear together in heterozygotes, codominance occurs. The two alleles of a
pair at a specific locus are not identical but the expression of both is observed.
Codominance is clearly different than incomplete dominance. An example of
codominance is the ABO blood typing system used to determine the type of human
blood. It is common knowledge that a blood transfusion can only take place between
two people who have compatible types of blood. Human blood is separated into
different classifications because of the varying proteins contained in each blood type's
red blood cells. These proteins are there to identify whether or not the blood in the
individual's body is it's own and not something the immunity system should destroy.
The protein's structure is controlled by three alleles; i, IA and IB. The first allele is, i, the
recessive of the three, and IA and IB are both codominant when paired together. If the
recessive allele i is paired with IB or IA, it's expression is hidden and is not shown.
When the IB and IA are together in a pair, both proteins A and B are present and
expressed.
The ABO system is called a multiple allele system for there are more than two possible
allele pairs for the locus. The individual's blood type is determined by which combination
of alleles he/she has. There are four possible blood types in order from most common to
most rare: O, A, B and AB. The O blood type represents an individual who is
homozygous recessive (ii) and does not have an allele for A or B. Blood types A and B
are codominant alleles. Codominant alleles are expressed even if only one is present.
The recessive allele i for blood type O is only expressed when two recessive alleles are
present. Blood type O is not apparent if the individual has an allele for A or B.
Individuals who have blood type A have a genotype of IAIA or IAi and those with blood
type B, IBIB or IBi, but an individual who is IAIB has blood type AB.
Blood Type Chart
Parent 1
Parent 2
Child
Type O (i i)
Type O
Type O (i i)
Type A (IA)
Type A (IA)
Type A (IAIA)
Type B (IB)
Type B (IB)
Type B (IBIB)
Type A (IA)
Type O (i i)
Type A (IAi)
Type B (IB)
Type O (i i)
Type B (IBi)
Type A (IA)
Type B (IB)
Type AB (IAIB)
Polygenic Traits: (Example Eye Color, Height, Skin Color)
1. Modifier genes: (example: eye color) Eye color is not a simple trait controlled by
different alleles of one gene. Rather, eye color in controlled by at least 3 genes: EYCL, the
Green/blue eye color gene (probably located on chromosome 19); EYCL2, the central brown eye
color gene (possibly located on chromosome 15), and EYCL3 , the Brown/blue eye color gene
located on chromosome 15. These three genes contribute to the typical patterns of inheritance of
brown, green, and blue eye colors
For EYCL3, B = brown or b = blue eyes. However, grey eye color, hazel eye color, and multiple
shades of blue, brown, green, and grey come from the combined action of all three genes PLUS
the action of modifier genes that control how much pigment is deposited in the iris of the eye.
Eye color at birth is often blue, and later turns to a darker color. Why eye color can change after
birth not known (it is most likely due to changes in gene expression, but why?!) An additional
gene for green is also postulated, and there are reports of blue eyed parents producing brown
eyed children, which the three known genes can't easily explain. Mutations, modifier genes that
suppress brown, and additional brown genes are all potential explanations....so don't get worried
if your eye color doesn't seem to fit with the rest of your family!!!
Sex-Linked Traits: (Color blindness, hemophilia, muscular dystrophy)
Sex linkage is the phenotypic expression of an allele that is related to the chromosomal
sex of the individual. This mode of inheritance is in contrast to the inheritance of traits on
autosomal chromosomes, where both sexes have the same probability of expressing the trait.
Since, in humans, there are many more genes on the X than there are on the Y, there are many
more X-linked traits than there are Y-linked traits.
In mammals, the female is the homozygous sex, having two X chromosomes (XX), while the
male is heterozygous, having one X and one Y chromosome (XY). Genes that are present on the
X or Y chromosome are called sex linked genes.
Let’s do hemophilia and color blindness crosses.
Pedigree examples next.