Chromosomes and Gender
Chromosomes are long, stringy aggregates of genes that carry heredity information. Gender is determined by the presence or absence of certain chromosomes.

Law of Segregation
The principles that govern heredity were discovered by a monk named Gregor Mendel in the 1860's. One of these principles is now called Mendel's law of segregation.

Independent Assortment
This principle of heredity formulated by Gregor Mendel states that traits are transmitted to offspring independently of one another.

Incomplete dominance:
A form of intermediate inheritance in which heterozygous alleles are both expressed, resulting in a combined phenotype. See below example

R = allele for red flowers W = allele for white flowers red x white ---> pink RR x WW ---> 100% RW

The trick is to recognize when you are dealing with a question involving incomplete dominance. There are two steps to this:
1) Notice that the offspring is showing a 3rd phenotype. The parents each have one, and the offspring are different from the parents.
2) Notice that the trait in the offspring is a blend (mixing) of the parental traits.


Co-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.

When it comes to punnett squares & symbols, it's the same as incomplete dominance. Use capital letters for the allele symbols. My example cross from above would look like so:

R = allele for red flowers W = allele for white flowers red x white ---> red & white spotted RR x WW ---> 100% RW

In class we have seen some examples can also use markers (I) and have a superscript to show the different trait as in human blood type AB, in which two types of protein ("A" & "B") appear together on the surface of blood cells..

A very very very very very common phenotype used in questions about co-dominance is roan fur in cattle. Like the one found in our hand out pakages from class.
Cattle can be red (RR = all red hairs), white (WW = all white hairs), or roan (RW = red & white hairs together). A good example of codominance.
Another example of codominance is

b) What percent of kittens would have tan fur if a tabby cat is crossed with a black cat?

The concepts presented here do not make Mendel's concept less meaningful in any way. They are simply information that has been discovered & added to his ingenious work, making this ball of gathered string and twine we call science just a wee bit bigger, ...and more colorful!