Genes contain information about a specific characteristic or trait and can either be dominant or recessive. Genes are found on chromosomes and each gene has a designated place on every chromosome, called a locus. Not all copies of a gene are identical and alternative forms of a gene, called alleles, lead to the alternative form of a trait. Alleles are a way of identifying the two members of a gene pair which produce opposite contrasting phenotypes. An allele of a gene is it's partner gene, for example b is an allele of B and vice versa. When the alleles are identical, the individual is homozygous for that trait. While if the pair is made of two different alleles, the individual is heterozygous. A homozygous pair of can be either dominant (AA, BB) or recessive (aa, bb). Heterozygous pairs are made up of one dominant and one recessive allele (Aa, Bb). In heterozygous individuals only one allele, the dominant, gains expression while the other allele, the recessive, is hidden but still present. Capital letters represent dominant genes and lower case letters, recessive genes. The word genotype is was created to identify genes of an individual and phenotype for the expression of the trait and genes. Phenotype and genotype are terms used to describe the difference between the visible expression of the trait vs. the actual gene makeup. An individual which expresses a dominant trait may carry a recessive allele, but the recessive expression is hidden by it's dominant partner.

In Mendel's garden pea experiment, he also crossed plants that had two different pairs of alleles. He made a dihybrid cross of round, yellow seeds vs. wrinkled green seeds. The genes for round and yellow are dominant over their alleles of wrinkled and green. In the F1 generation all seeds were round and yellow as expected. Out of 556 plants in the F2 generation, 315 were round and yellow, 108 were round and green, 101 were wrinkled and yellow and 35 were wrinkled and green. When brought down to its lowest terms, it comes to a ratio of nearly 9:3:3:1. Mendel observed that the results were the same as the product of two monohybrid crosses. This lead to the law of probability which states that "the chance of two or more independent events occurring together is the product of the chances of their separate occurrences." For example, crosses between parents that differ in three traits, called trihybrid crosses, are a combination of three monohybrid crosses together. Mendel also noticed that the pairs of alleles separated and behaved independently with respect to the other pair. Mendel then wrote the law of independent assortment which observes that independent combinations of different pairs of alleles may occur. Mendel believed that a single gene was responsible for one single trait all by itself. We now know that many genes have control over the production of traits. Individuals inherit genes from their parents, not traits. Another important fact is that genes behave as separate units and traits are the product of complex gene interaction.

Recessive Genes
Recessive genes can only be expressed in homozygous (aa) individuals. There are more heterozygous (Aa) carriers than homozygous (aa) carriers who actually express the trait. All three genotypes (AA, aa, Aa) are possible throughout any population. Even in carriers that are not phenotypically expressed (Aa), the recessive allele can be identified in a cross.

The three criteria for identifying recessive genes:

• The first appearance of the recessive trait within a family usually is in the children of the unaffected parents.
• 25% of the children will be express trait.
• Both males and females can express the trait unless it is a recessive sex linked gene.

Dominant Genes
If a gene (A) is completely dominant, AA and Aa are phenotypically alike. Phenotypes specified by single gene substitutions are called dominants and those that require homozygous combinations for expression are called recessives. Dominants are easier to find than recessives, for dominants are fully expressed when paired with either allele. The individual's genotype may be homozygous or heterozygous if they express a dominant trait. In dominant the trait will be expressed in all generations.

The 4 criteria for identifying dominant genes:

• If the trait if dominant, it will be expressed in all generations.
• The trait is passed from the affected parent to about 50% of his/her children.
• Any parent that does not express the trait does not transmit it to any of his/her children.
• Both males and females can express and transmit the trait.

Incomplete Dominance
Alleles are not always recessive or dominant, but have a range of dominance. In simple or complete dominance the heterozygote, even though genetically different, has the exact same phenotype as one of the homozygotes. This leads to the conclusion that Aa is equal to AA, phenotypically speaking. The recessive gene is present in the heterozygote but hidden by the dominant. Dominance is then considered a physiological effect.

In Mendel's experiments all the chosen genes showed complete dominance, except flowering time. One of his plants flowered early, one late and surprisingly two flowered somewhere in the middle. Even though the genotype ratio remains the same, it is the phenotype ratio of 3:1 dominant-recessive that changes to 1:2:1. The absence of complete dominance makes every genotype different. The examples of dominance in the garden peas were flower color and seed shape. But in other plants the flower color is not necessarily one color or the other, meaning that the plant may express a color between the two. Ever since the time of Mendel, examples of partial of incomplete dominance have been discovered in animals and plants. In incomplete dominance the heterozygote shows a phenotype which in between the homozygous recessive and homozygous dominant phenotypes.

Snapdragon Crosses

An excellent example of incomplete dominance are snapdragon flowers. When one crosses a red flowered snapdragon with a white flowered, all of the F1 generation have pink heterozygous flowers. It appears that the red and white colors were mixed together two create a pink pigment, but this proves to be untrue when you cross two plants from the F1 generation. The F2 generation have all three colors; red, pink and white, with a ratio of 1:2:1. This is a definite exception to the 3:1 ratio that is observed with all examples of complete dominance. Incomplete dominance causes a distortion of the normal phenotypic ratio. For one to fully understand the possibility of pink flowers, remember that the gene for flower color controls the amount of pigment in the flower petals. Each allele is a code for a specific amount of pigment. When both alleles for pigment are present the petals have a dark red color due to the heavy production of pigment. On the other hand if none of the alleles for pigment exist, the flower is then white. When one of the alleles is present, only half the pigment is produced, creating a pink shade. If the heterozygous phenotype (Rr) coincides with the phenotype of one of the homozygotes phenotypic effect of the heterozygote (rr) can then be termed incomplete 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.

Multiple Alleles
The source of new alleles was recognized as a gene change or a mutation. Alleles are located in corresponding parts of homologous chromosomes, only one member of a pair can be present in a given chromosome and only two are present in a cell of a diploid. Alleles are genes that are members of the same gene pair, each kind of allele affecting a trait differently than the other. A diploid organism has, by its definition, only two alleles at one time, yet exceptions to the rules do appear. Many examples were found where more than two alternative alleles, also called multiple alleles, are present. In these cases two or more different mutations must have taken place at the same locus but in different individuals or at different times. Multiple alleles are alternative states at the same locus. The different alleles of a series are usually represented by the same symbol. Subscripts and superscripts are used to identify different members of a series of alleles. Most alleles produce variations of the same trait, but some produce very different phenotypes.

The most famous example of multiple alleles was discovered in rabbits. It was known that Albino rabbits were produced on occasion in variously colored rabbit populations. After conducting a monohybrid cross between a colored and Albino rabbit, it was discovered that the members of a pair of alternative genes, either c or c+, must be responsible for colored or albino rabbits. A cross of homozygous colored (c+ c+) and albino (c c) rabbits was made and the F1 generation were all colored, while the F2 generation had three colored and one albino. This showed that one pair of alleles was involved, the wild c+ and the mutant allele c. It was determined that c+ was dominant over c.