there are three possible phenotypes, incomplete dominance or codominance

Incomplete Dominance (Blended Inheritance) Meaning. In Mendel's pea experiment, dominance was essentially complete. Consequently, there was practically no difference between the homozygous and the heterozygous plants in the expression of a dominant character. For instancea (TT) tall pea plant was almost similar to a (Tt) tall pea plant. However, this is not true of all characters or organisms. There are characters or alleles that are not dominant or recessive. In such cases, both the alleles of the contrasting conditions of a character express as a blend (mixture).

With the result, the hybrid produced by crossing two pure individuals does not resemble either of them, but is midway between them. The expression of the traits of two pure parents as an intermediate condition or fine mixture in the Fi hybrids is known as incomplete or partial dominance. It is also called blended or intermediate or mosaic inheritance. It is due to -the fact that the dominant character or gene is not ■ in a position to completely suppress the recessive \ one. With the result, the heterozygote has a dif-ferent phenotype (as well as a different genotype) -from homozygotes for either allele.

Andalusian fowl.

1. Four-O'clock Plant (Figs and 5.24). A cross between a plant pure for red flowers and a

lethal alleles Examples of diseases caused by recessive lethal alleles are cystic fibrosis, Tay-Sachs disease, sickle-cell anemia, and brachydactyly cystic fibrosisTay-Sachs diseasesickle-cell anemia brachydactyly The lethal alleles modify the 3: 1 phenotypic ratio into 3 : 0. A mutant form of a gene that eventually results in the death of an organism if expressed in the phenotype. Most lethal genes are recessive; sickle-cell anaemia results from a recessive lethal gene that causes the production of abnormal and inefficient haemoglobin.

Dominant Lethal Allele - Quickly eliminated from the population, because usually causes death before the individual can reproduce. In 1905, Lucien Cuénot observed unusual patterns when studying inheritance of a coat color gene in mice. After mating two yellow mice, he observed that the offspring never showed a normal 3:1 phenotypic ratio. Instead, Cuénot always observed a 2:1 ratio, with two yellow mice for every one non-yellow mouse (Cuénot, 1905; Paigen, 2003). Cuénot thus determined that yellow coat color was the dominant phenotypic trait, and by using test crosses, he showed that all his yellow mice were heterozygotes. However, from his many crosses, Cuénot never produced a single homozygous yellow mouse.

Defination of epistasis - A double mutant where one mutation masks the phenotype of another mutation. epistasis Note that epistasis is not the same thing as dominance. With epistasis a mutation in one gene masks the expression of a different gene. With dominance, one allele of a gene masks the expression of another allele of the same gene.

9:7 phenotypic ratio Duplicate recessive epistasis Flower Color in Peas Complete dominance at both gene pairs; however, when either gene is homozygous recessive, it hides the effect of the other gene

Many years after Bateson first described this 9:7 phenotypic ratio in pea plants, researchers were finally able to determine the two genes responsible for it (Dooner et al., 1991). These genes control flower color by controlling pea plant biochemistry, in particular that related to pigment compounds called anthocyanins. In peas, there is a two-step chemical reaction that forms anthocyanins; gene C is responsible for the first step, and gene P is responsible for the second (Figure 2).

If either step is nonfunctional, then no purple pigment is produced, and the affected pea plant bears only white flowers. The dominant C and P alleles code for functional steps in anthocyanin production, whereas the recessive c and p alleles code for nonfunctional steps. Thus, if two recessive alleles occur for either gene, white flowers will resul tt

Female Gametes CPCpcPcp Male Gametes CP CCPPCCPpCcPPCcPp Cp CCPp CCpp CcPp Ccpp cP CcPPCcPp ccPPccPp cp CcPp CcppccPpccpp Table 2: Results of the Cross Between Two Pea Plants with Genotype CcPp Primula Petal Color

dominant suppression epistasis 13:3 phenotypic ratio Primula plant Complete dominance at both gene pairs; however, when either gene is dominant, it hides the effects of the other gene

Female Gametes KDKdkDkd Male Gametes KD KKDDKKDdKkDDKkDd Kd KKDd KKdd KkDd Kkdd kD KkDDKkDdkkDDkkDd kd KkDd Kkdd kkDdkkdd Table 3: Results of the Cross Between Two Primula Plants with Genotype KdDd

Duplicate dominant epistasis 15:1 phenotypic ratio Wheat Kernel Color Complete dominance at both gene pairs; however, when either gene is dominant, it hides the effects of the other gene

Female Gametes ABAbaBab Male Gam etes AB AABBAABbAaBBAaBb Ab AABbAAbbAaBbAabb aB AaBBAaBbaaBBaaBb ab AaBbAabbaaBbaabb Table :Results of the Cross Between Two Wheat Plants with Genotype AaBb

In this cross, whenever a dominant allele is present at either locus, the biochemical conversion occurs, and a colored kernel results. Thus, only the double homozygous recessive genotype produces a phenotype with no color, and the resulting phenotypic ratio of color to noncolor is 15:1.

Dominant epistasis 12:3:1 phenotypic ratio Complete dominance at both gene pairs; however, when one gene is dominant, it hides the phenotype of the other gene Fruit Color in Squash

the gene symbols are W=white and w=colored. At the second gene yellow is dominant to green, and the symbols used are G=yellow, g=green. If the dihybrid is selfed, three phenotypes are produced in a 12:3:1 ratio. The following table explains how this ratio is obtained.

GenotypeFruit ColorGene Actions 9 W_G_White Dominant white allele negates effect of G allele 3 W_ggWhite Dominant white allele negates effect of G allele 3 wwG_Yellow Recessive color allele allows yellow allele expression 1 wwggGreen Recessive color allele allows green allele expression

RatioDescription Name(s) of Relationship (Used by Some Authors) 9:7 Complete dominance at both gene pairs; however, when either gene is homozygous recessive, it hides the effect of the other gene Duplicate recessive epistasis 12:3:1 Complete dominance at both gene pairs; however, when one gene is dominant, it hides the phenotype of the other gene Dominant epistasis 15:1 Complete dominance at both gene pairs; however, when either gene is dominant, it hides the effects of the other gene Duplicate dominant epistasis 13:3Complete dominance at both gene pairs; however, when either gene is dominant, it hides the effects of the other gene Dominant and recessive epistasis Table 4: Examples of Digenic Epistatic Ratios

MULTIPLE ALLELES if there are 4 or more possible phenotypes for a particular trait, then more than 2 alleles for that trait must exist in the population. We call this "MULTIPLE ALLELES".

An excellent example of multiple allele inheritance is human blood type. Blood type exists as four possible phenotypes: A, B, AB, & O. There are 3 alleles for the gene that determines blood type.

The ABO system in humans is controlled by three alleles, usually referred to as I A, I B, and I O (the "I" stands for isohaemagglutinin). I A and I B are codominant and produce type A and type B antigens, respectively, which migrate to the surface of red blood cells, while I O is the recessive allele and produces no antigen. The blood groups arising from the different possible genotypes are summarized in the following table codominant antigens recessive

Genotype Blood Group I A A I A I O A I B B I B I O B I A I B AB I O O

Polygenic traits are the result of the interaction of several genes. For instance, phenotypes like high blood pressure (hypertension) are not the result of a single "blood pressure" gene with many alleles (a 120/80 allele, a 100/70 allele, a 170/95 allele, etc.) The phenotype is an interaction between a person's weight (one or more obesity genes), cholesterol level (one or more genes controlling metabolism), kidney function (salt transporter genes), smoking (a tendency to addiction), and probably lots of others too. Each of the contributing genes can also have multiple alleles. Polygenic traits

Hair color in humans is a polygenic trait. Eye color is, too.