incomplete dominance or codominance there are three possible

incomplete dominance or codominance there are three possible phenotypes,

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.

lethal alleles 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; Examples of diseases caused by recessive lethal alleles are cystic fibrosis, Tay-Sachs disease, sickle-cell anemia, and brachydactyly The lethal alleles modify the 3: 1 phenotypic ratio into 3 : 0. 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.

epistasis Defination of epistasis - A double mutant where one mutation masks the phenotype of another mutation. 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.

Duplicate recessive epistasis 9: 7 phenotypic ratio 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

Table 2: Results of the Cross Between Two Pea Plants with Genotype Cc. Pp Primula Petal Color Female Gametes CP Cp c. P cp CP CCPp Cc. PP Cc. Pp Cp CCPp Male Gametes c. P cp Cc. PP Cc. Pp CCpp Cc. Pp Ccpp cc. PP cc. Pp ccpp

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

Table 3: Results of the Cross Between Two Primula Plants with Genotype Kd. Dd Female Gametes KD KKDD KD Male Gametes Kd k. D kd KKDd Kk. DD Kk. Dd Kd KKDd KKdd Kk. Dd Kkdd k. D Kk. Dd kd Kk. Dd Kkdd kk. DD kk. Dd kkdd

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

Table : Results of the Cross Between Two Wheat Plants with Genotype Aa. Bb AB Male Ab Gam a. B etes ab Female Gametes AB Ab a. B ab AABB AABb Aa. BB Aa. Bb AAbb Aa. Bb Aabb Aa. BB Aa. Bb aa. BB aa. Bb Aabb aa. Bb 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 Fruit Color in Squash Complete dominance at both gene pairs; however, when one gene is dominant, it hides the phenotype of the other gene

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.

Genotype 9 W_G_ 3 W_gg 3 ww. G_ 1 wwgg Fruit Color Gene Actions White Dominant white allele negates effect of G allele Yellow Recessive color allele allows yellow allele expression Green Recessive color allele allows green allele expression

Table 4: Examples of Digenic Epistatic Ratios Ratio Description 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 Complete dominance at both gene pairs; however, when one gene is dominant, it hides the phenotype of the other gene Complete dominance at both gene pairs; however, when either gene is dominant, it hides the effects of the other gene Duplicate recessive epistasis 12: 3: 1 15: 1 13: 3 Dominant epistasis Duplicate dominant epistasis Dominant and recessive epistasis

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 IA, IB, and IO (the "I" stands for isohaemagglutinin). IA and IB are codominant and produce type A and type B antigens, respectively, which migrate to the surface of red blood cells, while IO is the recessive allele and produces no antigen. The blood groups arising from the different possible genotypes are summarized in the following table

Blood Group Genotype IA IA IA IO IB IB IB IO IA IB IO IO A A B B AB 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.

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