Animal Genetics Deviation from simple dominance Mendel, during his life, made a lot of experiments and in all of them all traits had clear dominantrecessive patterns. However, in nature, lack o strict dominance is widespread. The absence of dominance of one member, of a pair of alleles over the other is quite common in most organisms. If dominance were universal, the heterozygote would always have the same phenotype as the dominant homozygote. Incomplete dominance We refer to incomplete dominance (partial dominance) as these cases in which the phenotype of the heterozygote falls on some scale between the two homozygotes. To understand this, we will see it with one example: In four o clock plants (Mirabilis jalapa) we can Gross a plant with red flower petales with another with white petales, the offspring have pink flower petales. If these pink flowered F 1 plants are crossed, the F 2 plants appear in a ratio of 1:2:1, having red, pink or white petals, respectively. The pink flowered plants are heterozygotes in which a colour is intermediate between the red and white colours of the homozygotes. In this case, there is an Allele for red pigment colour (R 1 ) and another Allele that results in no colour (R 2, the flower petals have a white background colour). Flowers in heterozygotes (R 1 R 2 ) have about half the red pigment of the flowers in red homozygotes (R 1 R 1 ) because the heterozygotes have only one copy of the Allele producing colour whereas homozygotes have two copies of it.
Codominance Another exception to simple dominance is the full expression of two alternative alleles in a heterozygote, resulting in a phenotype in which the presence of both alleles can be detected. This is called codominance (the situation in which a heterozygote shows the phenotype effects of both alleles fully and equally). The best example of the codominance, and the most universal example, is the effect of the gene that determine the A, B, AB or 0 human blood groups, which are an example of multiple alleles too. Multiple alleles result from different mutations of the same gene. Blood type is determined by the type of polysaccharides (polymer of sugars) present on the surface of red blood cells. Two different polysaccharides, A and B, can be formed. The A type is synthesized by an enzyme coded by the I A allele, and the B type by an enzyme coded by the I B allele. People of genotype I A /I A produced cells having only the A polysaccharide and are said to have blood type A. Those of genotype I B /I B have red cells only with the B polysaccharide and have blood type B.
Heterozygous I A /I B people have red cells with both A and B polysaccharides and have blood type AB. The I A /I B genotype illustrates codominance of these alleles because both alleles are expressed. A third Allele I 0 codes for a defective enzyme that produces neither the A nor the B type of polysaccharide. The I 0 allele is recessive to both I A and I B. I A /I 0 heterozygotes have blood type A. I B /I 0 heterozygotes have blood type B. The homozygous recessives I 0 /I 0, which lack both the A and the B polysaccharides, are said to have blood type 0. Example Coat color in the short horn breed of cattle or in horses. In a cross of red coat cattle with white coat cattle, progeny of roan coat is obtained. In roan coat, the red and white hairs occur in definite patches but no hair has intermediate color of red and white. Pleiotropy Pleiotropy is the effect of a single gene on more than one characteristic. An example is the "frizzle-trait" in chickens. The primary result of this gene is the production of defective feathers. Secondary results are both good and bad; good include increased adaptation to warm temperatures, bad include increased metabolic rate, decreased egg-laying, changes in heart, kidney and spleen. Cats that are white with blue eyes are often deaf, white cats with a blue and an yellow-orange eye are deaf on the side with the blue eye. Sickle-cell anemia is a human disease originating in warm lowland tropical areas where malaria is common. Sickle-celled individuals suffer from a number of problems, all of which are pleiotropic effects of the sickle-cell allele. Gene interactions and modified Mendelian ratios The collaboration of several different genes in the production of one phenotypic character (or related group of characters). Epistasis is the interaction between two or more genes to control a single phenotype. Interaction involves one gene masking or modifying the phenotypic expression of another gene. No new phenotypes are produced by this type of gene interaction. A gene that masks
another gene s expression is said to be epistatic, and a gene whose expression is masked is said to be hypostatic The ratio of phenotype 9:3:3:1 observed in the F 2 offspring of parents dihybrid, from epistasis changes in relationship that are different combination of group 9:3:3:1. When epistasis take place between two gene loci, the number of phenotypes appearing in the offspring of parents dihybrid be less than four. There are six types of reports commonly recognized, three of whom have three phenotypes and the other three only two. 1. Epistasis dominant (12:3:1) When the dominant allele at a locus, for example the A allele, produces a phenotype without taking into account the condition of allelic locus, it is said that the locus A is the epistasis locus B. Moreover, since the A dominant allele is able to express all the same, in the presence of B as b, this is a case of epistasis dominant. Only when the genotype of homozygous recessive individuals is at the epistasis locus (aa), the hypostasis locus alleles (B or b) can express themselves. So genotypes A-B- and A-bb produce the same phenotype, while aab- and aabb produce two other phenotypes. The classic 9:3:3:1 is changed to 12:3:1. 2. Recessive epistasis (9:3:4) If the recessive genotype at a locus (for example aa) suppresses the expression of alleles in the B- locus, is said that the A- loci presents a recessive epistasis on locus B-.The alleles of the hypostasis B - locus can tell only if the dominant allele is present in the locus A-. The genotypes A-B- and A-bb produce two other phenotypes. The relationship 9:3:3:1 becomes 9:3:4. 3. Gene with double cumulative effect (9:6:1) If the dominant condition (homozygous or heterozygous) at each of the two locus (but not both) has the same phenotype, the ratio of F 2 becomes 9:6:1. For example, when epistasis genes are involved in the production of various quantities of a substance can be as a pigment, it can be considered that the dominant genotypes of each locus, independently produce one unit of pigment. So genotypes A-Bb and aab- produce one unit of each pigment and then have the same phenotype. The genotype aabb not produce pigment, but in A-Bgenotype effect is cumulative and is produced two units of pigment. 4. Duplicate gene action (15:1) The relationship 9:3:3:1 is changed to 15:1, if the dominant alleles of both loci each produce the same phenotype without cumulative effect.
5. Complementary gene action (9:7) Where are identical phenotypes produced by both genotypes homozygous recessive, the ratio of F 2 becomes 9:7. The genotypes aab-, A-bb and aabb produce a single phenotype: both dominant alleles if they are present together complement each other and produce a different phenotype. 6. Dominant suppression (13:3) There are only two phenotypes in the F 2 when a dominant genotype on a locus (for example A-) and the recessive genotype on locus (bb) produce the same phenotype effect. So A-B-, A-bb and aabb produce a single phenotype and aab- producing another in report 13:3. Example: Coat Color in Horses Yet another type of epistasis occurs when one gene interacts with another to modify - but not mask - a phenotype. For example, in horses, the extension gene determines whether an animal's coat color will be red or black; here, the dominant allele E produces black pigment in the coat, while the recessive allele e produces red pigment. All horses with genotype ee are therefore red, yet there are
many different types of red horses. These differences exist because of the action of epistatic modifier genes. One such modifier gene is called cream dilution. The cream dilution gene has two alleles: C Cr and C. The C Cr allele is semidominant; it dilutes red to yellow in the heterozygous state and red to pale cream in the heterozygous state. On the other hand, the C allele has no diluting effect on coat color. Thus, horses with genotype eecc are chestnut colored, and they have reddish-brown coats, tails, and manes. In contrast, horses with one copy of the C CR allele (genotype eecc CR ) are palomino (i.e., they have a gold coat with a white mane and tail), while horses with two copies of the C CR allele (genotype ee C CR C CR ) are cremello (i.e., basically white or cream colored). Polygenic Inheritance of complex traits Polygenic inheritance is a pattern responsible for many features that seem simple on the surface. Many traits such as height, shape, weight, color, and metabolic rate are governed by the cumulative effects of many genes. Polygenic traits are not expressed as absolute or discrete characters, as was the case with Mendel's pea plant traits. Instead, polygenic traits are recognizable by their expression as a gradation of small differences (a continuous variation). The results form a bell shaped curve, with a mean value and extremes in either direction. Conclusion The F 1 phenotype generated by each pair of alleles defines the dominance relationship between these alleles. One allele is not always completely dominant or completely recessive to another. With incomplete dominance, the F 1 hybrid phenotype resembles neither parent. With codominance, the F 1 hybrid phenotype contains observable components from both parents. Many allele pairs are codominant at the level of protein production. Two or more genes may interact in several ways to affect the production of a single trait. It is often possible to anive.at an understanding of these interactions by observing characteristic deviations from traditional Mendelian phenotypic ratios. In epistasis, the action of an allele at one gene can hide traits normally caused by the expression of alleles at another gene. One gene can contribute to multiple traits (pleiotropy); for such a gene, the dominance relation between any two alleles can vary according to the particular phenotype under consideration. A continuous trait can ha ve any value of expression between two extremes. Most traits of this type are polygenic, that is, determined by the interactions of multiple genes. The document was created: 28. 01. 2017 05:46:44 Source: http://web2.mendelu.cz/af_291_projekty2/vseo/