Basic Genetics '4 ) Dominant Genes

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Dominant Genes If a gene (A) is completely dominant, AA and Aa are phenotypically alike. Phenotypesspecified by single gene substitutions are called dominants and those thatrequire homozygous combinations for expression are called recessives. Dominantsare easier to find than recessives, for dominants are fully expressed whenpaired with either allele. The individual's genotype may be homozygous orheterozygous if they express a dominant trait. In dominant the trait will beexpressed 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. Insimple or complete dominance the heterozygote, even though geneticallydifferent, has the exact same phenotype as one of the homozygotes. This leadsto the conclusion that Aa is equal to AA, phenotypically speaking. Therecessive gene is present in the heterozygote but hidden by the dominant. Dominanceis then considered a physiological effect.In Mendel's experiments all the chosen genes showedcomplete dominance, except flowering time. One of his plants flowered early,one late and surprisingly two flowered somewhere in the middle. Even though thegenotype ratio remains the same, it is the phenotype ratio of 3:1 dominant-recessivethat changes to 1:2:1. The absence of complete dominance makes every genotypedifferent. The examples of dominance in the garden peas were flower color andseed shape. But in other plants the flower color is not necessarily one coloror the other, meaning that the plant may express a color between the two. Eversince the time of Mendel, examples of partial of incomplete dominance have beendiscovered in animals and plants. In incomplete dominance the heterozygoteshows a phenotype which in between the homozygous recessive and homozygousdominant phenotypes. Snapdragon Crosses An excellent example of incomplete dominance aresnapdragon flowers. When one crosses a red flowered snapdragon with a whiteflowered, all of the F1 generation have pink heterozygous flowers. It appearsthat 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. TheF2 generation have all three colors; red, pink and white, with a ratio of1:2:1. This is a definite exception to the 3:1 ratio that is observed with allexamples of complete dominance. Incomplete dominance causes a distortion of thenormal phenotypic ratio. For one to fully understand the possibility of pinkflowers, remember that the gene for flower color controls the amount of pigmentin the flower petals. Each allele is a code for a specific amount of pigment. Whenboth alleles for pigment are present the petals have a dark red color due tothe heavy production of pigment. On the other hand if none of the alleles forpigment 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 heterozygousphenotype (Rr) coincides with the phenotype of one of the homozygotesphenotypic effect of the heterozygote (rr) can then be termed incompletedominance.Codominance and Blood Types Two alleles in a gene pair are each associated with different substances. Whenboth substances appear together in heterozygotes, codominance occurs. The twoalleles of a pair at a specific locus are not identical but the expression ofboth is observed. Codominance is clearly different than incomplete dominance.An example of codominance is the ABO blood typing stem used to determine thetype of human blood. It is common knowledge that a blood transfusion can onlytake place between two people who have compatible types of blood. Human bloodis separated into different classifications because of the varying proteinscontained in each blood type's red blood cells. These proteins are there toidentify whether or not the blood in the individual's body is it's own and notsomething the immunity system should destroy.The protein's structure is controlled by threealleles; i, IA and IB. The first allele is, i, the recessive of the three, andIA and IB are both codominant when paired together. If the recessive allele iis paired with IB or IA, it's expression is hidden and is not shown. When theIB and IA are together in a pair, both proteins A and B are present andexpressed.The ABO system is called a multiple allele system forthere are more than two possible allele pairs for the locus. The individual'sblood type is determined by which combination of alleles he/she has. There arefour possible blood types in order from most common to most rare: O, A, B andAB. 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 codominantalleles. Codominant alleles are expressed even if only one is present. Therecessive allele i for blood type O is only expressed when two recessivealleles are present. Blood type O is not apparent if the individual has anallele for A or B. Individuals who have blood type A have a genotype of IAIA orIAi and those with blood type B, IBIB or IBi, but an individual who is IAIB hasblood type AB.