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●Codominance:a case in which there are only two possible phenotypes (the dominant and the recessive).○Codominance occurs when the effects of both alleles appear in the heterozygote (one allele is not dominated by the other).Codominance is especially common with traits that are polygenic (controlled by multiple alleles). One example ofmultiple-allele inheritance is the inheritance of the ABO blood type. There are three alleles of the gene for blood type: the Aallele, the B allele, and the O allele. The A allele codes for a modification of the erythrocyte surface protein randomly designated"A". Similarly, the B allele codes for modifications that produce the marker protein "B". The O allele codes for no modified markerprotein, effectively a null allele. If one of your alleles is A and the other is A or O, you have type A blood. Similarly, if you have twoB alleles or a B and an O allele, your blood type is B. If you have an A allele and a B allele, you have type AB blood. If you arehomozygous for O, you have type O blood. Notice that both A and B are inherited as dominant alleles, and O is inherited as arecessive allele.
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●Incomplete dominance is characterized by a third phenotype for the heterozygous genotype. Often this third phenotype is a blending of thetwo homozygous phenotypes. In the case of hair pattern, the HH phenotype is curly, while the H'H' phenotype is straight. The heterozygousgenotype is wavy--a blend of both curly and straight. The example pedigree shows a possible pedigree of this trait. Notice the "blended"outcomes for heterozygotes. This pedigree is based on each allele being inherited according to the probabilities that a Punnett Square wouldpredict. Actual families may differ somewhat because allele inheritance doesn't always follow the predicted probabilities.●The top image shows the possible blood types of offspring of parents with various blood types (shown in black above the parents). Thepossible blood types of the child are shown in blue. The A blood type could have the genotypes of AA or AO and the B blood type could havethe genotypes of BB or BO, resulting in more possible blood types for the child. The bottom image shows a possible pedigree for blood typebased on codominance inheritance patterns.Sex-Linked Inheritance:●Many genes for sex-linked traits are present on the X chromosomesbut are absent from Y chromosomes, so males (XY)) have only one copyof the alleles found on the X chromosome.○This produces pattern of heredity called X-linked inheritance- the most common type of sex-linked inheritance.○While Y-linked inheritance is possible, it’s extremely rare because the Y chromosome is small and doesn’t carry many genes.■One example of sex-linked inheritance is red-green color blindness, the most common type of color blindness. Thiscondition is characterized by defects in one of the opsingenes that cause a deficiency in either red- or green-sensitive
cones. As a result, red and green are seen as the same color (either red or green, depending on which cone is present).
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