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4. Phenotypes and Gene Interactions.pdf

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Biology (Sci)
BIOL 202
Tamara Western

Phenotypes and Gene Interactions: Phenotypes come from the interaction between alleles within genes, and also between different genes for biochemical, developmental and signalling pathways  We have looked at dominance before, but now we will look at the three different pathways in how it could work: o Complete dominance is the typical example we have seen before o Incomplete dominance, like red, white and pink flowers o Codominance, in which more than one phenotype can be expressed at the same time Complete dominance can be seen in recessive mutations, which we have seen before, at the molecular level  The homozygous wild type has two dominant alleles, which means that each chromosome can create a fully functional protein, resulting in full function  In the heterozygous individual, one of the genes is mutated, resulting in the production of a non-functional protein, but the other copy of the gene is fine o In this case, the 1 dose of protein from the working allele is enough to maintain normal function or phenotype, known as haplosufficiency  The homozygous recessive individual is the only one with a different phenotype because it has NONE of the function protein because both loci are mutated Incomplete dominance can be seen in two instances; either with Haploinsufficieny or with dominant negatives  Haploinsufficieny is basically the same as haplosufficiency, except that the 1 dose of protein in a heterozygote is not enough to maintain a normal phenotype o This can be seen, for example, in the red, white and pink flowers we have already looked at  The red flowers have 2 doses (WW), meaning they made two doses of color pigment, while the ww plants make none, resulting in the white color  The pink flowers only make 1 dose of red pigment, which is not enough to the full red color o Haploinsufficieny can also be seen in polygenic trait dosages, like we did before for height; different doses of the protein can cause different height gradients, not just tell short and medium  Dominant negative has a different mechanism; when you have 2 doses (e.g. AA), then the proteins all form proper dimers which can have a normal phenotype o In a recessive homozygote, aa, the altered proteins will form another dimer, which could have a different function, or no function  Other mechanisms in the body may make up for the non-functional proteins, or not, which would result in a recessive lethal mutation o However, in a heterozygote, the altered protein will bind to the normal protein and will prevent the normal protein from doing its function o An example of a dominant negative mutation is brittle bond disease, which is caused in a mutation for the collagen gene  The “bad” protein wraps itself around the wild type and prevents the proper folding, which leads to improper function Codominance is when both genes’ products can be expressed if they are both present in the cell  The most common example of this is with blood types, in which the different alleles cause the expression of a different antigen on the red blood cell surface o The I allele causes the expression of a certain type of antigen (anti-A), while the I allele produces anti-B, and i, the recessive allele, producesA A antiAen  I /I or I /i have a blood type A because the produce only anti-A antigens B B B  I /I or I /i have blood type B as they only produce anti-B  I /I are type AB because they produce BOTH antigens  i/i are type O because they have no antigens  This is essentially the basis of codominance; the heterozygotes are capable of expressin
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