BIOL 1090 Chapter Notes - Chapter p 63-70: Cinnabar, Peptide, Chinchilla
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13 Oct 2016
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Me(cid:374)del"s e(cid:454)peri(cid:373)e(cid:374)ts esta(cid:271)lished that ge(cid:374)es (cid:272)a(cid:374) e(cid:454)ist i(cid:374) alter(cid:374)ate for(cid:373)s. Genes can exist in more than two allelic states, and each allele can have a different effect on the phenotype. An allele is dominant if it has the same phenotypic effect in heterozygotes as in homozygotes that is, the genotypes aa and aa and phenotypically indistinguishable. Sometimes a heterozygote has a phenotype different from that of either of its associated homozygotes. An example of this is flower colour in the snapdragon, white and red varieties are homozygous for different alleles of a colour determining gene and when crossed they produce heterozygotes that have pink flowers. The allele for red colour (w) is therefore said to be incompletely, or partially, dominant over the allele for the white colour (w). The most likely explanation is that the intensity of pigmentation in this species depends on the amount of a product specified by the colour gene.
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These all relate to exceptions to the inheritance patterns encountered by Mendel.â
Why do multiple and lethal alleles often result in modifications of the classic Mendelian monohybrid and dihybrid ratios?
Select the four correct statements.
-When an essential gene is mutated, it can result in a lethal phenotype. There are no classic Mendelian monohybrid and dihybrid ratios. |
-In the case of codominance, heterozygotes produce gene products from both alleles of a gene. Classic Mendelian monohybrid and dihybrid ratios are modified by codominance. |
-In the case of incomplete dominance, the phenotype of the heterozygote is distinct from and often intermediate to the phenotypes of homozygous individuals. Classic Mendelian monohybrid and dihybrid ratios are modified by incomplete dominance. |
-Genes exist in a large number of allelic versions and a diploid organism has two homologous gene loci that may be occupied by different alleles of the same gene. This can result in many different phenotypes for traits, which may not follow typical Mendelian ratios. |
-When an essential gene is mutated, it can result in a lethal phenotype. This results in a modification of classic Mendelian ratios. |
-The phenotype of the heterozygous genotype is distinct from and often intermediate to the phenotypes of the homozygous genotypes. The joint expression of both alleles in a heterozygote is called codominance. There are no classic Mendelian monohybrid and dihybrid ratios. |
-Genes exist in a large number of allelic versions, but in a diploid organism, only one allele of the gene can occupy one homologous gene loci. Classic Mendelian inheritance cannot explain this phenomenon. |
-Each gene produces a unique gene product. The effect of one allele in a heterozygote completely masks the effect of the other. Classic Mendelian genetics cannot explain this phenomenon. |