BIOL 121 Lecture : Biol 121- 2010.02.10- Genetics- Beyond Mendel (Ch. 13).docx
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13 extending mendel"s rules from genotype to phenotype. Alleles are either dominant or recessive, each allele codes for different phenotypes. There is no intermediate phenotype (e. g. black and white only, no grey) When an individual is heterozygous for a gene (one dominant allele, one recessive), the individual will have the dominant phenotype. The individual will only have the recessive phenotype if the individual has two recessive alleles. When ratios are not what is expected, then complete dominance is not the case. It will be either incomplete dominance or codominance instead. e. g. four o" clock flowers. F1: expect all purple (if purple is dominant), but instead get all lavender. This is the intermediate phenotype (lavender = b/t purple and white) F2: 1 purple : 2 lavender : 1 lavender (rr : 2 rr : rr same genotype as in complete dominance inheritance, but different phenotype)
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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. |