Bio190-ExtendingMendel - 4 Relationships between genotype...

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4. Relationships between genotype and phenotype is rarely as simple as Mendel’s Peas 0. Mendel had good fortune to choose a system that was relatively simple genetically 0. Each character is controlled by a single gene 1. Each gene has only two alleles, one of which is completely dominant to the other 2. The heterozygous F 1 offspring always looked like one of the parental varieties a. However, there is a range of dominant/recessive relationships between alleles Some alleles show Incomplete dominance where heterozygotes show a distinct intermediate phenotype, not seen in homozygotes - Characteristics kind of blend…mask each other. 0. Offspring of a cross between heterozygotes will show three phenotype ratios: 1:2:1 (same as the genotype ratio) 1. This is not blended inheritance because the traits are separable (particulate) as seen in further crosses 0. An example is seen in flower color of snapdragons A cross between a white-flowered plant and a red-flowered plant will produce all pink F 1 offspring Self-pollination of the F 1 offspring produces 25% white, 25% red, and 50% pink offspring 0. Complete and incomplete dominance are part of a spectrum of relationships among alleles 1. Codominance occurs when two alleles affect the phenotype in separate, distinguishable ways - Each characteristic can be seen The blood groups of humans are due to the presence of specific molecules on the People of type M (genotype MM) have one type of molecule on their red blood cells, people of type N (genotype NN) have the other, and people of type MN (genotype MN) have both molecules present 2. Dominant/recessive relationships are often traced to the presence or absence of a functional gene (—> functional protein) People with Tay-Sachs disease lack a functioning enzyme to metabolize gangliosides (a lipid), which thus accumulate in the brain, harming brain cells, and ultimately leading to death Children with two Tay-Sachs alleles have the disease, while heterozygotes are “normal” (but produce less functional enzyme)
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0. Note: Because an allele is dominant does not necessarily mean that it is more common in a population than the recessive allele For example, polydactyly, in which individuals are born with extra fingers or toes, is due to an allele dominant to the recessive allele for five digits per appendage However, the recessive allele is far more prevalent than the dominant allele in the
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Bio190-ExtendingMendel - 4 Relationships between genotype...

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