Ch5FINAL - Chapter Five Extensions and Modifications of...

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Chapter Five: Extensions and Modifications of Basic Principles COMPREHENSION QUESTIONS *1. How do incomplete dominance and codominance differ? Incomplete dominance means the phenotype of the heterozygote is intermediate to the phenotypes of the homozygotes. Codominance refers to situations in which both alleles are expressed and both phenotypes are manifested simultaneously. *2. What is incomplete penetrance and what causes it? Incomplete penetrance occurs when an individual with a particular genotype does not express the expected phenotype. Environmental factors, as well as the effects of other genes, may alter the phenotypic expression of a particular genotype. 3. Explain how dominance and epistasis differ. Dominance is an allelic interaction of two alleles of a single gene or locus, in which the phenotype corresponds only to the dominant allele. Epistasis results from the interaction of two or more genes or loci, in which the phenotype of the organism is governed by the genotype at the epistatic gene or locus, masking the genotype of the other. 4. What is a recessive epistatic gene? Recessive epistasis occurs when homozygous recessiveness of the epistatic gene masks the interacting gene or genes. In the example from the text, being homozygous recessive at the locus for deposition of color in hair shafts (ee) completely masked the effect of the color locus regardless of whether it had the dominant black (B-) or recessive brown (bb) allele. *5. What is a complementation test and what is it used for? Complementation tests are used to determine whether different recessive mutations affect the same gene or locus (are allelic) or whether they affect different genes. The two mutations are introduced into the same individual by crossing homozygotes for each of the mutants. If the progeny show a mutant phenotype, then the mutations are allelic (in the same gene). If the progeny show a wild-type (dominant) phenotype, then the mutations are in different genes and are said to complement each other because each of the mutant parents can supply a functional copy (or dominant allele) of the gene mutated in the other parent. 6. What is genomic imprinting? Genomic imprinting refers to different expression of a gene depending on whether it was inherited from the male parent or the female parent. 7. What characteristics do you expect to see in a trait that exhibits anticipation? Traits that exhibit anticipation become stronger or more pronounced, or are expressed earlier in development, as they are transmitted to each succeeding generation.
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Chapter Five: Extensions and Modifications of Basic Principles *8. What characteristics are exhibited by a cytoplasmically inherited trait? Cytoplasmically inherited traits are encoded by genes in the cytoplasm. Because the cytoplasm usually is inherited from a single (most often the female) parent, reciprocal crosses do not show the same results. Cytoplasmically inherited traits often show great variability because different egg cells (female gametes) may have differing proportions of cytoplasmic alleles from random sorting of mitochondria (or plastids in plants).
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