Lecture 6 Answers - Lecture 6: Study Questions and Answers...

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: Study Questions and Answers 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. What is gene interaction? What is the difference between an epistatic gene and a hypostatic gene? Gene interaction is the determination of a single trait or phenotype by genes at more than one locus; the effect of one gene on a trait depends on the effects of a different gene located elsewhere in the genome. One type of gene interaction is epistasis. The alleles at the epistatic gene mask or repress the effects of alleles at another gene. The gene whose alleles are masked or repressed is called the hypostatic gene. 4. What is a recessive epistatic gene? Recessive epistasis occurs when the epistatic gene in a homozygous recessive state 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 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). 7. Palomino horses have a golden yellow coat, chestnut horses have a brown coat,
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This note was uploaded on 05/07/2011 for the course BIOLOGY 352 taught by Professor Townsend during the Spring '08 term at San Diego State.

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Lecture 6 Answers - Lecture 6: Study Questions and Answers...

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