Lecture 26 exceptions

Lecture 26 exceptions - L ectur e 26 - Extensi ons a nd...

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Lecture 26 - Extensions and Exceptions to Mendelian I nheritance Campbell, Chapter 14 pp 271-285 1. Given a simple family pedigree, deduce the genotypes for some of the family members. 2. Explain how phenotypic expression of the heterozygote differs when carrying alleles that are incompletely or co- dominant. 3. Explain why certain diseases (for instance Tay-Sachs) are considered recessive at the level of the organism, but not at the molecular level. 4. Explain the molecular basis for a dominant versus recessive allele. 5. Describe the inheritance of the ABO blood system and explain why the I A and I B alleles are said to be co-dominant. 6. Define and give examples of epistasis. 7. Describe and give examples of how environmental conditions can influence phenotypic expression. 8. Distinguish between the specific and broad interpretations of the terms phenotype and genotype. ADVI CE: Doing the problems at the end of the chapter will help you understand genetics. You will see similar types of problems on the exam. Learning Objectives
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THE RELATIONSHIP BETWEEN GENOTYPE AND PHENOTYPE IS OFTEN NOT SO SIMPLE. THERE ARE MANY EXAMPLES OF EXTENSIONS OR WHAT APPEAR TO BE EXCEPTIONS TO THE PATTERN OF INHERITANCE THAT MENDEL OBSERVED. I NH ERI TANCE OF CH ARACTERS DETERMI NED BY A SI NGLE GENE CAN DEVI ATE FROM SI MPLE MENDELI AN PATTERNS WH EN: 1. ALLELES ARE NOT COMPLETELY DOMI NANT OR RECESSI VE. 2. A PARTI CULAR GENE H AS MORE TH AN TWO ALLELES. 3. A SI NGLE GENE AFFECTS MORE TH AN ONE PH ENOTYPE.
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What is a dominant allele? I n a heterozygote, the dominant allele is the allele that is fully expressed as the phenotype. Remember that alleles are just alternative forms of the same gene. Therefore if the gene product is a protein, then two different alleles encode altered forms of the same protein I n some cases dominance may simply be a function of gene dosage - A Tay- Sachs heterozygote makes half the amount of hexosaminidase as the homozygous wild type individual. This is an example of complete dominance at the phenotypic level but incomplete dominance at the biochemical level. Dominant alleles are not necessarily found more frequently among the population For instance, the polydactyly allele that leads to growth of extra fingers or toes is dominant, yet is found only rarely in the population digits are recessive pp homozygotes.
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Lecture 26 exceptions - L ectur e 26 - Extensi ons a nd...

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