PCB3063 Exam 2 Study Guide Chapter 4 Section 4 1 Alleles Alter Phenotypes in Different Ways Recall that o Alleles are alternative forms of the same gene A a The allele that occurs more frequently in a population is sometimes referred to as the wild type allele and it if often but not always dominant o Mutation is the source of all alleles To understand the various modes of inheritance we must first examine the potential function of alleles New phenotypes due to phenotypes result from changes in the functional activity of the cellular product specified by that gene These include o Loss of function mutation a mutation in this gene may ultimately change the concentration of this enzyme and reduce or eliminate its affinity for the substrate If this loss is complete the mutation has resulted in what is called a null allele o Gain of function mutation a mutation in this gene enhances the function of the wild type product and it generally results in dominant alleles since one copy in a diploid organism is sufficient to alter the normal phenotype o Neutral mutations the gene product presents no change to either the phenotype or the evolutionary fitness of the organism Section 4 2 Geneticists Use a Variety of Symbols for Alleles Common nomenclature in genetics o Recessive is lowercased and italicized d dwarf o Dominant is uppercase and italicized D tall Common nomenclature for Drosophila o Is used to discriminate between wild type and mutant traits o For example ebony is a recessive body color mutation and gray is the normal type in fruit flies Wild type allele normal e Ebony allele mutant e So e e homozygote wild type e e heterozygote wild type and e e homozygote mutant Common gene functions and disorders o cdk yeast gene for cyclin dependent kinase o leu a bacterial gene mutation that interrupts the biosynthesis of the amino acid leucine o leu a wild type functional version of the gene o dnaA a bacterial gene involved in DNA synthesis o BRCA1 human genes in ALL CAPS breast cancer disease Section 4 3 Neither Allele is Dominant in Incomplete or Partial Dominance A cross between parents with contrasting traits may generate offspring with an intermediate phenotype For example if the P1 parents of red snapdragons X white snapdragons all of the F1 generation is pink leading to the incomplete or partial dominance The F1 generation crosses with itself to produce a F2 phenyotpic ratio of 1 Red 2 Pink 1 White A human example of the incomplete dominace is Tay Sachs disease in which homozygous recessive individuals are severly affected with a fatal lipid storage disorder when the enzyme hexosaminidase is absent The heterozygous is viable but with only 50 percent of the activity found in normal individuals Section 4 4 In Codominance the Influence of Both Alleles in a Heterozygote is Clearly Evident Codominance occurs when joint expression of two alleles of a gene in a heterozygote results in phenotypic detection of both gene products Inheritance in which two alleles of a gene pair in a heterozygote both have full phenotypic expression o An example is the MN blood group Section 4 5 Multiple Alleles of a Gene May Exist in a Population When three or more alleles of the same gene are found multiple alleles are present that The simplest case of multiple alleles is that in which three alternative alleles of one gene create a unique mode of inheritance exist the ABO Blood Group in humans o The ABO system is characterized by the presence of antigens on the surface of red blood cells o Four phenotypes are present A antigen A phenotype B antigen B phenotype A and B antigens AB phenotype or neither antigen O phenotype Genotype Antigen Phenotype IAIA IAi IBIB IBi IAIB ii A A B B AB O A A B B AB O The Bombay Phenotype o Both of the A and B antigens are derived from a precursor molecule called the H substance to which one or two terminal sugars are added The H substance is an inadequate substrate for the enzyme and this condition results in the expression of blood type O called the Bombay Phenotype The mother in this Bombay phenotype was able to pass the IB allele even though she was type O Section 4 6 Lethal Alleles Represent Essential Genes Essential genes are those that are required for survival Mutation resulting in the synthesis of a nonfunctional protein can often be tolerated in the heterozygote but not the recessive allele The recessive lethal allele result in a homozygous recessive individuals that do not survive Some homozygous lethal alleles can result in a new distinctive mutant phenotype A mutation in mice causes a yellow coat color that varies from the normal agouti coat phenotype But if the homozygous recessive allele is present in the yellow mice death occurs Some mutations may behave as a dominant lethal allele where one copy results in death Huntington s disease is due to an autosomal dominant allele In the heterozygote the onset of the disease is delayed until adulthood These affected individuals undergo gradual nervous and motor degenerations until they die Typical onset of this disease is around the age of 40 Section 4 7 Combinations of Two Gene Pairs with Two Modes of Inheritance Modify the 9 3 3 1 Ratio When two modes of inheritance occur together Mendel s principle of independent assortment applies as long as the genes are not linked on the same chromosome A mating between two humans both of whom are heterozygous for albinism and blood type AB the result is not 9 3 3 1 Section 4 8 Phenotypes are Often Affected by More Than One Gene The term gene interaction is often used to describe the idea that several genes influence a particular characteristic The cellular function of numerous gene products contributes to the development of a common phenotype Epigenesis a developmental concept whereby each step of development increases the complexity of this sensory organ and is under the control and influence of one or more genes o One example of epigenesis and multiple gene interactions involves the formation of the inner ear in mammals Mutations that interrupt many of the steps of ear development lead to a common phenotype hereditary deafness The mutant phenotype is described as a heterogeneous trait reflecting the many genes involved In epistasis the expression of gene or gene pairs masks or modifies the expression of another gene or gene pair o Homozygous presence of a recessive allele prevents epistatic or overrides the expression of alleles at another loci hypostatic This effect can either be
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