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Chapter 4

by: Brittany Woody

Chapter 4 PCB3603

Brittany Woody

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About this Document

These are lecture, PowerPoint, and book notes for chapter 4.
Dr. W. Brad Barbazuk
Class Notes
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This 5 page Class Notes was uploaded by Brittany Woody on Tuesday September 20, 2016. The Class Notes belongs to PCB3603 at University of Florida taught by Dr. W. Brad Barbazuk in Fall 2016. Since its upload, it has received 28 views. For similar materials see Genetics in Genetics at University of Florida.


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Date Created: 09/20/16
Sunday, September 11, 2016 Chapter 4 Extensions to Mendelian Genetics - Modifications of the classic 3:1 or 9:3:3:1 ratios usually result from alleles that do not adhere to simple dominance/recessive, when more than one pair of genes influences a single character, or when traits in questions are linked to the X-chromosome - Allele is an alternate form of a gene (e.g. one form responsible for white flowers, another for purple) - Alleles responsible for the phenotype most often in a population (what we arbitrarily designate as ‘normal’) is the ‘wild-type’ allele; wild type alleles are often, but not always dominant (implies that most mutant alleles are recessive); mutant allele specifies an altered gene product; mutation is the source of new alleles (for better or for worse) - Often a mutation causes a reduction or loss of the wild-type function. e.g. a mutation in a gene whose product is an enzyme may change conformation or active site – resulting in reduced or loss of catalytic activity. - Loss of function alleles have some function; null alleles have no function; neutral mutation has a nucleotide change that does not change amino acid sequence (i.e. UCU -> UCC both Serine). - Phenotypic traits may be influenced by more than one gene and the allelic forms of each gene involved (consider an enzymatic pathway culminating in the production of an important molecule e.g. amino acid, or pigment) - Dominant alleles are usually indicated either by: • an italic uppercase letter (i.e D vs d for WT vs. dwarf) • letters (Wr) (uppercase signifies dominant form vs. wr for wrinkled) - Recessive alleles are usually indicated either by: • an italic lowercase letter (d) • an italic letter or group of letters with the + superscript (Wr ) (signifies the wt wing vs. the dominant wrinkled wing) - If no dominance exists, italic uppercase letters and superscripts are used to denote 1 2 W R alternative alleles (R , R , C , C ) 1 Sunday, September 11, 2016 - Incomplete dominance (aka partial dominance): a cross between contrasting traits produces an intermediate phenotype; neither allele is dominant in incomplete (partial) dominance - Assume alleles are of a gene that makes a pigment, and white is null; only produces ½ the pigment of the homozygous wildtype - Codominance: 2 alleles of a gene are responsible for producing 2 distinct and detectable gene products –> note the distinction between this case and incomplete dominance; this is a case of joint expression of 2 different alleles • e.g. MN blood group; carbohydrate antigen on the surface of blood cells – providing biochemical and immunological identity to individuals; 2 forms, N and M - Codominant inheritance is characterized by distinct expression of the gene product of both alleles . - There is a large amount of information stored within each gene, so there is no reason to assume that there are only 2 alleles per gene; simplest case of a multi-allele locus occurs when 3 alleles for a gene exist • classic and simple example: ABO blood groups; like the MN blood types, the ABO system is characterized by the presence of antigens on the surface of RBC; these are distinct from the MN antigen, and are under the control of a different locus • A and B antigens are carbohydrate groups; the I allele is responsible for an enzyme that can add the terminal sugar N-acetylgalactosamine (AcGalNH) to the H B substance; the I allele is responsible for a modified enzyme that cannot add N- acetylgalactosamine but instead can add a terminal galactose; the O phenotype results from an absence of either terminal sugar GENOTYPE ANTIGEN PHENOTYPE I IA A A I i A A I IB B B I i B B A B I I AB AB ii None O 2 Sunday, September 11, 2016 - Lethal mutations define essential genes; recessive lethal mutations usually have a loss of function mutation that creates a non-functional product; tolerated as a heterozygous where one allele will produce sufficient amounts of gene product - In some cases that lethal allele will have a distinctive (though on lethal) phenotype as a heterozygote; such an allele is recessive lethal, but dominant with respect to the on lethal phenotype - Dominant lethal mutation: one copy of the allele is lethal; sometimes because one copy of the allele may not provide the critical threshold of product needed; or, the new mutation may override the function of the wild type allele; Huntington’s chorea and Progeria (1 in 4 million) - Combinations of two gene pairs involving two modes of inheritance modify the 9:3:3:1 ratio: • Mendel’s principle of independent assortment applies to situations in which two modes of inheritance occur simultaneously, provided that the genes controlling each character are not linked on the same chromosome. The probability of each phenotype arising in a cross can be determined by the • forked-line method or by Punnett square, assuming that the genes under consideration undergo independent assortment. - Many traits characterized by a distinct phenotype are affected by more than one gene; in gene interaction, the cellular function of numerous gene products contributes to the development of a common phenotype - Epistasis occurs when alleles at one gene locus masks the effect of another gene (homozygous recessive at locus 1 overrides expression at locus 2, or presence of a dominant allele at locus 1 overrides expression at locus 2); or when two gene pairs complement each other such that one dominant allele is required at each locus to express a certain phenotype • The Bombay phenotype for ABO blood groups is an example of epistasis in which the homozygous recessive condition at one locus masks the expression of a second locus; the alleles at the H locus are epistatic to the alleles at the AB blood group locus (AB alleles are hypostatic to H locus) • In each case, distinct phenotypic classes are produced – each obvious from one another; genes involved are on different chromosomes, and thus sort independently; if A exhibits complete dominance, we can represent the diploid as 3 Sunday, September 11, 2016 A-, where the ‘-’ implies that either allele (A or a) could be present without phenotypic consequence; all P1 crosses involve homozygous individuals - When studying a single characteristic, a ratio expressed in 16 (e.g., 3:6:3:4) parts suggests that epistasis is occurring - Complementation testing: a test to determine if 2 mutations with similar phenotypes are alleles of the same gene- or represent alleles of different genes - Expression of a single gene may have multiple effects; pleiotropy occurs when expression of a single gene has multiple phenotypic effects, and it is quite common; examples are Marfan syndrome and porphyria variegate - Genes present on the X chromosome exhibit unique patterns of inheritance due to the presence of only one X chromosome in males, referred to as hemizygosity (not homozygous or heterozygous) Drosophila eye color was one of the first examples of X-linkage described (Thomas • H. Morgan); demonstrated that the inheritance pattern of the white-eye trait in Drosophila was clearly related to the sex of the parent carrying the mutant allele; concluded that the white locus is on the X chromosome -Sex-limited inheritance occurs in cases where the expression of a specific phenotype is absolutely limited to one sex; the genes are autosomal -In sex-influenced inheritance, the sex of an individual influences the expression of a phenotype that is not limited to one sex or the other; the genes are autosomal -In sex-limited and sex-influences inheritance, expression of autosomal genes responsible for a certain phenotype depends on the hormone constitution of the individual -Penetrance: percentage if individuals that show at least some degree of phenotype -Expressivity: reflects range of expression of the mutant phenotype; often phenotypes range in severity despite identical genotypes - If genetic background is constant in breeding, variation must be environmental 4 Sunday, September 11, 2016 - Modifiers of phenotype: genetic background and environmental effects; temperature sensitivity and suppression • Position effect: physical location of a gene in relation to other genetic material may influence it’s expression; can be seen if a region of a chromosome is relocated or rearranged such that it is now close to chromosomal regions that are prematurely condensed and “genetically inert,” referred to as heterochromatin - Mutations affected by temperature are called temperature-sensitive mutations, a type of conditional mutation - Nutritional mutations may prevent the phenotype from reflecting the genotype; phenyketonuria and lactose intolerance are human examples - Not all genetic traits become apparent at the same time during an organism life span; the age at which a mutant allele reveals a notable phenotype depends on events during the normal sequence of growth and development - As a result of genetic anticipation, some heritable disorders exhibit a progressively earlier age of onset with a increased severity in each generation (myotonic dystrophy) - In cases of genomic (parental) imprinting, phenotypic expression may depend on the parental origin of the chromosome; imprinting is thought to occur before or during gamete formation and may involve DNA methylation 5


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