CHAPTER 2 Transmission Genetics Heritage from Mendel The study of the patterns of inheritance from generation to generation is known as transmission genetics in eukaryotic organisms this is known as Mendelian genetics 2 1 Mendel took a distinctly modern view of transmission genetics Each parent contributed a number of separate and distinct elements of heredity factors that remained unchanged from one generation to the next Mendel worked with seven pairs of true breeding varieties where the plants produced only progeny like themselves when allowed to self fertilize The outcome of a genetic cross does not depend on which trait is present in the male and which is present in the female reciprocal crosses yield the same result The wrinkled mutation causes an inborn error in starch synthesis The normal gene encodes an enzyme starch branching enzyme I SBEI required to synthesize a branched chain form of starch known as amylopectin which helps peas shrink uniformly The most common form of a trait occurring in a natural population is the wild type The SBEI gene has become interrupted by insertion of a DNA sequence called a transposable element which are capable of moving from one location to another within a chromosome Many spontaneous mutations result from the insertion of transposable elements into a gene Gel electrophoresis is used to separate DNA molecules of different sizes Shorter fragments move faster and farther than long fragments W shorter moves farther A morphological trait is manifest plainly shown and readily perceived A molecular trait is only perceived by means of special methods such as gel electrophoresis 2 2 Genes come in pairs separate in gametes and join randomly in fertilization Genes are physical entities that come in pairs The paired genes separate segregate in the formation of reproductive cells The Principle of Segregation in the formation of gametes the paired hereditary determinants separate in such a way that each gamete is equally likely to contain either member of the pair Gametes unite at random in fertilization In a testcross the relative proportion of the different gametes produced by the heterozygous parent can be observed directly in the proportion of phenotypes of the progeny because the recessive parent contributes only recessive alleles 2 3 The alleles of different genes segregate independently The green color defect in an enzyme necessary to break down the green pigment chlorophyll The Principle of Independent Assortment segregation of the members of any pair of alleles is independent of the segregation of other pairs in the formation of reproductive cells 2 4 Chance plays a central role in Mendelian genetics The addition rule applies to mutually exclusive possibilities Addition Rule the probability of the realization of one or the other of two mutually exclusive possibilities A or B is the sum of their separate probabilities The multiplication rule applies to independent possibilities A sibship is a group of offspring from the same parents Multiplication Rule the probability of two independent possibilities A and B being realized simultaneously is given by the product of their separate probabilities 2 5 The results of segregation can be observed in human pedigrees In a pedigree a family tree that shows phenotypes circles females squares males Huntington disease is a dominant disease characterized by progressive nerve degeneration that results in severe physical and mental disability In a Mendelian dominant allele the trait affects both sexes every affected person has an effected parent and approximately half of the offspring of affected persons are affected Rare diseases traits that are dominant heterozygous Albinism is a recessive trait characterized by lack of pigment in the skin hair and iris In a Mendelian recessive allele the trait affects both sexes most affected persons have parents who are not affected carriers approximately of the children of heterozygous parents are affected and the parents of affected individuals are often relatives Mating between relatives constitutes inbreeding The trait to taste phenylthiocarbamide PTC is a simple Mendelian dominant inherited trait The molecular basis of the taster polymorphism is now known to reside in a taste receptor protein known as hTAS2R38 allele PAV taste AVI not taste PAV associated with bitter broccoli taste overall PTC structure is from a class of molecules called glucosinolates that most likely evolved as a chemical defense against plant eating insects 2 6 Dominance is property of a pair of alleles in relation to a particular attribute of phenotype When a gene affects multiple traits as most genes do a particular pair of alleles might show complete dominance for some traits but not others The total phenotype of an organism consists of many different physical and biochemical attributes and dominance may be observed for some of these attributes and not for others thus dominance is a property of a pair of alleles in relation of a particular phenotype Flower color in snapdragons Antirrhinum illustrates incomplete dominance Incomplete dominance is when the phenotype of the heterozygous genotype is between the phenotypes of the homozygous genotypes more frequent for morphological traits The human ABO blood groups illustrate both dominance and codominance Codominance means that the heterozygous genotype exhibits traits associated with both homozygous genotypes more frequent for molecular traits than morphological The O i allele has a single base deletion in codon 86 that shifts the translational reading frame of the mRNA resulting in an incomplete inactive enzyme An antibody is a protein that is made by the immune system in response to a stimulating molecule called an antigen and is specifically capable of binding to that one antigen Mechanism called tolerance prevents organism from producing antibodies against own antigens People of blood type O make both anti A and anti B antibodies those of blood type A make anti B antibodies those of type B make anti A antibodies those of type AB make neither Type AB is the universal recipient and type O is the universal donor A mutant gene is not always expressed in exactly the same way Variable expressivity refers to genes expressed to different degrees in different organisms Ex Inherited genetic diseases Penetrance refers to the proportion of organisms whose phenotype matches their genotype for a given trait A Penetrance of less than 100 incomplete