Chapter 5 Extensions and Modifications of Basic Principles I. Multiple Alleles The ABO blood group has multiple alleles codominance and complete dominance. In codominance, both alleles are expressed simultaneously. In the ABO blood system, there are 3 alleles: I A I B i I A is dominant over i I B is dominant over i I A and I B are co-dominant Universal donor (type O) and universal recipient (type AB) 1. Dominance is a function of interaction between alleles. 2. Epistatsis is a function of interaction between genes. What pattern of inheritance is demonstrated in the following cross? 1
II. Sex limited and sex influenced inheritance What pattern of inheritance is demonstrated in the following cross? What pattern of inheritance is demonstrated in the cross above? What pattern of inheritance is demonstrated in the following cross? 2
What pattern of inheritance is demonstrated in the cross above? III. Dominance / Incomplete dominance IV. Gene interaction A. New genotypes can show up in the F 1 and F 2 generation. Eg. Comb type in chickens Walnut R-P Rose R-pp Pea rrp- Single rrpp What are the phenotypes of these parents, and what F 1 and F 2 offspring would you expect from the following crosses? RRPP x rrpp walnut x single F 1 : all walnut Note that your first hypothesis here would probably be a single gene where walnut is dominant. F 2 : walnut: rose: pea: single What are the phenotypes of these parents, and what F 1 and F 2 offspring would you expect from the following crosses? RRpp x rrpp rose x pea F 1 : all walnut Note that a novel phenotype appears in this generation. F 2 : walnut: rose: pea: single 3
9 3 3 1 aab- A-B- A-bb aabb B. Epistasis occurs when one gene masks the effect of a second gene. Gene interaction and/or epistasis may produce offspring ratios that are variations of a 9:3:3:1. 9:3:3:1 12:3:1 10:3:3 9:6:1 9:3:4 15:1 13:3 12:4 10:6 9:7 Dominant epistasis white: yellow: green W--- white: wwy- yellow: wwyy green Duplicate recessive epistasis pigmented: white A-B- pigmented: A-bb or aab- or aabb white 4
Two pathways that would yield a 9:7 (color : white) ratio. gene A gene B Color gene A (C?) gene B Color with two pathways you can also get a 9:3:3:1 ratio, but A- B- BLUE purple red blue white PURPLE aa aa B- PURPLE bb PURPLE BLUE BLUE If the functional a gene is recessive? blue white purple red If both functional genes are recessive? white blue red purple How do we get so many different ratios? A- PINK red pink white white 9 3 4 We have already seen that if the intermediate product is white (colorless) you get a : ratio. B- Recessive epistasis 5
and with a slight variation? aa PINK white white red pink 12 3 1 What ratio do you get if the intermediate product is white? B- but if these are duplicate pathways A- B- red red red white red : white Examples of Epistatic Ratios Sheperd s purse 15 triangular : 1 oval Summer squash 9 disk: 6 sphere: 1 elongate Fowl color 13 white: 3 colored Corn kernel color 9 purple: 7 white 1. Genes are discrete units that control the phenotype of organisms. 2. Inheritance follows the rules of segregation and independent assortment 3. Dominance is a function of interaction between alleles. 4. Genes control the production of enzymes and thus the function of biochemical pathways. 5. Epistatsis is a function of interaction between genes. Complementation test Determines whether two independently isolated mutations are at the same loci or different loci. Flower color Wile type is red, mutant is white. Consider two independently isolated white mutants, where wild type is dominant: w c w c 6
Cross them. Two possible outcomes Allelic: no complementation w Two possible outcomes Allelic: Different genes: no complementation complementation w c W c c C w V. Lethal alleles In a cross between two mutant lines (with the same phenotype), complementation results in a wild type phenotype and indicates that two mutations are in different genes. This is called a complementation test. Lethal alleles result in missing classes in a genetic cross Eg. 2:1 phenotypic ratio in a monohybrid cross VI. Non-Mendelian inheritance A. Maternal effects: influence of mother's genotype on phenotype of offspring B. Cytoplasmic inheritance: extra-nuclear genes that are found in chloroplasts and mitochondria 7
Snail shell coiling dextral (right hand) sinistral (left hand) Start with two true-breeding lines Dextral female x sinistral male: all dextral F1 Sinstral female x dextral male: all sinistral F1 Give a hypothesis about how this trait is controlled. A. Maternal effects Dextral female X sinistral male: dextral F1 selfing produces all dextral all Sinstral female X dextral male: all sinistral F1 selfing produces all dextral Genotype of mother determines cleavage pattern in egg, and that determines direction of coiling She is sinistral because her mother was ss. They are dextral because their mother was s + s. What will their offspring be? For traits determined by genetic maternal effects, the genotype of the mother (rather than of the gamete she contributes) determines the phenotype of the offspring through mrna in the egg. 8
B. Cytoplasmic inheritance organelles or particles Mitochondria and chloroplasts have own genome 1. Mitochondrial genome Rules of extranuclear inheritance 1. Uniparental inheritance leads to differences between reciprocal crosses. 2. Genes cannot be mapped to nuclear chromosomes. 3. Ratios associated with Mendelian traits cannot be found. 4. Extranuclear inheritance is persists despite nuclear substitution. Homoplasmic: all mitochondria identical or heteroplasmic: mitochondrial DNAs have more than one sequence Inheritance is uniparental; usually maternal. V. Imprinting: For traits determined by cytoplasmic inheritance, genes are encoded in the cytoplasm (usually in the mitochondria and chloroplast). These organelles usually show uniparental inheritance. Phenotype of offspring depends upon maternal or paternal source of altered allele i.e. which parent contributed the allele Mutants act like a dominant, but only through one parent. 9
Prader-Willi/Angelman syndrome Results from deletion of 15q11-15q13. But sometimes you got one phenotype and sometimes another. Angelman syndrome: jerky, repetitive, lurching body movements; seizures; incoherent speech; loud bursts of laughter; large mouths; red cheeks Prader-Willi syndrome: mild to moderate mental retardation; lack muscle tone; insatiably hungry so become obese; tiny hands and gonads. AS PWS Prader-Willi/Angelman syndrome Results from deletion of 15q11-15q13. Two different imprinted genes in the same region. Maternally inheritted deletion causes Angelman s Paternal deletion causes Prader-Willi For traits determined by genomic imprinting, only the allele inherited from the parent of one sex is expressed. VI. Anticipation: The severity increases or the age of onset gets early when you look across generations. Eg: Grandfather age of onset: 50 Mother age of onset: 35 Child age of onset: 5 Ascertainment bias? NO 10
For traits that show anticipation, mutant alleles are unstable and may change over even one generation. They are caused by trinucleotide repeats; more repeats results in earlier and/or more severe expression of the mutant phenotype. Mutations due to trinucleotide repeat expansion: (CAG) n Huntington s Several types of ataxia Fragile X (non-coding) Grandfather age of onset: 50 (CAG) 25 Mother age of onset: 35 (CAG) 50 Child age of onset: 5 (CAG) 120 VII. Environmental effects on gene expression Expression of many genes is modified by the environment. This is known as a gene x environment interaction. This is so common as to be virtually universal. 11