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BIOL 239
Christine Dupont

Biology 139 EXAM REVIEW Mendelian Genetics Why garden peas? Easy to cross fertilize Large numbers of offspring Short growing season Terms Phenotype - observable characteristic (largely determined by genotype). Commonly referred to as a trait Genotype - genetic make-up Monohybrid Cross matings between individuals that only differ in one trait Gene unit of inheritance Allele alternative form of a single gene Polymorphic More then one wildtype allele; allele frequency greater than 1% Monomorphic Only one wildtype allele; only one allele above 1% frequency Mendels law of segregation: The two alleles for each trait separate (segregate) during gamete formation then unite at random, one from each parent, at fertilization. At the DNA level, alleles vary in nucleotide sequence. This can result in: o New amino acid sequence o Change in the amount of protein o E.g. Garden peas R gives the pea a round shape because this version of the Starch Branching Enzyme is normal. The r allele as a different amino acid sequence and the enzyme is non-functional (i.e. no branched starch). Genetic Crosses Test Cross Breeding an unknown genotype with the homozygous recessive will reveal its genotype. If it was homozygous dominant, all progeny are heterozygous and have the same phenotype. If it was heterozygous, we would see a 1:1 phenotypic ratio. The law of product: The probability of two or more independent events occurring together is the product of the probabilities that each event will occur by itself. The law of sum: The probability of either of two mutually exclusive events occurring is the sum of their individual probabilities Dihybrid Cross Matings of individuals that differ in 2 traits. More phenotypic classes are seen because there are more allele combinations for independent assortment. Mating between dihybrids produces a 9:3:3:1 phenotypic ratio. Multihybrid Cross Matings that differ in 3 or more traits o Huge punnet square. Use a simpler method. E.g., Aa Bb Cc Dd x Aa Bb Cc Dd o Probability of an AA bb Cc Dd offspring: x x x =1/64 o Modifications of Mendellian Ratios Crossing When a gene has more than 2 alleles, reciprocal crosses help determine dominance relationships by crossing pure breeding lines. A dominance series shows most dominant to least dominant. Types of Dominance Incomplete Dominance The F H1brid doesnt resemble either parent (intermediate phenotype) o E.g. Flower pigments. A red flower and a white flower produce all pin1 F s. T2e F s will have a 1:2:1 phenotypic ratio. Codominance Both traits are expressed in the 1 Hybrid o E.g. A dotted parent and a spotted parent mate to make an 1 that is all dotted and spotted. The 2 generation also has a 1:2:1 ratio. Blood type is also an example of codominance. o I = makes enzyme to add A sugar B o I = makes enzyme to add B sugar o I or i = no enzymes - no sugars added A B o I I = both enzymes to add both A + B sugars Alleles and Inheritance Allele Frequency frequency of a given allele within a population o Mutant Allele Less than 1% o Wild-type Allele Greater than 1% Pleiotropy Multiple phenotypic effects from one allele (E.g., sickle cell anemia) Recessive Lethal Homozygotes for this allele die2 F is 2:1 (when heterozygotes are crossed). o E.g. The yellow allele in mice is the dominant for colour, but recessive for lethality. Multifactorial Inheritance Arising from the action of two or more genes (polygenic), or from interactions between genes and the environment. F is 9:3:3:1 (same as dihybrid cross) 2 Complementary Gene Action Two or more genes can work in tandem, in the same biochemical pathway to produce a particular trait.2F is 9:7 Heterogeneous Trait - A mutation at any one of a number of genes can give rise to the same phenotype Complementation Testing determines if 1 gene or 2 or more are involved in producing a trait. o If the mutation is on a different gene, complementation can happen. o No complementation on the same gene. Epistasis - A gene interaction in which the effects of an allele at one gene hide the effects of alleles at another gene Recessive when both alleles are recessive, it hides expression. Dominant dominant allele hides expression. Hypothesis Testing Can determine mode of inheritance fromF ratios (but not in humans) 2 o Must use pedigrees in humans Penetrance vs Expressivity Penetrance percentage of the population with a particular genotype, that demonstrate the expected trait Expressivity the degree or intensity with which a particular genotype is expressed in a phenotype within a population o E.g. Retinoblastoma 75% penetrance (25% do not develop the disease) of those who do get the disease, some get in it only one eye (30% expressivity) Sex-linked traits Due to genes ON the X or Y chromosome Sex-limited traits - affect a structure or process that is found in one sex but not the other Sex-influenced traits - show up in both sexes but their expression may differ between the two sexes Note: the environment can also influence the phenotype of a given genotype. (Himalayan coat) Conditional lethality occurs with a particular type of allele which is lethal under only certain conditions o Conditions are said to be permissive or restrictive Pedigree Analysis Many traits do not show the Mendelian pattern of inheritance because most traits in humans are influenced by more than one gene. Most single gene traits in humans are life-threatening or disabling Examples of single-gene traits: Thalassemia, Sickle-cell disease and PKU. Thalassemia: there is a mutation in front of the coding region for the B-globin chain of haemoglobin. Sickle-cell anemia: there is a change in the amino acid sequence that changes the conformation of the B-globin protein. Albinism and Huntingtons are also single-gene traits seen in humans. Understanding pedigrees Symbols: The propositus is the first person to be investigated in the pedigree In a pedigree of a dominant trait affected individuals appear frequently in each generation. In a pedigree of a recessive trait affected individuals appear sporadically. Traits in pedigrees can be autosomal or sex linked Autosomal trait: trait due to a gene that is on a chromosome that is not involved with determining the sex of the individual. Sex linked trait: trait due to a gene that is present on a gene on the X or Y chromosome. X-linked dominant traits, i.e. colour blindness, are more prevalent in males then females. Females have a second X chromosome to compensate for the other X. Chromosomes Chromosomal theory of inheritance Hereditary information is on genes and genes are located on chromosomes. Egg and sperm contribute equally to the creation of offspring. In humans, female sex chromosomes are XX In humans, male sex chromosomes are XY Each parent contributes one of the sex chromosomes, either an X or a Y. Problems during chromosome donation by parents can cause differences in females and males.
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