Mendelian Genetics II
Mendelian Genetics II BIO 105 Cr.4
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Date Created: 11/16/15
Lecture 28 Mendelian Genetich Both dominant and recessive phenotypes exist in humans just as in peas 0 Some of these traits are considered disorders 0 Genotype organism s alleles o Phenotype manifestation of genotype Polydactyly is a rare dominant condition two alleles D causes trait ol does not cause trait DD and Dd are possible genotypes 0 Dominant traits DO NOT necessarily appear more frequently in a population Dominance means that it only takes a single copy of the gene for the disease phenotype to be expressed 0 Why does the dominant allele determine phenotype o In many cases the normal gene encodes a protein that has some positive function in the normal cell 0 Other mutant alleles represent a broken nonfunctional version of the gene 0 Often just having one allele make a functional protein is good enough so the phenotype will be wildtype Wildtype represents the majority allele found in the population at large and it may not always be dominant to the mutant alleles The wildtype allele represents the normal functional version of the gene found in the majority of the individuals in a population Some mutant alleles represent a protein that now has greater activity or an unregulated activity compared to the wildtype Since the mutant allele results in a greater or unregulated activity that supersedes that of the normal allele its phenotype will be expressed The presence of at least one copy of a dominant allele results in the presence of hyperfunctional protein in the cell which can cause the mutant phenotype However if a cell is homozygous recessive then they produce a normal or appropriately regulated protein that produces the wildtype phenotype 0 An example of a dominant allele that is not wildtype 0 Growth factor is a small protein release by cells to trigger other cells to divide undergo mitosisO 0 Binding of growth factor causes receptor to change conformation and dimerize o Signaling Intracellular part now binds other proteins to trigger cell division 0 Cancer is often caused by mutations that make growth factor receptor signaling too active dominant allele 0 The receptor has a mutation which causes it to be dimerized even when no growth factor is present 0 Any particular locus can have many different alleles present in a given population including quotnormalwildtypequot mutant dominant and mutant recessive traits Remember that any particular diploid organism can only have two alleles t is the collective gene pool that organism belongs to that has the many alleles When things differ from Mendel o Alleles can interact with each other in interesting ways Incomplete Dominance looks like neither parent Codominance looks like both parents Multiple Alleles Pleiotropy 0 Many traits are controlled by more than one gene Epistasis Polygenic Inheritance Dominance is not always complete 0 Crosses between truebreeding strains can produce hybrid with phenotypes different from both parent So far we have looked at traits that are clearly dominant or recessive But because many different alleles exist in a given population the way that the particular alleles interact may not always be strictly dominant or recessive 0 Red ower and white ower pink ower 0 Why is purple dominant and red isn t Purple one functional copy P is enough to ensure 100 activation of the enzymeencoding genes resulting in a quotfully purplequot petal regardless of whether the genotype is PP or Pp Pink One functional copy W is only enough enzyme activity to accumulate quot50 redquot pigment Red It s only once two W alleles 2 functioning copies are present that there is sufficient enzyme activity to produce quot100 redquot pigment such that the owers look red In Incomplete Dominance dominant alleles do not completely mask recessive alleles F1 looks like neither parent The amount of red pigment present is dependent on the amount of enzymes present which is dependent on the number of wildtype copies of the gene This is a dosage effect 0 Red WW l Amount of gene product 2x 0 Pink Ww l x 0 White ww I 0 o The c locus of domesticated cats encodes an enzyme called tyrosinase that performs the rst step in the melanin production pathway Cats with a cbc genotype have a Burmese or a sepia coat due to melanin deposition into the hairs of the coat Cats with a 66 genotype have a Siamese or near albino coat due to almost no melanin deposition into the hair shafts Cats with a 68 genotype have a quotTonkinesequot or mink colored coat Blood groups are based on a carbohydrate pattern on blood cells 0 There are two codominant alleles at the lloci that encode enzymes with different substrate speci city 0 Both the IA and 3 alleles encode enzymes that modify the baseantigen by adding sugar but they add different kinds of sugar 0 There is also an i allele that encodes a defective enzyme that cannot modify the Hantigen resulting in the OAntigen I 0 Both the activeenzyme encoding alleles MAUR are a dominant to the defective allele i but they are codominant with each other such that an 1A 13 individual will show both A and B antigens on the surface of the red blood cells 0 ln codominance the effects of different alleles are equally detectable in heterozygotes Both alternative traits are expressed in the F1 of a cross of two pure breeding parents Do variations on dominance relations negate Mendel s law of segregation o Dominance relations affect phenotype and have no bearing on the segregation of alleles Alleles still segregate randomly Gene products control expression of phenotypes differently Mendel s law of segregation still applies 0 Interpretation of phenotypegenotype relation is more complex Multiple Alleles o A gene can have more than two alleles o How many do we inherit Just twobut can pull from more than two in the gene pool COO o How many blood types Three I IA I B 39 i 0 ABO blood types in humans 4 blood group phenotypes A B AB and O Each pair has a different dominance relationship Biochemical basis 0 Complex membrane anchored molecules that cause a variation in the structure of the sugar polymer on the cell surface 0 3 possibilities Sugar type A B or none 0 The simplest rule we often use is quotone gene one traitquot But this is not always the case 0 Sometimes the same gene will control multiple traits Mendel s seedcolor gene Y is also involved in the recycling of chlorophyll from old dying leaves The same gene is involved in two different phenotypes Cases where a single gene is involved in multiple traits called pleiotopy White coat color and blue eyes in dogs is often co inherited with deafness o Pleitropic genes have mutant phenotypes that disrupt more than one process even seemingly unrelated such as pigment formation and ear development Pleiotropy o Heredity congenital deafness in dogs and cats is associated with defects in neural crest cells 0 Neural crest cells are precursors for both Melanocytes required for pigmentation of the coat and eyes And structures of the inner ear 0 Sometimes the same trait will be under control of multiple genes 0 Often complex traits will be under the control of multiple genes polygenic inheritance resulting in a pattern of inheritance that falls along a continuum of phenotypes o For example skin color in humans is controlled by multiple genes at least three that each contribute in a quantitative way to the phenotype of the organism