Presentation on theme: "Extensions of Mendelian Genetics"— Presentation transcript:
1 Extensions of Mendelian Genetics Mendelian genetics seems to be relevant to only a small set of heritable featuresFor only a few characters there are…Only 2 versions of an allele (green or yellow)1 gene codes for a single external character1 allele is completely dominant to the otherThe basic patterns of segregation & independent assortment apply to more complex patterns of inheritance
2 Different Types of Dominance Complete dominancePhenotype of heterozygote & HomoD are indistinguishableThe pattern with which you are already familiarIncomplete dominancePhenotype of heterozygote is in between the 2 Homo phenotypesExample: pink snapdragons
4 So incomplete dominance does NOT provide evidence for “blending” theories
5 Codominance Codominance Phenotype of heterozygote is separate & distinguishable from Homozygous Dominant & Homozygous RecessiveExample: AB blood type or Rhododendron flower
6 Dominance & PhenotypeThe observed dominance/recessiveness of alleles depends on the level of the investigationConsider Tay-Sachs diseaseBrain cells of the baby do not metabolize certain lipidsAs lipids accumulate, seizures, blindness, and mental degenerationDeath occurs within a few years of conception
7 Tay-Sachs Disease At the Organismal level, the disease is recessive Only children with 2 copies of the recessive trait will have the maladyHeterozygote is not afflicted – they produce some lipid- metabolizing enzyme, though not as much as in HomoDSo intermediate enzyme productionThis suggests the biochemical level, the disease is an example of incomplete dominanceWhich is Tay-Sachs: dominance or incomplete dominance?
8 Prevalence & Dominance PolydactylyExtra fingers or toes1 of 400 in the USThe allele for polydactyly is dominant, but rarely presentRecessive homozygotes (HomoR) are found 399 out of 400 instances
9 Blood TypingOnly 2 alleles existed for Mendel’s peas, but this is not typical for most traitsConsider ABO blood group in humansA refers to the “A” membrane carbohydrate & type A bloodB refers to (seriously, I’m not writing this down)O means neither A or B carbohydrate is foundAB means both A & B are foundBUT the A and B alleles are codominant and are both expressed if an individual inherits both alleles
11 EpistasisA gene at one locus alters the phenotypic expression of a gene at a second locusExample: Mouse fur colorBb or BB = Black bb = brownIf HomoR for (C) gene [cc], then no fur color (albino or white)Regardless of fur color specified by brown-black geneIf NOT HomoR for (c) gene [Cc or CC], then can be brown (bb) or black (Bb or BB)
12 What is the phenotype of… BBcc?BbCc?bbCC?Bbcc?BBCC?bbcc?
13 Pleiotrophy Single gene has multiple effects Should be unsurprising given intricate molecular and cellular interactions for development of an organismPhenylketonuriaMental RetardationReduced skin and hair pigmentation
14 Polygenic Inheritance The additive effect of 2 or more genes on 1 phenotypic characterCalled quantitative characters since there is a continuum of gradationsNormal curve of phenotypesExample: human skin pigmentation is determined by at least 3 separately inherited genesAABBCC = DarkAaBbCc = Intermediateaabbcc = Light
16 Pedigree AnalysisFamily tree describing the interrelationships of parents & children across the generations
17 Recessively Inherited Disease Requires 2 copies of the recessive allele (Homozygous Recessive) to express the malityHeterozygotes are called carriersNormal phenotype, but may transmit disease to offspringExamples:Cystic FibrosisTay Sachs diseaseSickle-cell disease
18 Cystic Fibrosis Recessive autosomal disease Common in those of European descent1 of 2,500 affected, but 1 of 25 are carriersAffects Chloride ion transport between a cell and extracellular fluidIf untreated, most die before 5th birthdayTypically, patients live until their 20s or 30s with efficacious treatment
19 Sickle-Cell Anemia Recessive autosomal disease African descent Affects Hemoglobin protein in RBCsLow blood oxygen = hemoglobin molecules clump together forming sickle shaped RBCsSickle-celled RBCs clump together creating chronic vascular occlusion of small vesselsExample of incomplete dominanceHeterozygotes are usually normal but will show some symptoms during prolonged periods of reduced blood oxygenation
20 Dominant AllelesMost harmful alleles are recessive, but some human diseases are due to dominant allelesOnly require one copy of the allele to be expressedHypothesis: if there is a lethal disease carried on a dominant allele, it would have burned out its carriers by now.UNLESS, the lethal disease carried by a dominant allele is one that affects organisms of advanced ageLike Huntington’s disease
21 Examples of Dominant Allele Disease Achondroplasia – form of dwarfismHeterozygous individual = dwarf1 in 25,000 have achondroplasia, so 99% of the population are HomoRHuntington’s diseaseCaused by a lethal dominant alleleDegenerative disease of nervous systemUsually only affects those > 40 yrs old
22 Genetic TestingPedigree analysis gives some info about risk to offspringThere are also tests to identify carriers of certain genetic diseasesAmniocentesis – amniotic fluid is removed and then cells contained in the fluid are cultured to identify certain chromosomal defects via karyotypeChorionic villus sampling (CVS) – placental tissue is removed for same purpose as amnio, but results are available far faster, & can be performed earlier in pregnancyKaryotype can be immediately obtained
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