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Lecture 3

CAS BI 216 Lecture 3: Genetics - Lecture 3 - Chapter 3 - (1/26/17)

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CAS BI 216
John Celenza

Lecture 3 – 1/27/17 (Chapter 3) Mutations Are the Source of New Alleles · Mutations o mutation = alterations in genetic material by chance § if occurring in gamete-producing cells => they are faithfully inherited § they make it possible to study genetic (transmission) · without mutations - its impossible to study genes that only come in one form · need two alleles to see the transmission of a gene · Allele Frequences - Monomorphic Genes o Total Number of Genes in Population = number of individuals x 2 o Allele frequency - percentage each allele of the gene in the total number of gene copies § Wild Type Alleles - most common alleles; assigned with a super script (+) · if it is present >1%; it is wild-type § Mutant Allele - rare allele (mutations arise from a new mutant allele) o Agouti gene - wild type allele (A) -> hair with black and yellow bands = dark grey (agouti) coloring § 14 mutant alleles of the agouti gene § (at) - recessive to wild-type -> black coat; yellow belly § (a) - recessive to A; produces pure black-coat · recessive alleles -> produce phenotypes that make it harder to survive § AA - is present at a frequency greater than 99% · Monomorphic - there is one wild-type allele that is most common · Allele Frequencies - Polymorphic Genes o some genes have more than one common allele - blood type o Polymorphic - having more than one common allele § example: Ia, Ib, and i all have comparable frequencies in a population o Common variants - refer to high-frequency alleles of a polymorphic gene One Gene May Contribute to Several Characteristics · Pleiotropy - (why men can become sterile) o Pleiotropy - single gene determines a more than one “seemingly unrelated” characteristics § each gene determine specific protein -> each protein can have a cascade of effects on an organism o Maori people of New Zealand - have lung problems and sterility § fault lies with recessive gene § normal dominant allele -> has protein needed for action of cilia and flagella § men who are homozygous recessive -> cilia don’t work; flagella can’t propel themselves forward § one gene affects a protein that affects both respiratory function and reproduction · Recessive Lethal Alleles o single variation of pleiotropy in alleles -> may not just affect phenotype but also viability y y o coat color inheritance in mice – A A -> is lethal § one of 14 mutant alleles -> leads to yellow color · AA -> A A = 1:1 ratio of two coat color among offspring o all yellow mice as agouti allele o yellow is dominant to agouti o all yellow mice are heterozygotes · A A x AA = 1:1 (A A:AA)y · A A x A A = 0:2:1 (2/3:1/3; A A, AA)y y y § pure bred A A is impossible -> it is lethal o recessive lethal allele - negatively affects survival of a homozygote o Note: easy to detect the lethal allele because it displays dominance to other phenotypes; not the same for all genes · Delayed Lethality o some homozygotes for a recessive allele don’t show effects until later in life § e.g. Tay-Sachs disease o can only be passed on by heterozygotes A Comprehensive Example - Sickle-Cell Disease · Sickle-Cell Disease o result of a faulty hemoglobin molecule o Hbα and Hpβ both produce · Multiple Alleles o β-globin - wild type => fuctioning β-globin protein o some mutations make hemoglobin less efficient o some cause beta-thalassemia · Pleiotropy o Hbβs allele of the beta-globi gen affects more than one trait § cause transformation after releasing oxygen -> form long fibers that deform into sickle shapes § leads to shortness of breath, fatigue -> anemia (low red blood cell count) o leads to resistance to malaria · Recessive Lethality o most homozygous for Hb-beta-s die early in childhood from heart failure · Different Dominance Relations o molecular - both alleles are codominant o cellular - affect of red blood cell shape - show either complete dominance or codominance depending on altitude § normal oxygen - Hb-beta-A is dominant § abnormal oxygen (drops) - some sickling occurs o malaria resistance - hb-beta-s is dominant § dominant because the cells break down before they can reproduce · 3.2 - Extensions to Mendel for Multifactorial Inheritence · most genes are multifactorial - affected by two are more genes o environment - all aspects of the outside world an organism comes into contact with Two Genes Can Interact to Determine One Trait · Dihybrid Cross o can show phenotypic ratios -> help determine if its one or two genes · Novel phenotypes from gene interactions o F2 - 9:3:3:1 - ratios - if you are looking at only one trait (lentil see color) o biochemistry - not 100% certain § Enzyme A -> tan pigment; Enzyme B -> grey pigment § if B is of
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