Presentation on theme: "Patterns of Inheritance"— Presentation transcript:
1 Patterns of Inheritance Chapter 9BIOL 1010Patterns of Inheritance
2 GENETICS: the scientific study of heredity Genome: complete set of an organism’s gene Gene: unit of heredity which codes for a proteinSiberian huskies: look alike because they share a similar genetic historyThick double coat, well-insulated ears, great staminaInbreeding: purebreds tend to concentrate ‘bad’ genes (i.e. PRA)A dog’s behavior is determined by its genes and by the environmentNature versus nurture
3 Gregor Mendel: “Father of Modern Genetics” Gained posthumous fame as the founder of geneticsWas the first person to analyze patterns of inheritance: “heritable factors”Deduced the fundamental principles of geneticsAustrian scientist and Augustinian friarMost famous for working with pea plant hybridization experiments in St. Thomas Abbey (geneticists have since ID’d the genes that controlled for traits Mendel usedCould completely control fertilization, short generation time, many offspring, easily identifiable traits1866: Published work (seven years after Darwin’s Origin of Species)Rejected at first; Darwin and other scientists unsuccessful at trying to explain inheritanceNot until the 1900’s that the importance of his work was realizedResearch was experimentally and mathematically rigorous, has stood the test of time
4 Rules of ProbabilityRule of Multiplication: probability of a compound event is the product of the separate probabilities of the independent eventsP(A and B) = P(A)*P(B)Rule of Addition: if events are mutually exclusive, then the probability of either/or is the sum of the separate probabilities of the independent eventsP(A or B) = P(A) + P(B)The probability of event A AND event B occurring is the chance of event A occurring multiplied by the chance of event B occuringThe probability of event A OR event B occurring is the chance of event A occurring added to the chance of event B occuring
5 Genetic NomenclatureHeredity: transmission of traits from one generation to the nextPhenotype (Character): heritable feature (i.e. flower color) based on genotype (genetic makeup)Trait: variant of a characterWild-type: variant found most often in natureTrue-breeding: purebred, offspring are identical to the parentHybrids: offspring of two different true-breeding parentsThe seven characters of pea plants studied by MendelWild-type: doesn’t necessarily mean dominant. (i.e. freckles, a dominant trait, is less frequent in humans than no freckles, which is recessive)
6 Inherited Traits in Humans: Controlled by a Single Gene *Most other traits such as hair or eye color are multigenic (polygenic) traits
7 Genetic CrossesMonohybrid CrossP generation: two different pure-breeding parental plantsF1 hybrids: the first generation plants obtained from crossing two selected pure breeding plants.F1 hybrids do not produce seed that is the same as the parent plantsF2 hybrids: second generation plants (result of self or cross fertilization of F1 hybrids
8 Monohybrid CrossA monohybrid cross is a cross between purebred parents that differ in only one characteristicF1 generation: all show the trait of one parent (i.e. purple flowers)F2 generation: show the two traits of the parents in a 3:1 ratio (i.e. purple to white flowers)Monohybrid CrossFigure 9.5 Mendel's cross tracking one character (flower color). (Step 3)
9 Mendel’s Law of Segregation There are alternate versions of genesFor each inherited character, an organism inherits two alleles, one from each parentHomozygous: two identical allelesHeterozygous: two different allelesIf the two alleles differ (the individual is heterozygous)Dominant allele: determines the phenotype (character)Recessive allele: no noticeable effect on the phenotype (character)Law of segregation: A sperm or egg carries only one allele for each character because the allele pair segregates during meiosisBased on Mendel’s observations, he came up with four hypotheses
10 Punnett Squares Show the results of random fertilization Each axis shows possible alleles from each parent
11 Test Crosses: Determining Unknown Genotypes Mating of an individual of dominant phenotype but unknown genotype to a homozygous recessive individual
12 Modern Genetics Modern genetics Homologous chromosomes contain the same genes at the same loci but may contain different alleles (alternative forms of a gene)Gene locus (plural-loci): specific location of a gene on the chromosomeGene loci: BRCA1: 17q21.31 (long arm of chromosome 17, band 21, sub-band 31)
13 Dihybrid CrossA dihybrid cross is a cross between purebred parents that differ in two characteristicsF1 generation: all show the dominant trait of the two charactersF2 generation: show four combinations of traits in a 9:3:3:1 ratioFigure 9.5 Mendel's cross tracking one character (flower color). (Step 3)
14 Mendel’s Law of Independent Assortment Each pair of alleles assorts independently of the other pairs of alleles during gamete formation (the inheritance of one character has no effect on the inheritance of another)Based on Mendel’s observations, he came up with four hypotheses
15 Mendel’s Laws and Meiosis Chromosome Theory of Inheritance: genes are located at specific loci on chromosomes and that the behavior of chromosomes during meiosis and fertilization accounts for inheritance patternsLaw of segregation: two allele pairs segregate into different gametes during meiosisLaw of Independent Assortment: each pair of alleles for a particular character segregate independently of each otherHow can genes located on the same chromosome be assorted independently?
16 A family pedigree showing inheritance of free versus attached earlobes MaleFemaleAffected MaleAffected FemaleMatingbetween related individualsA family pedigree showing inheritance of free versus attached earlobes
17 Some Autosomal Disorders in Humans Recessive DisordersAlbinism (1/22,000)Cystic fibrosis (1/1,800)Phenylketonuria (1/10,000)Sickle cell disease (1/500)Tay Sachs disease (1/3,500)Dominant DisordersAchondroplasia (1/25,000)Alzheimer’s disease (early onset)Huntington’s disease (1/25,000)Hypercholesterolemia (1/500)Recessive alleles (even lethal) persist indefinitely in populations in heterozygous carriersAlbinism: lack of pigment in skin, hair and eyesCystic fibrosis: excess mucus in lungs, digestive tract, liver; increased susceptibility to infections, death in early childhood unless treated-30,000 in US, 70,000 worldwide, 1/25 people are carriersPhenylketonuria: lack enzyme necessary to break down phenylalanine; accumulation of phenylalanine in blood; lack of normal skin pigment, mental retardation unless treatedSickle cell disease: sickled red blood cells; damage to many tissuesTay Sachs disease: lipid accumulation in brain cells, mental deficiency, blindness, death in childhoodAchondroplasia: dwarfismAlzheimer’s disease: mental deterioration; usually strikes late in lifeHuntington’s disease: mental deterioration and uncontrollable movements; strikes in middle ageHypercholesterolemia: excess cholesterol in blood; heart disease
18 Inbreeding Increases the Likelihood that a Recessive Trait will be Inherited Inbreeding: mating between close blood relativesIncreases the likelihood of homozygosity of a recessive allele in children of inbred parentsMost genetic disorders are not evenly distributed across all ethnic groupsResult of prolonged isolation of certain populationsDdDDDDDdDDDdDdDdEarly inhabitants of Martha’s Vineyard (island off the coast of Massachusetts) letd to frequent marriages between close relativesFrequency of an allele that caused deafness was highInbreeding in dogs: many purebreds have known genetic defectsInbreeding in endangered species: not many mates to choose fromEuropean ancestry: cystic fibrosis, phenylketonuriaAshkenazi Jews: Tay Sachs diseaseAfrican ancestry: sickle cell diseasedd=deaf
19 Dominant DisordersOne allele is all that is required to cause the phenotypei.e. Achondroplasia: dwarfism (homozygosity=lethal)Dominant lethal alleles are rare in populationsi.e. Huntington’s disease: causes degeneration of the nervous system in middle ageFamily with and without achondroplasia: Amy Roloff and husband Matt (unrelated form of dwarfism) have four children, one with achondroplasia, three withoutAchondroplasia: dwarfism, head and torso develop normally, arms and legs are shortFibroblast growth factor receptor 3 which causes an abnormality of cartilage formation leading to shortened bonesWhy are dominant lethal alleles rare?
20 Genetic Testing Carrier screening Prenatal diagnostic testing: i.e. amniocentesisNewborn screeningGenealogical DNA testingPredicting adult-onset disordersEstimating risks of disease developmentConfirmational diagnosisForensic/identity testingAbout 200,000 new breast cancer cases are diagnosed each yearApproximately 10% of these are heritable; they run in familiesThere are options if the results of genetic testing show a positive result for a BRCA mutationGenetic counseling: (interdisciplinary field between biology and psychology)Prenatal Diagnostic TestingAmniocentesis: physician removes ~2tsp of amniotic fluid with a needle for genetic testingChorionic villus sampling: physician collects some placental tissue for genetic testingNewer methods: able to isolate fetal cells from mother’s bloodAshkenazi Jewish ancestry: higher risk for Tay Sachs, Fanconi anemia, cystic fibrosis and othersEthical issues?
21 Variations on Mendel’s Laws Incomplete dominance: F1 has an appearance in between the two parental phenotypesCodominance: both alleles are fully expressed in heterozygous individualsPleiotropy: single gene influences more than one characterPolygenic Inhertitance: additive effects of two or more genes on a single phenotypeEnvironmental Factors: non-genetic, non-hereditary factors that contribute to phenotypeMendel was lucky he chose peas and not snapdragonsEnvironment: weight, skin color, health and fitness, etc.
22 An Example of Incomplete Dominance HypercholesterolemiaHypercholesterolemia: gene that codes for LDL receptor is defective, cells unable to mop up excess LDL from bloodHH: normal individualsHh: blood cholesterol levels about 2x normal, may have heart attacks by mid-30shh: blood cholesterol levels about 5x normal, may have heart attacks as early as age 2
23 An Example of Codominance ABO Blood GroupsABO Blood Groups: three alleles for blood type, IA, IB and iClumping of donor blood cells can kill the recipientWhat blood type is the universal donor?What blood type is the universal acceptor?Blood types: Out of 100 people38 will be O positive7 will be O negative34 will be A positive6 will be A negative8 will be B positive2 will be B negative4 will be AB positive1 will be AB negative
24 An Example of Pleiotropy Sickle-Cell DiseaseSickle-cell disease: defective gene causes abnormal hemoglobin proteinsSickle cells are rapidly destroyed by the bodySickle cells don’t flow smoothly in blood and tend to clump and clog
25 An Example of Polygenic Inheritance Skin ColorSkin color: controlled by at least three genes
26 Linked GenesThomas Hunt Morgan: In 1916, published a paper on genes in Drosophila melanogaster (fruit fly)Found the recombinant frequency to be 17%, not 50%Punnett square on expected outcomes assuming independent assortment (575) and compare with actual outcomesCalculate recombination frequency (391/2300) and convert into cM (centiMorgans)Why are recombination frequencies generally not over 50%
27 Linked GenesGenetic recombination (crossing over): usually ensures that genes on the same chromosome still assort independentlyLinked genes: genes so close together on a chromosome that they do not assort independently but tend to travel togetherLinkage map: diagrams describing relative gene locations using recombination frequenciesThe shorter the distance between two genes, the less likely a crossover event will happen between them
28 Sex Chromosomes and Sex-Linked Genes Sex chromosomes: X and YXX FemaleXY MaleSex-linked gene: any gene located on a sex chromosomeX-linked genes: the X chromosome contains many more genes (~2000 genes) than the Y chromosome (~78 genes)Sex-linked disorders: disorders associated with a defective gene found on a sex (usually X) chromosomeWhy doesn’t the Y chromosome contain as many genes as the X chromosome?Why are none of the Y chromosome genes necessary for survival?
30 Sex-Linked Disorders in Humans Hemophilia: “The Royal Disease”Sex-linked recessive blood-clotting trait that may result in excessive bleeding and death after relatively minor cuts and bruisesFigure 9.32 Hemophilia in the royal family of Russia.Figure 9.32