3Monohybrid Cross for Flower Color Parents differ for one traitParents are true breedingAnalyze parents for two generations, F1 and F2Examine large number of progenyParentalsXPurple WhiteXF1 Generation100% PurpleF2 GenerationPurple and White Flowered Plants705 purple and 224 white3:1 Ratio
5Mendel’s Postulates (based on monohybrid crosses) Alternative versions of genes (alleles) account for variation in characteristicsEach individual has two alleles for each gene, one inherited from each parentIf the individual has two different alleles, one allele is dominant to the other which is recessiveThe two alleles randomly segregate during gamete formation with 50% of the gametes having one allele and 50% having the other allele.
6Important Terminology Genotype – the list of alleles an individual possesses.Phenotype – a measurable characteristic displayed by an individualHomozygote – an individual possessing two identical alleles for a geneHeterozygote – an individual possessing two different alleles for a gene
7Allele for purple flowers Fig. 14-4Allele for purple flowersHomologouspair ofchromosomesLocus for flower-color geneFigure 14.4 Alleles, alternative versions of a geneAllele for white flowers
15Variations on Mendel Allelic Interactions Sex Linkage Mendel focused on traits based clearly on dominant and recessive alleles.However , the expression pattern of genes for some traits are not so straight forward.Allelic InteractionsSex LinkageComplete/Partial LinkageOrganellular Genes
17Complete Dominance – the heterozygote is indistinguishable from one of the two homozygotes. Rh factor – a protein found on the surface of the blood cells in some people.Two AllelesRh+ - possess the Rh factorRh- - lacks the Rh factorHeterozygoteRh+,Rh- - possess the Rh factor and has positive type blood.
18Partial DominanceHypercholesterolemia – defect in LDL receptor mediated endocytosis.Two allelesHc+ - functional LDL receptorHc- defective LDL receptorHomozygotesHc-,Hc- super high serum cholesterol early onset of heart disease.Hc+,Hc+ normal serum cholesterol- typical risk of heart disease.Hc+,Hc- high serum cholesterol – high risks of heart disease.
19Codominance – Heterozygote expresses both alleles (with no blending as in partial dominance) Two non-identical alleles of a gene are both expressed in heterozygotes, so neither is dominant or recessive. Co-dominance may occur in multiple allele systems.Multiple Alleles – More than two alleles in the population, although any individual still has only two alleles.ABO Blood type – carbohydrate attached to surface protein of blood cells.Three AllelesIA – A type bloodIB – B type bloodIO – O type bloodSix genotypesHomozygotes HeterozygotesIA,IA IA,IBIB,IB IA,IOIO,IO IB,IOA typeB typeAB typeO type
20Pleiotropy – one gene may influence multiple traits. Cystic FibrosisTwo AllelesCF – healthy allele –functional Cl transporter (dominant)cf – cystic fibrosis allele – non functional Cl transporter (recessive)Homozygotes for cf exhibit several symptoms- mucus filled lungs- kidney failure- digestive problems- salty sweat
21Genes encoding products used throughout the body are the ones most likely to be pleiotropic. Sergei RachmaninoffMutations in the fibrillin gene cause a disorder called Marfan syndrome.Fibrillin (long fibers) impart elasticity to the heart skin, blood vessels, tendons and other parts of the body. In Marfan syndrome the fibrillin are defective or not present.The aorta is particularly affected, can rupture during exercise.Hard to diagnose, affected people are typically, tall, thin , loose jointed. 1 in 5000
22Marfan Syndrome. Become Aware Haris Charalambous, a 6’10’’ Centre, died during a light training session at Toledo University on 9 October He was 21 years of age. He began his basketball career at Manchester Magic at age 13 and left the club at 18 to take up his scholarship at Toledo. He was later diagnosed with Marfan Syndrome, which is a genetic disease associated with abnormally tall people. At the time, neither the club nor Toledo knew of this situationSyndrome are a tall thin physique, disproportionately long limbs,fingers and toes, flat feet, spinal curvature and abnormally shapednarrow chest.
23Multigenic Traits – traits influenced by more than one gene. Epistasis – Alleles at one gene mask the expression of alleles at a second gene.Example – black/ brown gene of mice - albino geneB – black is dominant C – allows pigment (dominant)b – brown is recessive c – prevents pigment (recessive)
241/4 1/4 1/4 1/4 1/4 1/4 1/4 1/4 BbCc BbCc Sperm Eggs BBCC BbCC BBCc FigBbCcBbCcSperm1/41/41/41/4BCbCBcbcEggs1/4BCBBCCBbCCBBCcBbCc1/4bCBbCCbbCCBbCcbbCc1/4BcFigure An example of epistasisBBCcBbCcBBccBbcc1/4bcBbCcbbCcBbccbbcc9: 3: 4
25Polygenic Inheritance – many genes influencing a single trait Results in a continuum of variation (height in humans, skin pigmentation in humans,Fat content of milk cows, yield of kernels in corn)# dark alleles1/ / / / /64 6/64 1/64
30EXPERIMENT RESULTS CONCLUSION Fig. 15-4 + w w + w w + + GenerationF1All offspring had red eyesGenerationRESULTSF2GenerationCONCLUSIONP+XwXwGenerationXY+wwSpermEggsF1++Figure 15.4 In a cross between a wild-type female fruit fly and a mutant white-eyed male, what color eyes will the F1 and F2 offspring have?+wwGenerationww+wSpermEggs+++wwF2wGeneration+wwww+w
32Reciprocal Crosses ♂ ♀ ♂ ♀ Fruit flies use XY sex chromosomes X-linked white eye geneR – normal red eyes (dominant)r - white eyes (recessive)White eye X Red eye Red eye X White eye♂ ♀ ♂ ♀Assume flies came from true breeding cultures
33♂ ♀ ♂ ♀ White eye X Red eye Red eye X White eye R Y r Y r Rr rY R Rr ♂ ♀ ♂ ♀rY RR RY rrSpermSpermR Yr YrRrrYRRrRYEggsEggs50%Redfemale50%Whitemale50%Redfemale50%Redmale
35(a) Is a widow’s peak a dominant or recessive trait? Fig b1st generation(grandparents)WwwwwwWw2nd generation(parents, aunts,and uncles)WwwwwwWwWwww3rd generation(two sisters)WWwwFigure 14.15a Pedigree analysisorWwWidow’s peakNo widow’s peak(a) Is a widow’s peak a dominant or recessive trait?