Genetic LinkagePowerPoint PresentationSlide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Genetic Linkage1. Complete linkage2. Independent assortment3. Partial (incomplete) linkage4. Detecting and estimating linkage distanceQuiz Sept. 13, 20131)How are map units, % recombinants and linkage related?2)If we have two loci, A and B, map a cross between a homozygous recessive and a homozygous dominant in the correct format (hint: ‘chromosomes’ should be stacked). Show what an F1 would look like and show all possible gametes for the F1•If complete linkage exists between two genes because of their close proximity and organisms heterozygous at both loci are mated, a unique F2 phenotypic ratio designated the linkage ratio results (1:2:1 – Ab:AB:aB)•Genes assort independently if they are on different chromosomes but show linkage if they are on the same chromosome•In complete linkage, only parental (noncrossover) gametes are produced •If crossing over between two linked genes occurs between two nonsister chromatids, both parental and recombinant (crossover) gametes are producedFigure 5.2•The degree of crossing over between any two loci on a single chromosome is proportional to the distance between them, known as the interlocus distance•Genes on the same chromosome are part of a linkage group•The number of linkage groups should correspond to the haploid number of chromosomes•The percentage of offspring resulting from recombinant gametes depends on the distance between the two genes on the chromosome•Synapsed chromosomes in meiosis wrap around each other to create chiasmata, X-shaped intersections, that are points of genetic exchange•Two genes located relatively close to each other along a chromosome are less likely to have a chiasma form between them, and it is less likely that crossing over will occur•The recombination frequencies between linked genes are additive, and the frequency of exchange is an estimate of the relative distance between two genes along the chromosome© 2012 Pearson Education, Inc.Figure 5.5•One map unit (mu) is defined as 1 percent recombination between two genes on a chromosome •Map units are often called centimorgans (cM) and are relative distances, not exact ones•A single crossover (SCO) alters linkage between two genes only if the crossover occurs between those two genes•The percentage of tetrads involved in an exchange between two genes is twice the percentage of recombinant gametes produced •When two linked genes are more than 50 map units apart, a crossover theoretically can be expected to occur between them in 100 percent of the tetradsyellow – white recombinant frequency = 0.5%white – miniature recombinant frequency = 34.5%yellow – miniature recombinant frequency = 35.4%In Drosophila the droopy gene is at map position 71 and the curved gene is at position 76. Both mutant traits are recessive. A heterozygous female was generated by mating a wildtype male with a double recessive female. If this heterozygous female was test crossed and 1000 progeny were examined, how many wildtype progeny would you expect?If the heterozygous fly was male, how many wildtype progeny would you expect?Two genes control leaf traits in corn. The Pale gene controls leaf color intensity. The dominant P allele generates bright green leaves while the recessive p allele gives pale green leaves. The Stripe gene controls yellow striping on leaves. The dominant ST allele generates stripes and the recessive st allele causes no striping. Design an experiment to test whether these genes independently assort or whether these genes are linked. What results would support independent assortment?What results would support linkage?Mate a stripe bright plant with plain pale plant to generate a dihybrid. Test cross the dihybrid by mating it with a pale plain plant. The results Pale plain 88Pale striped 13Bright plain 9Bright striped 93Are these genes independently assorting?If they are linked, what is the genetic distance between