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6.2 inheritance of linked genes

6.2 inheritance of linked genes. SBI3U Ms. De Sousa. After mendel. Walter Sutton , discovered that alleles on the same chromosome do not segregate independently . Some genes are inherited together because they are linked . .

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6.2 inheritance of linked genes

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  1. 6.2 inheritance of linkedgenes SBI3U Ms. De Sousa

  2. Aftermendel • Walter Sutton, discoveredthatalleles on the same chromosome do not segregateindependently. • Somegenes are inheritedtogetherbecausethey are linked.

  3. Scientistsdiscoveredthat the Law of Independent Assortmentdid not apply to all cases. Genesthat are located on the same chromosome tend to segregatetogether. Linkedgenes: genesthat are on the same chromosome and that tend to beinheritedtogether.

  4. linkedgenes • Each chromosome containsthousands of genes. • Linkage group: all of the geneslocated on one chromosome. • All genes in a linkage group are inheritedtogetherunlesscrossing over occurs.

  5. crossing over • Crossing over is a processthatoccurs in Prophase I. • Twohomologous chromosomes exchange genetic information. • If crossing over occurs, the linkage group isdisrupted. • Genesthatwereinheritedtogether, nowsegregateseparately due to the event of crossing over.

  6. Crossing over Recombinant alleles: linkedgenesthat have been separatedduringcrossing over.

  7. Chromosome mapping • Allelesthatseperateduring crossing-over do sowith a predictablefrequency. • The frequency varies betweendifferent pairs of linkedgenes. • Crossing-over ismore commonbetweenallelesthat are far apart on the chromosomes.

  8. Chromosome mapping • Knowingtheirfrequencies, enablesscientists to map the genes on the chromosome. • Thusthey are able to track the position of the geneson the chromosomes.

  9. Sex- linkedinheritance Thomas Hunt Morgan, conductedresearch on « DrosophilaMelanogaster » to enhanceSutton’stheory. Throughhisresults, hewas able to discoverthat certain traits are controlled by genesthat are linked on the sex chromosomes ( X and Y) . This phenomenonisknown as « Sex-linked trait ».

  10. Sexlinkedinheritance X X In the F1 generation, both males and femaleshadredeyes. Thusredeyeshad to be a dominant trait. X X X X X Y X Y X Y

  11. Sex-linkedinheritance X X In the F2 generation, All of the females have redeyes. 50% of the males have white eyes 50% of the males have redeyes. X X X X X Y X Y X Y

  12. Why are the males and females different? Thomas Morgan did not thinkthatthiswas a a coincidence. He hypothesizedthatperhapsthese chromosomes werelocated on the sex chromosomes, thusinfluencingwether the male or femalewouldinherita certain allele.

  13. Sex chromosomes There isonly one area thatissimilarwhichallowsit to pair as homologous chromosomes X and Y chromosomes differ in terms of theirlength and the type/number of genesthat are present.

  14. Sex chromosomes • The X chromosome contains200 genes • The Y chromosome onlycontains 100 genes • Sex chromosomes containsomegenesthat are unrelated to sexdetermination. • Sex-linkedgene: geneslocated on the X- or Y- chromosome • Eachsex-linkedgenecancontain dominant vs. recessivealleles

  15. Sex-linkedinheritance • X-linkedgenes: geneslocated on the X chromosome -Alleles are identified as eitherdominant or recessivethroughsuperscripts - XR , Xr • Y linkedgenes: geneslocated on the Y chromosome -Alleles are identified as either dominant or recessivethroughsuperscripts - YR , Yr

  16. Sex-linkedinheritance • Femalesinherittwo X chromosomes. One chromosome isinheritedfrom the mother and the otherfrom the father. • Malesinheritone X-chromosome and one Y chromosome. The X-chromosome must beinheritedform the mother and the Y-chromosome isinheritedfrom the father. • If a geneislocated on the X-chromosome, the males must inherit the trait fromtheirmother.

  17. Female: XR X____ Male: XR Y Female: XrXr Male: Xr Y

  18. Morgan’sExperimentalevidence XR XR All F1 females are heterozygous, and thus have redeyes due to its dominant nature. All F1 males have redeyesbecausetheyinherited the dominant allelefrom the mother. Boys only have one X chromosome, thusthey express whicheverallelesthey’veinheritedfrom the mother. Xr Xr Xr XR XR Y XR Y XR Y

  19. Experimentalevidence XR Xr F2: All females have redeyesbecause the dominant allelssupresses the recessive white allele 50% of the males have redeyesbecausethey have inherited (XR) 50% of males have white eyesbecausethey have inherited (Xr) XR Xr XR XR XR Xr Y Y Y XR

  20. Sex-linkedinheritance • Recessive X-linked: the femalewillonly express it if sheishomozygousrecessive ( XrXr) • Dominant X-linked: the femalewill express is if sheishomozygous dominant or heterozygous ( XR X_) • The male willalways express the phenotype of the X chromosome thatisinherited. **Because the male only has one chromosome heisconsideredhemizygous (not hereto- or homo-)

  21. X – linkedrecessive

  22. X – linked dominant

  23. Autosomalrecessive

  24. X – linked dominant

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