05 Inheritance

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05 Inheritance

  1. 1. Genetics & Inheritance
  2. 2. INHERITANCE OF TRAITS
  3. 3. Allele 1 Allele 2 GENE
  4. 4. The Genetic Lottery <ul><li>As every individual carries two alleles for a particular gene, the chance of inheriting a particular one of these alleles is 1 in 2 or ½ </li></ul><ul><li>So the chance of if inheriting your genotype for any trait is ½ x ½ = ¼ </li></ul>
  5. 5. Mendel’s Pea Plants TT TT tt tt Tt Tt Tt Tt Tt Tt TT tt
  6. 6. Punnet Squares for: Monohybrid Crosses 1 st Generation 2 nd Generation All Tt or Tt All Tall 1 TT : 2 Tt : 1 tt or ½ TT: Tt : ½ tt 3 Tall : 1 Dwarf Genotypic ratio Phenotypic ratio T T t Tt ¼ Tt ¼ t Tt ¼ Tt ¼ T t T TT ¼ Tt ¼ t Tt ¼ tt ¼
  7. 7. Dihybrid Crosses <ul><li>Consider two traits </li></ul><ul><li>W = straight brow, w = widows peak </li></ul><ul><li>D = detatched earlobes, d = attached </li></ul><ul><li>If we cross 2 people who are heterozygous for both traits, how would the punnet square look and what would be the phenotypic ratios? </li></ul><ul><ul><li>NOTE: Genotypic / phenotypic ratios can be represented as fractions, decimals, ratios or percentages </li></ul></ul>
  8. 8. Dihybrid Crosses <ul><li>1 WWDD: 1 WWdd: 2 WWDd: 2 wwDd: 4 WwDd: 2 WwDD: 2 Wwdd: 1 wwDD: 1 wwdd </li></ul><ul><li>9 straight/detached: 3 straight/attached: 3 widow/detached: 1 widow attached </li></ul>WD Wd wD wd WD WWDD straight/detached WWDd straight/detached WwDD straight/detached WwDd straight/detached Wd WWDd straight/detached WWdd straight/attached WwDd straight/detached Wwdd straight/attached wD WwDD straight/detached WwDd straight/detached wwDD widows/detached wwDd widows/detached wd WwDd straight/detached Wwdd straight/attached wwDd widows/detached wwdd widows/attached
  9. 9. Dihybrid Crosses <ul><li>1WWDD:1WWdd:2WWDd:2wwDd:4WwDd:2WwDD:2Wwdd:1wwDD:1wwdd </li></ul><ul><li>9straight/detached:3straight/attached:3widow/detached:1widow attached </li></ul>WD Wd wD wd WD WWDD straight/detached WWDd straight/detached WwDD straight/detached WwDd straight/detached Wd WWDd straight/detached WWdd straight/attached WwDd straight/detached Wwdd straight/attached wD WwDD straight/detached WwDd straight/detached wwDD widows/detached wwDd widows/detached wd WwDd straight/detached Wwdd straight/attached wwDd widows/detached wwdd straight/attached
  10. 10. Test Crosses <ul><li>Prior to it being possible to establish an individual’s genotype via molecular means, a test cross was used. </li></ul><ul><li>Mono or dihybrid test crosses involve breeding the unknown individual with a homozygous recessive. </li></ul><ul><li>Eg. An unknown black cat (DD or Dd) is mated with a grey cat (dd). </li></ul><ul><li>If just one grey kitten is produced, a Dd genotype of the unknown is confirmed </li></ul><ul><li>The general rule is that 16 consecutive black kittens will provide reasonable confidence that the unknown is DD </li></ul>
  11. 11. Linked Genes <ul><li>If two genes are said to be linked, this means that they are located in a simillar position on the same chromosome and have a higher chance of being passed on together. </li></ul><ul><li>Eg. RBC shape and RH blood group are linked </li></ul><ul><ul><li>RH gene : D = Rh + , d = Rh - </li></ul></ul><ul><ul><li>EL1 gene : E = eliptical RBCs, e = normal RBCs </li></ul></ul>
  12. 12. Linked Genes <ul><li>The only way the alleles of linked genes can be separated is through crossing over. </li></ul><ul><li>Based on the above, Sarah is far more likely to produce parental eggs ( DE or de ) than recombinant eggs ( De or dE ) </li></ul>
  13. 13. Linked Genes <ul><li>Based on the spacing between linked genes, a probability of recombination can be expressed. </li></ul><ul><li>In the case of RH and EL1, the probability of recombination is 0.01 for each type </li></ul>
  14. 14. Detecting Linkage <ul><li>Do a punnet square for a test cross with a known heterozygote (aabb x AaBb). </li></ul><ul><ul><li>Genotypic ratio for unlinked genes is approx. 1:1:1:1 </li></ul></ul><ul><li>If the genes are linked, instead you should see a greater proportion of parental rather than recombinant genotypes. </li></ul>Genotype AaBb aabb Aabb aaBb Unlinked 25% 25% 25% 25% Linked 44% 44% 6% 6%
  15. 15. Detecting Linkage <ul><li>A simple calculation can be used to either predict distance between loci or outcomes for linked genes </li></ul><ul><ul><li>So, in a litter of 16 mice, if 7 have brown, curly hair, 7 have black, straight hair, 1 has brown straight hair and 1 has black curly hair ... </li></ul></ul><ul><ul><li>We can estimate these gene loci to be </li></ul></ul><ul><ul><li>= 12.5 map units apart </li></ul></ul>Distance between loci =
  16. 16. What can you predict from the following test cross results? <ul><li>Independent assortment (unlinked) </li></ul><ul><li>Linked, with no crossing over </li></ul><ul><li>Linked with crossing over </li></ul><ul><li>Now draw a representation of the heterozygote’s chromosomes </li></ul>A a B b Genotype AaBb Aabb aaBb aabb Ratio 1.1 0.9 1.2 1.0 Genotype AaBb aabb Ratio 1.0 1.0 Genotype AaBb Aabb aaBb aabb Ratio 1.0 0.45 0.55 1.0
  17. 17. Pedigrees – guide to symbols
  18. 18. Mr Jellinek’s family tree – EYE COLOUR Can you work out the genotypes for each individual?
  19. 19. Mr Jellinek’s family tree – EYE COLOUR bb bb bb bb bb Bb Bb Bb Bb Bb Can you work out the genotypes for each individual? Bb
  20. 20. Is this genetic pedigree possible, if so, explain how
  21. 21. 1 2 4 3 Yes it is, known genotypes are shown below 5 6 7 8 bb bb bb bb bb Bb Bb Bb Bb Bb Bb Bb Bb
  22. 22. Autosomal Recessive <ul><li>Key Features </li></ul><ul><li>Presence of affected individuals can frequently skip a generation. </li></ul><ul><li>In large samples there will be an even distribution of affected males and females </li></ul><ul><li>It is possible for two normal individuals to have an affected child </li></ul>
  23. 23. Autosomal Dominant <ul><li>Key Features </li></ul><ul><li>It is not possible for the disease to skip a generation </li></ul><ul><li>Homozygous affecteds are rare as they will always have affected offspring, meaning that lineage is often short-lived. </li></ul><ul><li>In large samples there will be an even distribution of affected males and females </li></ul>
  24. 24. X-Linked Recessive <ul><li>That disease will frequently skip a generation as females can be carriers </li></ul><ul><li>Majority of affecteds are usually male </li></ul><ul><li>All sons of an affected female will be affected </li></ul><ul><li>All daughters of an affected male will be carriers </li></ul>Key features
  25. 25. X-Linked Dominant <ul><li>Key Features </li></ul><ul><li>Affected male will always pass on the trait to his daughters, but not his sons </li></ul><ul><li>In large samples there will be an even distribution of affected males and females </li></ul><ul><li>Can not skip a generation </li></ul>
  26. 26. Construct this pedigree
  27. 27. Did yours look like this? Susan Jim Jean Scott James Natasha Alan Kylie Alison Paul Anne Emma Colin

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