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Punnet Squares & Dihybrid Crosses

Punnet Squares & Dihybrid Crosses. Back to Basics. What are their phenotypes?. How would you describe their genotypes?. Punnet Squares. Which one is the female?. How do you know?. What if we wanted to look at two features at once?. Feather color -- ORANGE or blue

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Punnet Squares & Dihybrid Crosses

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  1. Punnet Squares & Dihybrid Crosses

  2. Back to Basics What are their phenotypes? How would you describe their genotypes?

  3. Punnet Squares Which one is the female? How do you know?

  4. What if we wanted to look at two features at once? Feather color -- ORANGE or blue Crest? -- CREST or no crest What are the possible genotypes of these two birds? A – Orange a - blue Which alleles are dominant? C – Crest c – no crest

  5. Dihybrid Cross Feather color -- ORANGE or blue Crest? -- CREST or no crest Genotype: Aacc Possible gametes: Ac; Ac; ac; ac Genotype: aaCc Possible gametes: aC; ac; aC; ac

  6. Dihybrid Cross Feather color -- ORANGE or blue Crest? -- CREST or no crest Genotype: Aacc Possible gametes: Ac; Ac; ac; ac Genotype: aaCc Possible gametes: aC; ac; aC; ac

  7. Dihybrid Cross What are the possible genotypes of the offspring? What are the possible phenotypes of the offspring? What are the phenotypic ratios?

  8. Another example MA = lots of melanin mA = small amount of melanin MB = lots of melanin mB = small amount of melanin mAmB MAmB MAMB mAMB

  9. Linked Genes

  10. Linked Genes • These are: • Genes that have loci that are physically close to each other on the same chromosome • less likely to be independently assorted (separated from each other) during crossing over in meiosis

  11. Linked Genes • These are: • Genes that have loci that are physically close to each other on the same chromosome • less likely to be independently assorted (separated from each other) during crossing over in meiosis

  12. Because… • A crossing over point is more likely to occur on a chromosome between two genes that are widely separated compared to genes that are closer together

  13. For example:

  14. So what?? • This means that: • Linked genes are almost always inherited together as a single unit from one of the two parents • A dihybrid cross cannot be used to estimate the probability of inheriting different combinations of traits

  15. So how can we tell if genes are linked? Test crosses always include one homo-zygous recessive parent and one hetero-zygous parent • By looking at the results of a test cross: • ABab x abab =ABab; Abab; aBab; abab • If all offspring occur in equal amounts, genes are not linked • If linked, the vast majority of the offspring will be: • ABab or abab • Because AB or ab will be inherited as single units from one parent whilst only an ab combination an be inherited from the other.

  16. So how can we predict outcomes of linked gene crosses? • This depends on the distance between the linked genes on the chromosome • This is measured by map units • The number of map units correlates to the percentage of offspring that have recombined traits

  17. For example • In the cross ABab x abab • If A and B are 6 map units apart the recombined alleles Ab and aB will only have a probability of making up 6% of the total offspring (3% each) Parent Gametes

  18. Additional examples • In the cross CDcd x cdcd • 8 map units apart • recombined alleles Cd and cD will only have a probability of making up 8% of the total offspring (4% each) Parent Gametes • In the cross EFef x efef • 12 map units apart • recombined alleles Ef and eF will only have a probability of making up 12% of the total offspring (6% each) Parent Gametes

  19. Family Pedigrees

  20. Pedigree Charts • Pedigrees are diagrams which demonstrate the inheritance of a particular trait within a family • Pedigrees can indicate the inheritance modes of particular traits

  21. Pedigree Symbols I, II, III, IV – 1st, 2nd, 3rd, 4th generations respectively

  22. Are males and females equally affected?

  23. Pedigrees of x-linked recessive traits: • Males affected more commonly • All sons of affected female will also be affected • All children of 2 affected individuals will also have the trait

  24. Are males and females equally affected? Do all affected individuals have at least one affected parent?

  25. Autosomal Dominant • Pedigrees of autosomal dominant traits: • Males and females affected equally (autosomal) • Affected individuals must have at least one affected parent • Traits cannot ‘skip’ generations • No carriers

  26. Are males and females equally affected? Do all affected individuals have at least one affected parent?

  27. Autosomal Recessive • Pedigrees of autosomal recessive traits: • All children of 2 affected parents will also have the trait • Some affected individuals might have 2 unaffected parents if both are carriers: • Can ‘skip’ generations

  28. X-linked Dominant • Pedigrees of x-linked dominant traits: • All daughters of affected males will also have the trait • Female with the trait can pass it on to both daughters and sons • No carriers

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